author | andy simpson <andrews@symbian.org> |
Fri, 08 Oct 2010 16:52:59 +0100 | |
changeset 119 | 931f2bbaabf7 |
parent 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
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************************************ |
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Idioms and Anti-Idioms in Python |
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************************************ |
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:Author: Moshe Zadka |
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This document is placed in the public domain. |
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.. topic:: Abstract |
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This document can be considered a companion to the tutorial. It shows how to use |
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Python, and even more importantly, how *not* to use Python. |
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Language Constructs You Should Not Use |
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====================================== |
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While Python has relatively few gotchas compared to other languages, it still |
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has some constructs which are only useful in corner cases, or are plain |
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dangerous. |
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from module import \* |
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--------------------- |
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Inside Function Definitions |
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^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
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``from module import *`` is *invalid* inside function definitions. While many |
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versions of Python do not check for the invalidity, it does not make it more |
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valid, no more then having a smart lawyer makes a man innocent. Do not use it |
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like that ever. Even in versions where it was accepted, it made the function |
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execution slower, because the compiler could not be certain which names are |
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local and which are global. In Python 2.1 this construct causes warnings, and |
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sometimes even errors. |
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At Module Level |
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^^^^^^^^^^^^^^^ |
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While it is valid to use ``from module import *`` at module level it is usually |
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a bad idea. For one, this loses an important property Python otherwise has --- |
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you can know where each toplevel name is defined by a simple "search" function |
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in your favourite editor. You also open yourself to trouble in the future, if |
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some module grows additional functions or classes. |
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One of the most awful question asked on the newsgroup is why this code:: |
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f = open("www") |
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f.read() |
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does not work. Of course, it works just fine (assuming you have a file called |
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"www".) But it does not work if somewhere in the module, the statement ``from os |
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import *`` is present. The :mod:`os` module has a function called :func:`open` |
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which returns an integer. While it is very useful, shadowing builtins is one of |
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its least useful properties. |
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Remember, you can never know for sure what names a module exports, so either |
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take what you need --- ``from module import name1, name2``, or keep them in the |
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module and access on a per-need basis --- ``import module;print module.name``. |
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When It Is Just Fine |
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^^^^^^^^^^^^^^^^^^^^ |
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There are situations in which ``from module import *`` is just fine: |
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* The interactive prompt. For example, ``from math import *`` makes Python an |
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amazing scientific calculator. |
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* When extending a module in C with a module in Python. |
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* When the module advertises itself as ``from import *`` safe. |
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Unadorned :keyword:`exec`, :func:`execfile` and friends |
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------------------------------------------------------- |
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The word "unadorned" refers to the use without an explicit dictionary, in which |
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case those constructs evaluate code in the *current* environment. This is |
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dangerous for the same reasons ``from import *`` is dangerous --- it might step |
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over variables you are counting on and mess up things for the rest of your code. |
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Simply do not do that. |
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Bad examples:: |
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>>> for name in sys.argv[1:]: |
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>>> exec "%s=1" % name |
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>>> def func(s, **kw): |
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>>> for var, val in kw.items(): |
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>>> exec "s.%s=val" % var # invalid! |
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>>> execfile("handler.py") |
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>>> handle() |
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Good examples:: |
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>>> d = {} |
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>>> for name in sys.argv[1:]: |
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>>> d[name] = 1 |
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>>> def func(s, **kw): |
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>>> for var, val in kw.items(): |
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>>> setattr(s, var, val) |
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>>> d={} |
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>>> execfile("handle.py", d, d) |
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>>> handle = d['handle'] |
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>>> handle() |
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|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
110 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
111 |
from module import name1, name2 |
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|
112 |
------------------------------- |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
113 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
114 |
This is a "don't" which is much weaker then the previous "don't"s but is still |
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|
115 |
something you should not do if you don't have good reasons to do that. The |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
116 |
reason it is usually bad idea is because you suddenly have an object which lives |
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|
117 |
in two separate namespaces. When the binding in one namespace changes, the |
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|
118 |
