author | bugtracker-ml@nttdocomo.com |
Tue, 06 Apr 2010 17:00:22 +0100 | |
changeset 56 | f24810eebc6b |
parent 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
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.. _tut-classes: |
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******* |
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Classes |
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******* |
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Python's class mechanism adds classes to the language with a minimum of new |
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syntax and semantics. It is a mixture of the class mechanisms found in C++ and |
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Modula-3. As is true for modules, classes in Python do not put an absolute |
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barrier between definition and user, but rather rely on the politeness of the |
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user not to "break into the definition." The most important features of classes |
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are retained with full power, however: the class inheritance mechanism allows |
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multiple base classes, a derived class can override any methods of its base |
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class or classes, and a method can call the method of a base class with the same |
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name. Objects can contain an arbitrary amount of private data. |
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In C++ terminology, all class members (including the data members) are *public*, |
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and all member functions are *virtual*. There are no special constructors or |
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destructors. As in Modula-3, there are no shorthands for referencing the |
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object's members from its methods: the method function is declared with an |
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explicit first argument representing the object, which is provided implicitly by |
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the call. As in Smalltalk, classes themselves are objects, albeit in the wider |
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sense of the word: in Python, all data types are objects. This provides |
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semantics for importing and renaming. Unlike C++ and Modula-3, built-in types |
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can be used as base classes for extension by the user. Also, like in C++ but |
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unlike in Modula-3, most built-in operators with special syntax (arithmetic |
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operators, subscripting etc.) can be redefined for class instances. |
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.. _tut-terminology: |
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A Word About Terminology |
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======================== |
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Lacking universally accepted terminology to talk about classes, I will make |
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occasional use of Smalltalk and C++ terms. (I would use Modula-3 terms, since |
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its object-oriented semantics are closer to those of Python than C++, but I |
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expect that few readers have heard of it.) |
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Objects have individuality, and multiple names (in multiple scopes) can be bound |
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to the same object. This is known as aliasing in other languages. This is |
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usually not appreciated on a first glance at Python, and can be safely ignored |
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when dealing with immutable basic types (numbers, strings, tuples). However, |
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aliasing has an (intended!) effect on the semantics of Python code involving |
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mutable objects such as lists, dictionaries, and most types representing |
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entities outside the program (files, windows, etc.). This is usually used to |
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the benefit of the program, since aliases behave like pointers in some respects. |
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For example, passing an object is cheap since only a pointer is passed by the |
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implementation; and if a function modifies an object passed as an argument, the |
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caller will see the change --- this eliminates the need for two different |
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argument passing mechanisms as in Pascal. |
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.. _tut-scopes: |
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Python Scopes and Name Spaces |
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============================= |
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Before introducing classes, I first have to tell you something about Python's |
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scope rules. Class definitions play some neat tricks with namespaces, and you |
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need to know how scopes and namespaces work to fully understand what's going on. |
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Incidentally, knowledge about this subject is useful for any advanced Python |
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programmer. |
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Let's begin with some definitions. |
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A *namespace* is a mapping from names to objects. Most namespaces are currently |
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implemented as Python dictionaries, but that's normally not noticeable in any |
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way (except for performance), and it may change in the future. Examples of |
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namespaces are: the set of built-in names (functions such as :func:`abs`, and |
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built-in exception names); the global names in a module; and the local names in |
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a function invocation. In a sense the set of attributes of an object also form |
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a namespace. The important thing to know about namespaces is that there is |
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absolutely no relation between names in different namespaces; for instance, two |
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different modules may both define a function "maximize" without confusion --- |
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users of the modules must prefix it with the module name. |
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By the way, I use the word *attribute* for any name following a dot --- for |
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example, in the expression ``z.real``, ``real`` is an attribute of the object |
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``z``. Strictly speaking, references to names in modules are attribute |
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references: in the expression ``modname.funcname``, ``modname`` is a module |
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object and ``funcname`` is an attribute of it. In this case there happens to be |
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a straightforward mapping between the module's attributes and the global names |
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defined in the module: they share the same namespace! [#]_ |
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Attributes may be read-only or writable. In the latter case, assignment to |
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attributes is possible. Module attributes are writable: you can write |
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``modname.the_answer = 42``. Writable attributes may also be deleted with the |
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:keyword:`del` statement. For example, ``del modname.the_answer`` will remove |
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the attribute :attr:`the_answer` from the object named by ``modname``. |
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91 |
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Name spaces are created at different moments and have different lifetimes. The |
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namespace containing the built-in names is created when the Python interpreter |
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starts up, and is never deleted. The global namespace for a module is created |
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when the module definition is read in; normally, module namespaces also last |
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until the interpreter quits. The statements executed by the top-level |
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invocation of the interpreter, either read from a script file or interactively, |
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are considered part of a module called :mod:`__main__`, so they have their own |
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global namespace. (The built-in names actually also live in a module; this is |
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called :mod:`__builtin__`.) |
