--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/symbian-qemu-0.9.1-12/python-2.6.1/Doc/whatsnew/2.1.rst Fri Jul 31 15:01:17 2009 +0100
@@ -0,0 +1,794 @@
+****************************
+ What's New in Python 2.1
+****************************
+
+:Author: A.M. Kuchling
+
+.. |release| replace:: 1.01
+
+.. $Id: whatsnew21.tex 50964 2006-07-30 03:03:43Z fred.drake $
+
+
+Introduction
+============
+
+This article explains the new features in Python 2.1. While there aren't as
+many changes in 2.1 as there were in Python 2.0, there are still some pleasant
+surprises in store. 2.1 is the first release to be steered through the use of
+Python Enhancement Proposals, or PEPs, so most of the sizable changes have
+accompanying PEPs that provide more complete documentation and a design
+rationale for the change. This article doesn't attempt to document the new
+features completely, but simply provides an overview of the new features for
+Python programmers. Refer to the Python 2.1 documentation, or to the specific
+PEP, for more details about any new feature that particularly interests you.
+
+One recent goal of the Python development team has been to accelerate the pace
+of new releases, with a new release coming every 6 to 9 months. 2.1 is the first
+release to come out at this faster pace, with the first alpha appearing in
+January, 3 months after the final version of 2.0 was released.
+
+The final release of Python 2.1 was made on April 17, 2001.
+
+.. ======================================================================
+
+
+PEP 227: Nested Scopes
+======================
+
+The largest change in Python 2.1 is to Python's scoping rules. In Python 2.0,
+at any given time there are at most three namespaces used to look up variable
+names: local, module-level, and the built-in namespace. This often surprised
+people because it didn't match their intuitive expectations. For example, a
+nested recursive function definition doesn't work::
+
+ def f():
+ ...
+ def g(value):
+ ...
+ return g(value-1) + 1
+ ...
+
+The function :func:`g` will always raise a :exc:`NameError` exception, because
+the binding of the name ``g`` isn't in either its local namespace or in the
+module-level namespace. This isn't much of a problem in practice (how often do
+you recursively define interior functions like this?), but this also made using
+the :keyword:`lambda` statement clumsier, and this was a problem in practice.
+In code which uses :keyword:`lambda` you can often find local variables being
+copied by passing them as the default values of arguments. ::
+
+ def find(self, name):
+ "Return list of any entries equal to 'name'"
+ L = filter(lambda x, name=name: x == name,
+ self.list_attribute)
+ return L
+
+The readability of Python code written in a strongly functional style suffers
+greatly as a result.
+
+The most significant change to Python 2.1 is that static scoping has been added
+to the language to fix this problem. As a first effect, the ``name=name``
+default argument is now unnecessary in the above example. Put simply, when a
+given variable name is not assigned a value within a function (by an assignment,
+or the :keyword:`def`, :keyword:`class`, or :keyword:`import` statements),
+references to the variable will be looked up in the local namespace of the
+enclosing scope. A more detailed explanation of the rules, and a dissection of
+the implementation, can be found in the PEP.
+
+This change may cause some compatibility problems for code where the same
+variable name is used both at the module level and as a local variable within a
+function that contains further function definitions. This seems rather unlikely
+though, since such code would have been pretty confusing to read in the first
+place.
+
+One side effect of the change is that the ``from module import *`` and
+:keyword:`exec` statements have been made illegal inside a function scope under
+certain conditions. The Python reference manual has said all along that ``from
+module import *`` is only legal at the top level of a module, but the CPython
+interpreter has never enforced this before. As part of the implementation of
+nested scopes, the compiler which turns Python source into bytecodes has to
+generate different code to access variables in a containing scope. ``from
+module import *`` and :keyword:`exec` make it impossible for the compiler to
+figure this out, because they add names to the local namespace that are
+unknowable at compile time. Therefore, if a function contains function
+definitions or :keyword:`lambda` expressions with free variables, the compiler
+will flag this by raising a :exc:`SyntaxError` exception.
+
+To make the preceding explanation a bit clearer, here's an example::
+
+ x = 1
+ def f():
+ # The next line is a syntax error
+ exec 'x=2'
+ def g():
+ return x
+
+Line 4 containing the :keyword:`exec` statement is a syntax error, since
+:keyword:`exec` would define a new local variable named ``x`` whose value should
+be accessed by :func:`g`.