binding in the other will not, so there will be a discrepancy between them. This |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
119 |
happens when, for example, one module is reloaded, or changes the definition of |
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120 |
a function at runtime. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
121 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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122 |
Bad example:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
123 |
|
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|
124 |
# foo.py |
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125 |
a = 1 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
126 |
|
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127 |
# bar.py |
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128 |
from foo import a |
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129 |
if something(): |
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130 |
a = 2 # danger: foo.a != a |
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131 |
|
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132 |
Good example:: |
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|
133 |
|
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|
134 |
# foo.py |
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135 |
a = 1 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
136 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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137 |
# bar.py |
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|
138 |
import foo |
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|
139 |
if something(): |
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|
140 |
foo.a = 2 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
141 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
142 |
|
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|
143 |
except: |
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|
144 |
------- |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
145 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
146 |
Python has the ``except:`` clause, which catches all exceptions. Since *every* |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
147 |
error in Python raises an exception, this makes many programming errors look |
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|
148 |
like runtime problems, and hinders the debugging process. |
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|
149 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
150 |
The following code shows a great example:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
151 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
152 |
try: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
153 |
foo = opne("file") # misspelled "open" |
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parents:
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|
154 |
except: |
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|
155 |
sys.exit("could not open file!") |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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changeset
|
156 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
157 |
The second line triggers a :exc:`NameError` which is caught by the except |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
158 |
clause. The program will exit, and you will have no idea that this has nothing |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
159 |
to do with the readability of ``"file"``. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
160 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
161 |
The example above is better written :: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
162 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
163 |
try: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
164 |
foo = opne("file") # will be changed to "open" as soon as we run it |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
165 |
except IOError: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
166 |
sys.exit("could not open file") |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
167 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
168 |
There are some situations in which the ``except:`` clause is useful: for |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
169 |
example, in a framework when running callbacks, it is good not to let any |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
170 |
callback disturb the framework. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
171 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
172 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
173 |
Exceptions |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
174 |
========== |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
175 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
176 |
Exceptions are a useful feature of Python. You should learn to raise them |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
177 |
whenever something unexpected occurs, and catch them only where you can do |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
178 |
something about them. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
179 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
180 |
The following is a very popular anti-idiom :: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
181 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
182 |
def get_status(file): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
183 |
if not os.path.exists(file): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
184 |
print "file not found" |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
185 |
sys.exit(1) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
186 |
return open(file).readline() |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
187 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
188 |
Consider the case the file gets deleted between the time the call to |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
189 |
:func:`os.path.exists` is made and the time :func:`open` is called. That means |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
190 |
the last line will throw an :exc:`IOError`. The same would happen if *file* |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
191 |
exists but has no read permission. Since testing this on a normal machine on |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
192 |
existing and non-existing files make it seem bugless, that means in testing the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
193 |
results will seem fine, and the code will get shipped. Then an unhandled |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
194 |
:exc:`IOError` escapes to the user, who has to watch the ugly traceback. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
195 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
196 |
Here is a better way to do it. :: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
197 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
198 |
def get_status(file): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
199 |
try: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
200 |
return open(file).readline() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
201 |
except (IOError, OSError): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
202 |
print "file not found" |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
203 |
sys.exit(1) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
204 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
205 |
In this version, \*either\* the file gets opened and the line is read (so it |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
206 |
works even on flaky NFS or SMB connections), or the message is printed and the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
207 |
application aborted. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
208 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
209 |
Still, :func:`get_status` makes too many assumptions --- that it will only be |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
210 |
used in a short running script, and not, say, in a long running server. Sure, |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
211 |
the caller could do something like :: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
212 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
213 |
try: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
214 |
status = get_status(log) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
215 |
except SystemExit: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