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The local namespace for a function is created when the function is called, and |
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103 |
deleted when the function returns or raises an exception that is not handled |
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104 |
within the function. (Actually, forgetting would be a better way to describe |
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105 |
what actually happens.) Of course, recursive invocations each have their own |
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106 |
local namespace. |
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107 |
|
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108 |
A *scope* is a textual region of a Python program where a namespace is directly |
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accessible. "Directly accessible" here means that an unqualified reference to a |
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110 |
name attempts to find the name in the namespace. |
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111 |
|
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112 |
Although scopes are determined statically, they are used dynamically. At any |
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113 |
time during execution, there are at least three nested scopes whose namespaces |
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114 |
are directly accessible: the innermost scope, which is searched first, contains |
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115 |
the local names; the namespaces of any enclosing functions, which are searched |
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116 |
starting with the nearest enclosing scope; the middle scope, searched next, |
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contains the current module's global names; and the outermost scope (searched |
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last) is the namespace containing built-in names. |
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119 |
|
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120 |
If a name is declared global, then all references and assignments go directly to |
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121 |
the middle scope containing the module's global names. Otherwise, all variables |
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found outside of the innermost scope are read-only (an attempt to write to such |
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a variable will simply create a *new* local variable in the innermost scope, |
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leaving the identically named outer variable unchanged). |
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125 |
|
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126 |
Usually, the local scope references the local names of the (textually) current |
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127 |
function. Outside functions, the local scope references the same namespace as |
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128 |
the global scope: the module's namespace. Class definitions place yet another |
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namespace in the local scope. |
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130 |
|
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131 |
It is important to realize that scopes are determined textually: the global |
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132 |
scope of a function defined in a module is that module's namespace, no matter |
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133 |
from where or by what alias the function is called. On the other hand, the |
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134 |
actual search for names is done dynamically, at run time --- however, the |
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135 |
language definition is evolving towards static name resolution, at "compile" |
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136 |
time, so don't rely on dynamic name resolution! (In fact, local variables are |
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137 |
already determined statically.) |
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138 |
|
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139 |
A special quirk of Python is that -- if no :keyword:`global` |
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140 |
statement is in effect -- assignments to names always go |
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141 |
into the innermost scope. Assignments do not copy data --- they just bind names |
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142 |
to objects. The same is true for deletions: the statement ``del x`` removes the |
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143 |
binding of ``x`` from the namespace referenced by the local scope. In fact, all |
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144 |
operations that introduce new names use the local scope: in particular, import |
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145 |
statements and function definitions bind the module or function name in the |
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146 |
local scope. (The :keyword:`global` statement can be used to indicate that |
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147 |
particular variables live in the global scope.) |
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148 |
|
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149 |
|
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150 |
.. _tut-firstclasses: |
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151 |
|
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152 |
A First Look at Classes |
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153 |
======================= |
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154 |
|
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155 |
Classes introduce a little bit of new syntax, three new object types, and some |
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156 |
new semantics. |
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157 |
|
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158 |
|
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159 |
.. _tut-classdefinition: |
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160 |
|
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161 |
Class Definition Syntax |
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162 |
----------------------- |
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163 |
|
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164 |
The simplest form of class definition looks like this:: |
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165 |
|
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166 |
class ClassName: |
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167 |
<statement-1> |
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168 |
. |
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169 |
. |
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170 |
. |
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|
171 |
<statement-N> |
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172 |
|
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|
173 |
Class definitions, like function definitions (:keyword:`def` statements) must be |
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|
174 |
executed before they have any effect. (You could conceivably place a class |
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|
175 |
definition in a branch of an :keyword:`if` statement, or inside a function.) |
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|
176 |
|
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|
177 |
In practice, the statements inside a class definition will usually be function |
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|
178 |
definitions, but other statements are allowed, and sometimes useful --- we'll |
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|
179 |
come back to this later. The function definitions inside a class normally have |
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|
180 |
a peculiar form of argument list, dictated by the calling conventions for |
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|
181 |
methods --- again, this is explained later. |
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|
182 |
|
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|
183 |
When a class definition is entered, a new namespace is created, and used as the |
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|
184 |
local scope --- thus, all assignments to local variables go into this new |
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|
185 |
namespace. In particular, function definitions bind the name of the new |
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|
186 |
function here. |
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|
187 |
|
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|
188 |
When a class definition is left normally (via the end), a *class object* is |
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|
189 |
created. This is basically a wrapper around the contents of the namespace |
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|
190 |
created by the class definition; we'll learn more about class objects in the |
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|
191 |
next section. The original local scope (the one in effect just before the class |
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|
192 |
definition was entered) is reinstated, and the class object is bound here to the |
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|
193 |
class name given in the class definition header (:class:`ClassName` in the |
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|
194 |
example). |
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195 |
|
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196 |
|
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197 |
.. _tut-classobjects: |