+
+This shouldn't be much of a limitation, since :keyword:`exec` is rarely used in
+most Python code (and when it is used, it's often a sign of a poor design
+anyway).
+
+Compatibility concerns have led to nested scopes being introduced gradually; in
+Python 2.1, they aren't enabled by default, but can be turned on within a module
+by using a future statement as described in PEP 236. (See the following section
+for further discussion of PEP 236.) In Python 2.2, nested scopes will become
+the default and there will be no way to turn them off, but users will have had
+all of 2.1's lifetime to fix any breakage resulting from their introduction.
+
+
+.. seealso::
+
+ :pep:`227` - Statically Nested Scopes
+ Written and implemented by Jeremy Hylton.
+
+.. ======================================================================
+
+
+PEP 236: __future__ Directives
+==============================
+
+The reaction to nested scopes was widespread concern about the dangers of
+breaking code with the 2.1 release, and it was strong enough to make the
+Pythoneers take a more conservative approach. This approach consists of
+introducing a convention for enabling optional functionality in release N that
+will become compulsory in release N+1.
+
+The syntax uses a ``from...import`` statement using the reserved module name
+:mod:`__future__`. Nested scopes can be enabled by the following statement::
+
+ from __future__ import nested_scopes
+
+While it looks like a normal :keyword:`import` statement, it's not; there are
+strict rules on where such a future statement can be put. They can only be at
+the top of a module, and must precede any Python code or regular
+:keyword:`import` statements. This is because such statements can affect how
+the Python bytecode compiler parses code and generates bytecode, so they must
+precede any statement that will result in bytecodes being produced.
+
+
+.. seealso::
+
+ :pep:`236` - Back to the :mod:`__future__`
+ Written by Tim Peters, and primarily implemented by Jeremy Hylton.
+
+.. ======================================================================
+
+
+PEP 207: Rich Comparisons
+=========================
+
+In earlier versions, Python's support for implementing comparisons on user-
+defined classes and extension types was quite simple. Classes could implement a
+:meth:`__cmp__` method that was given two instances of a class, and could only
+return 0 if they were equal or +1 or -1 if they weren't; the method couldn't
+raise an exception or return anything other than a Boolean value. Users of
+Numeric Python often found this model too weak and restrictive, because in the
+number-crunching programs that numeric Python is used for, it would be more
+useful to be able to perform elementwise comparisons of two matrices, returning
+a matrix containing the results of a given comparison for each element. If the
+two matrices are of different sizes, then the compare has to be able to raise an
+exception to signal the error.
+
+In Python 2.1, rich comparisons were added in order to support this need.
+Python classes can now individually overload each of the ``<``, ``<=``, ``>``,
+``>=``, ``==``, and ``!=`` operations. The new magic method names are:
+
++-----------+----------------+
+| Operation | Method name |
++===========+================+
+| ``<`` | :meth:`__lt__` |
++-----------+----------------+
+| ``<=`` | :meth:`__le__` |
++-----------+----------------+
+| ``>`` | :meth:`__gt__` |
++-----------+----------------+
+| ``>=`` | :meth:`__ge__` |
++-----------+----------------+
+| ``==`` | :meth:`__eq__` |
++-----------+----------------+
+| ``!=`` | :meth:`__ne__` |
++-----------+----------------+
+
+(The magic methods are named after the corresponding Fortran operators ``.LT.``.
+``.LE.``, &c. Numeric programmers are almost certainly quite familiar with
+these names and will find them easy to remember.)
+
+Each of these magic methods is of the form ``method(self, other)``, where
+``self`` will be the object on the left-hand side of the operator, while
+``other`` will be the object on the right-hand side. For example, the
+expression ``A < B`` will cause ``A.__lt__(B)`` to be called.
+
+Each of these magic methods can return anything at all: a Boolean, a matrix, a
+list, or any other Python object. Alternatively they can raise an exception if
+the comparison is impossible, inconsistent, or otherwise meaningless.
+
+The built-in :func:`cmp(A,B)` function can use the rich comparison machinery,
+and now accepts an optional argument specifying which comparison operation to
+use; this is given as one of the strings ``"<"``, ``"<="``, ``">"``, ``">="``,
+``"=="``, or ``"!="``. If called without the optional third argument,
+:func:`cmp` will only return -1, 0, or +1 as in previous versions of Python;
+otherwise it will call the appropriate method and can return any Python object.