216 |
status = None |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
217 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
218 |
So, try to make as few ``except`` clauses in your code --- those will usually be |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
219 |
a catch-all in the :func:`main`, or inside calls which should always succeed. |
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|
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So, the best version is probably :: |
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|
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def get_status(file): |
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return open(file).readline() |
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|
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The caller can deal with the exception if it wants (for example, if it tries |
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several files in a loop), or just let the exception filter upwards to *its* |
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228 |
caller. |
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|
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The last version is not very good either --- due to implementation details, the |
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file would not be closed when an exception is raised until the handler finishes, |
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and perhaps not at all in non-C implementations (e.g., Jython). :: |
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233 |
|
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def get_status(file): |
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fp = open(file) |
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try: |
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return fp.readline() |
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finally: |
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fp.close() |
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|
240 |
|
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|
241 |
|
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242 |
Using the Batteries |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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243 |
=================== |
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|
244 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
245 |
Every so often, people seem to be writing stuff in the Python library again, |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
246 |
usually poorly. While the occasional module has a poor interface, it is usually |
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|
247 |
much better to use the rich standard library and data types that come with |
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Python then inventing your own. |
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|
249 |
|
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A useful module very few people know about is :mod:`os.path`. It always has the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
251 |
correct path arithmetic for your operating system, and will usually be much |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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252 |
better then whatever you come up with yourself. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
253 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
254 |
Compare:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
255 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
256 |
# ugh! |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
257 |
return dir+"/"+file |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
258 |
# better |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
259 |
return os.path.join(dir, file) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
260 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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changeset
|
261 |
More useful functions in :mod:`os.path`: :func:`basename`, :func:`dirname` and |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
262 |
:func:`splitext`. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
263 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
264 |
There are also many useful builtin functions people seem not to be aware of for |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
265 |
some reason: :func:`min` and :func:`max` can find the minimum/maximum of any |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
266 |
sequence with comparable semantics, for example, yet many people write their own |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
267 |
:func:`max`/:func:`min`. Another highly useful function is :func:`reduce`. A |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
268 |
classical use of :func:`reduce` is something like :: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
269 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
270 |
import sys, operator |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
271 |
nums = map(float, sys.argv[1:]) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
272 |
print reduce(operator.add, nums)/len(nums) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
273 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
274 |
This cute little script prints the average of all numbers given on the command |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
275 |
line. The :func:`reduce` adds up all the numbers, and the rest is just some |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
276 |
pre- and postprocessing. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
277 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
278 |
On the same note, note that :func:`float`, :func:`int` and :func:`long` all |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
279 |
accept arguments of type string, and so are suited to parsing --- assuming you |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
280 |
are ready to deal with the :exc:`ValueError` they raise. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
281 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
282 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
283 |
Using Backslash to Continue Statements |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
284 |
====================================== |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
285 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
286 |
Since Python treats a newline as a statement terminator, and since statements |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
287 |
are often more then is comfortable to put in one line, many people do:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
288 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
289 |
if foo.bar()['first'][0] == baz.quux(1, 2)[5:9] and \ |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
290 |
calculate_number(10, 20) != forbulate(500, 360): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
291 |
pass |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
292 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
293 |
You should realize that this is dangerous: a stray space after the ``\`` would |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
294 |
make this line wrong, and stray spaces are notoriously hard to see in editors. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
295 |
In this case, at least it would be a syntax error, but if the code was:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
296 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
297 |
value = foo.bar()['first'][0]*baz.quux(1, 2)[5:9] \ |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
298 |
+ calculate_number(10, 20)*forbulate(500, 360) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
299 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
300 |
then it would just be subtly wrong. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
301 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
302 |
It is usually much better to use the implicit continuation inside parenthesis: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
303 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
304 |
This version is bulletproof:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
305 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
306 |
value = (foo.bar()['first'][0]*baz.quux(1, 2)[5:9] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
307 |
+ calculate_number(10, 20)*forbulate(500, 360)) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
308 |