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198 |
|
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199 |
Class Objects |
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200 |
------------- |
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201 |
|
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202 |
Class objects support two kinds of operations: attribute references and |
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203 |
instantiation. |
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204 |
|
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205 |
*Attribute references* use the standard syntax used for all attribute references |
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206 |
in Python: ``obj.name``. Valid attribute names are all the names that were in |
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|
207 |
the class's namespace when the class object was created. So, if the class |
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208 |
definition looked like this:: |
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209 |
|
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210 |
class MyClass: |
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"""A simple example class""" |
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i = 12345 |
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def f(self): |
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return 'hello world' |
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|
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then ``MyClass.i`` and ``MyClass.f`` are valid attribute references, returning |
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an integer and a function object, respectively. Class attributes can also be |
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assigned to, so you can change the value of ``MyClass.i`` by assignment. |
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:attr:`__doc__` is also a valid attribute, returning the docstring belonging to |
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the class: ``"A simple example class"``. |
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|
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Class *instantiation* uses function notation. Just pretend that the class |
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object is a parameterless function that returns a new instance of the class. |
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For example (assuming the above class):: |
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|
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x = MyClass() |
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|
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creates a new *instance* of the class and assigns this object to the local |
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variable ``x``. |
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|
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The instantiation operation ("calling" a class object) creates an empty object. |
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Many classes like to create objects with instances customized to a specific |
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initial state. Therefore a class may define a special method named |
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:meth:`__init__`, like this:: |
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|
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def __init__(self): |
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self.data = [] |
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|
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When a class defines an :meth:`__init__` method, class instantiation |
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automatically invokes :meth:`__init__` for the newly-created class instance. So |
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in this example, a new, initialized instance can be obtained by:: |
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|
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x = MyClass() |
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|
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Of course, the :meth:`__init__` method may have arguments for greater |
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flexibility. In that case, arguments given to the class instantiation operator |
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are passed on to :meth:`__init__`. For example, :: |
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|
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>>> class Complex: |
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... def __init__(self, realpart, imagpart): |
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... self.r = realpart |
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... self.i = imagpart |
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... |
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>>> x = Complex(3.0, -4.5) |
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>>> x.r, x.i |
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(3.0, -4.5) |
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|
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|
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.. _tut-instanceobjects: |
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|
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Instance Objects |
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---------------- |
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263 |
|
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Now what can we do with instance objects? The only operations understood by |
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instance objects are attribute references. There are two kinds of valid |
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attribute names, data attributes and methods. |
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267 |
|
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*data attributes* correspond to "instance variables" in Smalltalk, and to "data |
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members" in C++. Data attributes need not be declared; like local variables, |
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they spring into existence when they are first assigned to. For example, if |
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``x`` is the instance of :class:`MyClass` created above, the following piece of |
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code will print the value ``16``, without leaving a trace:: |
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|
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x.counter = 1 |
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while x.counter < 10: |
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x.counter = x.counter * 2 |
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print x.counter |
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del x.counter |
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279 |
|
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The other kind of instance attribute reference is a *method*. A method is a |
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function that "belongs to" an object. (In Python, the term method is not unique |
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to class instances: other object types can have methods as well. For example, |
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list objects have methods called append, insert, remove, sort, and so on. |
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However, in the following discussion, we'll use the term method exclusively to |
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mean methods of class instance objects, unless explicitly stated otherwise.) |
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286 |
|
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.. index:: object: method |
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288 |
|
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Valid method names of an instance object depend on its class. By definition, |
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all attributes of a class that are function objects define corresponding |
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methods of its instances. So in our example, ``x.f`` is a valid method |
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reference, since ``MyClass.f`` is a function, but ``x.i`` is not, since |
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``MyClass.i`` is not. But ``x.f`` is not the same thing as ``MyClass.f`` --- it |
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is a *method object*, not a function object. |
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295 |
|
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296 |
|
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.. _tut-methodobjects: |
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298 |
|
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299 |
Method Objects |
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300 |
-------------- |
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301 |
|
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Usually, a method is called right after it is bound:: |
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303 |
|
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x.f() |
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305 |
|
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In the :class:`MyClass` example, this will return the string ``'hello world'``. |
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However, it is not necessary to call a method right away: ``x.f`` is a method |
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object, and can be stored away and called at a later time. For example:: |
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309 |
|
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xf = x.f |
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while True: |
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print xf() |
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|
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will continue to print ``hello world`` until the end of time. |
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|
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What exactly happens when a method is called? You may have noticed that |
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``x.f()`` was called without an argument above, even though the function |
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definition for :meth:`f` specified an argument. What happened to the argument? |
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|
319 |
Surely Python raises an exception when a function that requires an argument is |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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diff
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|
320 |
called without any --- even if the argument isn't actually used... |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
321 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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diff
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|
322 |
Actually, you may have guessed the answer: the special thing about methods is |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
323 |
that the object is passed as the first argument of the function. In our |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
324 |
example, the call ``x.f()`` is exactly equivalent to ``MyClass.f(x)``. In |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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diff
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|
325 |
general, calling a method with a list of *n* arguments is equivalent to calling |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
326 |
the corresponding function with an argument list that is created by inserting |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
327 |
the method's object before the first argument. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
328 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
329 |
If you still don't understand how methods work, a look at the implementation can |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
330 |
perhaps clarify matters. When an instance attribute is referenced that isn't a |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
331 |
data attribute, its class is searched. If the name denotes a valid class |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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|
332 |
attribute that is a function object, a method object is created by packing |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
333 |
(pointers to) the instance object and the function object just found together in |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
334 |
an abstract object: this is the method object. When the method object is called |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
335 |
with an argument list, it is unpacked again, a new argument list is constructed |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
336 |
from the instance object and the original argument list, and the function object |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
337 |
is called with this new argument list. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
338 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
339 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
340 |
.. _tut-remarks: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
341 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
342 |
Random Remarks |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
343 |
============== |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
344 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
345 |
.. These should perhaps be placed more carefully... |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
346 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
347 |
Data attributes override method attributes with the same name; to avoid |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
348 |
accidental name conflicts, which may cause hard-to-find bugs in large programs, |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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|
349 |
it is wise to use some kind of convention that minimizes the chance of |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
350 |
conflicts. Possible conventions include capitalizing method names, prefixing |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
351 |
data attribute names with a small unique string (perhaps just an underscore), or |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
352 |
using verbs for methods and nouns for data attributes. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
353 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
354 |
Data attributes may be referenced by methods as well as by ordinary users |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
355 |
("clients") of an object. In other words, classes are not usable to implement |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
356 |
pure abstract data types. In fact, nothing in Python makes it possible to |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
357 |
enforce data hiding --- it is all based upon convention. (On the other hand, |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
358 |
the Python implementation, written in C, can completely hide implementation |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
359 |
details and control access to an object if necessary; this can be used by |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
360 |
extensions to Python written in C.) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
361 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
362 |
Clients should use data attributes with care --- clients may mess up invariants |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
363 |
maintained by the methods by stamping on their data attributes. Note that |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
364 |
clients may add data attributes of their own to an instance object without |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
365 |
affecting the validity of the methods, as long as name conflicts are avoided --- |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
366 |
again, a naming convention can save a lot of headaches here. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
367 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
368 |
There is no shorthand for referencing data attributes (or other methods!) from |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
369 |
within methods. I find that this actually increases the readability of methods: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
370 |
there is no chance of confusing local variables and instance variables when |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
371 |
glancing through a method. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
372 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
373 |
Often, the first argument of a method is called ``self``. This is nothing more |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
374 |
than a convention: the name ``self`` has absolutely no special meaning to |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
375 |
Python. (Note, however, that by not following the convention your code may be |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
376 |
less readable to other Python programmers, and it is also conceivable that a |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
377 |
*class browser* program might be written that relies upon such a convention.) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
378 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
379 |
Any function object that is a class attribute defines a method for instances of |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
380 |
that class. It is not necessary that the function definition is textually |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
381 |
enclosed in the class definition: assigning a function object to a local |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
382 |
variable in the class is also ok. For example:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
383 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
384 |
# Function defined outside the class |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
385 |
def f1(self, x, y): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
386 |
return min(x, x+y) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
387 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
388 |
class C: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
389 |
f = f1 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
390 |
def g(self): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
391 |
return 'hello world' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
392 |
h = g |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
393 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
394 |
Now ``f``, ``g`` and ``h`` are all attributes of class :class:`C` that refer to |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
395 |
function objects, and consequently they are all methods of instances of |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
396 |
:class:`C` --- ``h`` being exactly equivalent to ``g``. Note that this practice |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
397 |
usually only serves to confuse the reader of a program. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
398 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
399 |
Methods may call other methods by using method attributes of the ``self`` |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
400 |
argument:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
401 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
402 |
class Bag: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
403 |
def __init__(self): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
404 |
self.data = [] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
405 |
def add(self, x): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
406 |
self.data.append(x) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
407 |
def addtwice(self, x): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
408 |
self.add(x) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
409 |
self.add(x) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
410 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
411 |
Methods may reference global names in the same way as ordinary functions. The |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
412 |
global scope associated with a method is the module containing the class |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
413 |
definition. (The class itself is never used as a global scope!) While one |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
414 |
rarely encounters a good reason for using global data in a method, there are |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
415 |
many legitimate uses of the global scope: for one thing, functions and modules |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
416 |
imported into the global scope can be used by methods, as well as functions and |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
417 |
classes defined in it. Usually, the class containing the method is itself |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
418 |
defined in this global scope, and in the next section we'll find some good |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
419 |
reasons why a method would want to reference its own class! |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
420 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
421 |
Each value is an object, and therefore has a *class* (also called its *type*). |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
422 |
It is stored as ``object.__class__``. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
423 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
424 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
425 |
.. _tut-inheritance: |
2fb8b9db1c86
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426 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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427 |
Inheritance |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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428 |
=========== |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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429 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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430 |
Of course, a language feature would not be worthy of the name "class" without |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
431 |
supporting inheritance. The syntax for a derived class definition looks like |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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432 |
this:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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433 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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434 |
class DerivedClassName(BaseClassName): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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435 |
<statement-1> |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
436 |
. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
437 |
. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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438 |
. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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439 |
<statement-N> |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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440 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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441 |
The name :class:`BaseClassName` must be defined in a scope containing the |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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442 |
derived class definition. In place of a base class name, other arbitrary |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
443 |
expressions are also allowed. This can be useful, for example, when the base |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
444 |
class is defined in another module:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
445 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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446 |
class DerivedClassName(modname.BaseClassName): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
447 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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448 |
Execution of a derived class definition proceeds the same as for a base class. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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449 |
When the class object is constructed, the base class is remembered. This is |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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450 |
used for resolving attribute references: if a requested attribute is not found |
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451 |
in the class, the search proceeds to look in the base class. This rule is |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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452 |
applied recursively if the base class itself is derived from some other class. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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453 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
454 |
There's nothing special about instantiation of derived classes: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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|
455 |
``DerivedClassName()`` creates a new instance of the class. Method references |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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456 |
are resolved as follows: the corresponding class attribute is searched, |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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457 |
descending down the chain of base classes if necessary, and the method reference |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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458 |
is valid if this yields a function object. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
459 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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460 |
Derived classes may override methods of their base classes. Because methods |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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|
461 |
have no special privileges when calling other methods of the same object, a |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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462 |
method of a base class that calls another method defined in the same base class |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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463 |
may end up calling a method of a derived class that overrides it. (For C++ |
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martin.trojer@nokia.com
parents:
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|
464 |
programmers: all methods in Python are effectively ``virtual``.) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
465 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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466 |
An overriding method in a derived class may in fact want to extend rather than |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
467 |
simply replace the base class method of the same name. There is a simple way to |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
468 |
call the base class method directly: just call ``BaseClassName.methodname(self, |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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|
469 |
arguments)``. This is occasionally useful to clients as well. (Note that this |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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470 |
only works if the base class is defined or imported directly in the global |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
471 |
scope.) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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472 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
473 |
Python has two builtin functions that work with inheritance: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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474 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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475 |
* Use :func:`isinstance` to check an object's type: ``isinstance(obj, int)`` |
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|
476 |
will be ``True`` only if ``obj.__class__`` is :class:`int` or some class |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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|
477 |
derived from :class:`int`. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
478 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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|
479 |
* Use :func:`issubclass` to check class inheritance: ``issubclass(bool, int)`` |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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|
480 |
is ``True`` since :class:`bool` is a subclass of :class:`int`. However, |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
481 |
``issubclass(unicode, str)`` is ``False`` since :class:`unicode` is not a |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
482 |
subclass of :class:`str` (they only share a common ancestor, |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
483 |
:class:`basestring`). |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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|
484 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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|
485 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
486 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
487 |
.. _tut-multiple: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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|
488 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
489 |
Multiple Inheritance |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
490 |
-------------------- |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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|
491 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
492 |
Python supports a limited form of multiple inheritance as well. A class |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
493 |
definition with multiple base classes looks like this:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
494 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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changeset
|
495 |
class DerivedClassName(Base1, Base2, Base3): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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|
496 |
<statement-1> |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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changeset
|
497 |
. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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|
498 |
. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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changeset
|
499 |
. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
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changeset
|
500 |
<statement-N> |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
501 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
502 |
For old-style classes, the only rule is depth-first, left-to-right. Thus, if an |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
503 |
attribute is not found in :class:`DerivedClassName`, it is searched in |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
504 |
:class:`Base1`, then (recursively) in the base classes of :class:`Base1`, and |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
505 |
only if it is not found there, it is searched in :class:`Base2`, and so on. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
506 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
507 |
(To some people breadth first --- searching :class:`Base2` and :class:`Base3` |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
508 |
before the base classes of :class:`Base1` --- looks more natural. However, this |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
509 |
would require you to know whether a particular attribute of :class:`Base1` is |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
510 |
actually defined in :class:`Base1` or in one of its base classes before you can |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
511 |
figure out the consequences of a name conflict with an attribute of |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
512 |
:class:`Base2`. The depth-first rule makes no differences between direct and |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
513 |
inherited attributes of :class:`Base1`.) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
514 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
515 |
For :term:`new-style class`\es, the method resolution order changes dynamically |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
516 |
to support cooperative calls to :func:`super`. This approach is known in some |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
517 |
other multiple-inheritance languages as call-next-method and is more powerful |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
518 |
than the super call found in single-inheritance languages. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
519 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
520 |
With new-style classes, dynamic ordering is necessary because all cases of |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
521 |
multiple inheritance exhibit one or more diamond relationships (where one at |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
522 |
least one of the parent classes can be accessed through multiple paths from the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
523 |
bottommost class). For example, all new-style classes inherit from |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
524 |
:class:`object`, so any case of multiple inheritance provides more than one path |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
525 |
to reach :class:`object`. To keep the base classes from being accessed more |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
526 |
than once, the dynamic algorithm linearizes the search order in a way that |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
527 |
preserves the left-to-right ordering specified in each class, that calls each |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
528 |
parent only once, and that is monotonic (meaning that a class can be subclassed |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
529 |
without affecting the precedence order of its parents). Taken together, these |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
530 |
properties make it possible to design reliable and extensible classes with |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
531 |
multiple inheritance. For more detail, see |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
532 |
http://www.python.org/download/releases/2.3/mro/. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
533 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
534 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
535 |
.. _tut-private: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
536 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
537 |
Private Variables |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
538 |
================= |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
539 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
540 |
There is limited support for class-private identifiers. Any identifier of the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
541 |
form ``__spam`` (at least two leading underscores, at most one trailing |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
542 |
underscore) is textually replaced with ``_classname__spam``, where ``classname`` |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
543 |
is the current class name with leading underscore(s) stripped. This mangling is |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
544 |
done without regard to the syntactic position of the identifier, so it can be |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
545 |
used to define class-private instance and class variables, methods, variables |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
546 |
stored in globals, and even variables stored in instances. private to this class |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
547 |
on instances of *other* classes. Truncation may occur when the mangled name |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
548 |
would be longer than 255 characters. Outside classes, or when the class name |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
549 |
consists of only underscores, no mangling occurs. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
550 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
551 |
Name mangling is intended to give classes an easy way to define "private" |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
552 |
instance variables and methods, without having to worry about instance variables |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
553 |
defined by derived classes, or mucking with instance variables by code outside |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
554 |
the class. Note that the mangling rules are designed mostly to avoid accidents; |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
555 |
it still is possible for a determined soul to access or modify a variable that |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
556 |
is considered private. This can even be useful in special circumstances, such |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
557 |
as in the debugger, and that's one reason why this loophole is not closed. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
558 |
(Buglet: derivation of a class with the same name as the base class makes use of |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
559 |
private variables of the base class possible.) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
560 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
561 |
Notice that code passed to ``exec``, ``eval()`` or ``execfile()`` does not |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
562 |
consider the classname of the invoking class to be the current class; this is |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
563 |
similar to the effect of the ``global`` statement, the effect of which is |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
564 |
likewise restricted to code that is byte-compiled together. The same |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
565 |
restriction applies to ``getattr()``, ``setattr()`` and ``delattr()``, as well |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
566 |
as when referencing ``__dict__`` directly. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
567 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
568 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
569 |
.. _tut-odds: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
570 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
571 |
Odds and Ends |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
572 |
============= |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
573 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
574 |
Sometimes it is useful to have a data type similar to the Pascal "record" or C |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
575 |
"struct", bundling together a few named data items. An empty class definition |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
576 |
will do nicely:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
577 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
578 |
class Employee: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
579 |
pass |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
580 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
581 |
john = Employee() # Create an empty employee record |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
582 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
583 |
# Fill the fields of the record |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
584 |
john.name = 'John Doe' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
585 |
john.dept = 'computer lab' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
586 |
john.salary = 1000 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
587 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
588 |
A piece of Python code that expects a particular abstract data type can often be |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
589 |
passed a class that emulates the methods of that data type instead. For |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
590 |
instance, if you have a function that formats some data from a file object, you |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
591 |
can define a class with methods :meth:`read` and :meth:`readline` that get the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
592 |
data from a string buffer instead, and pass it as an argument. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
593 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
594 |
.. (Unfortunately, this technique has its limitations: a class can't define |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
595 |
operations that are accessed by special syntax such as sequence subscripting |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
596 |
or arithmetic operators, and assigning such a "pseudo-file" to sys.stdin will |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
597 |
not cause the interpreter to read further input from it.) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
598 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
599 |
Instance method objects have attributes, too: ``m.im_self`` is the instance |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
600 |
object with the method :meth:`m`, and ``m.im_func`` is the function object |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
601 |
corresponding to the method. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
602 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
603 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
604 |
.. _tut-exceptionclasses: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
605 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
606 |
Exceptions Are Classes Too |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
607 |
========================== |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
608 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
609 |
User-defined exceptions are identified by classes as well. Using this mechanism |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
610 |
it is possible to create extensible hierarchies of exceptions. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
611 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
612 |
There are two new valid (semantic) forms for the raise statement:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
613 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
614 |
raise Class, instance |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
615 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
616 |
raise instance |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
617 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
618 |
In the first form, ``instance`` must be an instance of :class:`Class` or of a |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
619 |
class derived from it. The second form is a shorthand for:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
620 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
621 |
raise instance.__class__, instance |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
622 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
623 |
A class in an except clause is compatible with an exception if it is the same |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
624 |
class or a base class thereof (but not the other way around --- an except clause |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
625 |
listing a derived class is not compatible with a base class). For example, the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
626 |
following code will print B, C, D in that order:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
627 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
628 |
class B: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
629 |
pass |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
630 |
class C(B): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
631 |
pass |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
632 |
class D(C): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
633 |
pass |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
634 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
635 |
for c in [B, C, D]: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
636 |
try: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
637 |
raise c() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
638 |
except D: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
639 |
print "D" |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
640 |
except C: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
641 |
print "C" |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
642 |
except B: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
643 |
print "B" |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
644 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
645 |
Note that if the except clauses were reversed (with ``except B`` first), it |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
646 |
would have printed B, B, B --- the first matching except clause is triggered. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
647 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
648 |
When an error message is printed for an unhandled exception, the exception's |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
649 |
class name is printed, then a colon and a space, and finally the instance |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
650 |
converted to a string using the built-in function :func:`str`. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
651 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
652 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
653 |
.. _tut-iterators: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
654 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
655 |
Iterators |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
656 |
========= |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
657 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
658 |
By now you have probably noticed that most container objects can be looped over |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
659 |
using a :keyword:`for` statement:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
660 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
661 |
for element in [1, 2, 3]: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
662 |
print element |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
663 |
for element in (1, 2, 3): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
664 |
print element |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
665 |
for key in {'one':1, 'two':2}: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
666 |
print key |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
667 |
for char in "123": |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
668 |
print char |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
669 |
for line in open("myfile.txt"): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
670 |
print line |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
671 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
672 |
This style of access is clear, concise, and convenient. The use of iterators |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
673 |
pervades and unifies Python. Behind the scenes, the :keyword:`for` statement |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
674 |
calls :func:`iter` on the container object. The function returns an iterator |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
675 |
object that defines the method :meth:`next` which accesses elements in the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
676 |
container one at a time. When there are no more elements, :meth:`next` raises a |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
677 |
:exc:`StopIteration` exception which tells the :keyword:`for` loop to terminate. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
678 |
This example shows how it all works:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
679 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
680 |
>>> s = 'abc' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
681 |
>>> it = iter(s) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
682 |
>>> it |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
683 |
<iterator object at 0x00A1DB50> |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
684 |
>>> it.next() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
685 |
'a' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
686 |
>>> it.next() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
687 |
'b' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
688 |
>>> it.next() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
689 |
'c' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
690 |
>>> it.next() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
691 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
692 |
Traceback (most recent call last): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
693 |
File "<stdin>", line 1, in ? |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
694 |
it.next() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
695 |
StopIteration |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
696 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
697 |
Having seen the mechanics behind the iterator protocol, it is easy to add |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
698 |
iterator behavior to your classes. Define a :meth:`__iter__` method which |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
699 |
returns an object with a :meth:`next` method. If the class defines |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
700 |
:meth:`next`, then :meth:`__iter__` can just return ``self``:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
701 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
702 |
class Reverse: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
703 |
"Iterator for looping over a sequence backwards" |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
704 |
def __init__(self, data): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
705 |
self.data = data |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
706 |
self.index = len(data) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
707 |
def __iter__(self): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
708 |
return self |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
709 |
def next(self): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
710 |
if self.index == 0: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
711 |
raise StopIteration |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
712 |
self.index = self.index - 1 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
713 |
return self.data[self.index] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
714 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
715 |
>>> for char in Reverse('spam'): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
716 |
... print char |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
717 |
... |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
718 |
m |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
719 |
a |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
720 |
p |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
721 |
s |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
722 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
723 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
724 |
.. _tut-generators: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
725 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
726 |
Generators |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
727 |
========== |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
728 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
729 |
:term:`Generator`\s are a simple and powerful tool for creating iterators. They |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
730 |
are written like regular functions but use the :keyword:`yield` statement |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
731 |
whenever they want to return data. Each time :meth:`next` is called, the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
732 |
generator resumes where it left-off (it remembers all the data values and which |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
733 |
statement was last executed). An example shows that generators can be trivially |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
734 |
easy to create:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
735 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
736 |
def reverse(data): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
737 |
for index in range(len(data)-1, -1, -1): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
738 |
yield data[index] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
739 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
740 |
>>> for char in reverse('golf'): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
741 |
... print char |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
742 |
... |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
743 |
f |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
744 |
l |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
745 |
o |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
746 |
g |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
747 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
748 |
Anything that can be done with generators can also be done with class based |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
749 |
iterators as described in the previous section. What makes generators so |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
750 |
compact is that the :meth:`__iter__` and :meth:`next` methods are created |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
751 |
automatically. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
752 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
753 |
Another key feature is that the local variables and execution state are |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
754 |
automatically saved between calls. This made the function easier to write and |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
755 |
much more clear than an approach using instance variables like ``self.index`` |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
756 |
and ``self.data``. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
757 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
758 |
In addition to automatic method creation and saving program state, when |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
759 |
generators terminate, they automatically raise :exc:`StopIteration`. In |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
760 |
combination, these features make it easy to create iterators with no more effort |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
761 |
than writing a regular function. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
762 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
763 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
764 |
.. _tut-genexps: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
765 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
766 |
Generator Expressions |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
767 |
===================== |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
768 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
769 |
Some simple generators can be coded succinctly as expressions using a syntax |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
770 |
similar to list comprehensions but with parentheses instead of brackets. These |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
771 |
expressions are designed for situations where the generator is used right away |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
772 |
by an enclosing function. Generator expressions are more compact but less |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
773 |
versatile than full generator definitions and tend to be more memory friendly |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
774 |
than equivalent list comprehensions. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
775 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
776 |
Examples:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
777 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
778 |
>>> sum(i*i for i in range(10)) # sum of squares |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
779 |
285 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
780 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
781 |
>>> xvec = [10, 20, 30] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
782 |
>>> yvec = [7, 5, 3] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
783 |
>>> sum(x*y for x,y in zip(xvec, yvec)) # dot product |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
784 |
260 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
785 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
786 |
>>> from math import pi, sin |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
787 |
>>> sine_table = dict((x, sin(x*pi/180)) for x in range(0, 91)) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
788 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
789 |
>>> unique_words = set(word for line in page for word in line.split()) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
790 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
791 |
>>> valedictorian = max((student.gpa, student.name) for student in graduates) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
792 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
793 |
>>> data = 'golf' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
794 |
>>> list(data[i] for i in range(len(data)-1,-1,-1)) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
795 |
['f', 'l', 'o', 'g'] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
796 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
797 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
798 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
799 |
.. rubric:: Footnotes |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
800 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
801 |
.. [#] Except for one thing. Module objects have a secret read-only attribute called |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
802 |
:attr:`__dict__` which returns the dictionary used to implement the module's |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
803 |
namespace; the name :attr:`__dict__` is an attribute but not a global name. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
804 |
Obviously, using this violates the abstraction of namespace implementation, and |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
805 |
should be restricted to things like post-mortem debuggers. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
806 |