+
+There are also corresponding changes of interest to C programmers; there's a new
+slot ``tp_richcmp`` in type objects and an API for performing a given rich
+comparison. I won't cover the C API here, but will refer you to PEP 207, or to
+2.1's C API documentation, for the full list of related functions.
+
+
+.. seealso::
+
+ :pep:`207` - Rich Comparisions
+ Written by Guido van Rossum, heavily based on earlier work by David Ascher, and
+ implemented by Guido van Rossum.
+
+.. ======================================================================
+
+
+PEP 230: Warning Framework
+==========================
+
+Over its 10 years of existence, Python has accumulated a certain number of
+obsolete modules and features along the way. It's difficult to know when a
+feature is safe to remove, since there's no way of knowing how much code uses it
+--- perhaps no programs depend on the feature, or perhaps many do. To enable
+removing old features in a more structured way, a warning framework was added.
+When the Python developers want to get rid of a feature, it will first trigger a
+warning in the next version of Python. The following Python version can then
+drop the feature, and users will have had a full release cycle to remove uses of
+the old feature.
+
+Python 2.1 adds the warning framework to be used in this scheme. It adds a
+:mod:`warnings` module that provide functions to issue warnings, and to filter
+out warnings that you don't want to be displayed. Third-party modules can also
+use this framework to deprecate old features that they no longer wish to
+support.
+
+For example, in Python 2.1 the :mod:`regex` module is deprecated, so importing
+it causes a warning to be printed::
+
+ >>> import regex
+ __main__:1: DeprecationWarning: the regex module
+ is deprecated; please use the re module
+ >>>
+
+Warnings can be issued by calling the :func:`warnings.warn` function::
+
+ warnings.warn("feature X no longer supported")
+
+The first parameter is the warning message; an additional optional parameters
+can be used to specify a particular warning category.
+
+Filters can be added to disable certain warnings; a regular expression pattern
+can be applied to the message or to the module name in order to suppress a
+warning. For example, you may have a program that uses the :mod:`regex` module
+and not want to spare the time to convert it to use the :mod:`re` module right
+now. The warning can be suppressed by calling ::
+
+ import warnings
+ warnings.filterwarnings(action = 'ignore',
+ message='.*regex module is deprecated',
+ category=DeprecationWarning,
+ module = '__main__')
+
+This adds a filter that will apply only to warnings of the class
+:class:`DeprecationWarning` triggered in the :mod:`__main__` module, and applies
+a regular expression to only match the message about the :mod:`regex` module
+being deprecated, and will cause such warnings to be ignored. Warnings can also
+be printed only once, printed every time the offending code is executed, or
+turned into exceptions that will cause the program to stop (unless the
+exceptions are caught in the usual way, of course).
+
+Functions were also added to Python's C API for issuing warnings; refer to PEP
+230 or to Python's API documentation for the details.
+
+
+.. seealso::
+
+ :pep:`5` - Guidelines for Language Evolution
+ Written by Paul Prescod, to specify procedures to be followed when removing old
+ features from Python. The policy described in this PEP hasn't been officially
+ adopted, but the eventual policy probably won't be too different from Prescod's
+ proposal.
+
+ :pep:`230` - Warning Framework
+ Written and implemented by Guido van Rossum.
+
+.. ======================================================================
+
+
+PEP 229: New Build System
+=========================
+
+When compiling Python, the user had to go in and edit the :file:`Modules/Setup`
+file in order to enable various additional modules; the default set is
+relatively small and limited to modules that compile on most Unix platforms.
+This means that on Unix platforms with many more features, most notably Linux,
+Python installations often don't contain all useful modules they could.
+
+Python 2.0 added the Distutils, a set of modules for distributing and installing
+extensions. In Python 2.1, the Distutils are used to compile much of the
+standard library of extension modules, autodetecting which ones are supported on
+the current machine. It's hoped that this will make Python installations easier
+and more featureful.
+
+Instead of having to edit the :file:`Modules/Setup` file in order to enable
+modules, a :file:`setup.py` script in the top directory of the Python source
+distribution is run at build time, and attempts to discover which modules can be
+enabled by examining the modules and header files on the system. If a module is
+configured in :file:`Modules/Setup`, the :file:`setup.py` script won't attempt
+to compile that module and will defer to the :file:`Modules/Setup` file's
+contents. This provides a way to specific any strange command-line flags or
+libraries that are required for a specific platform.
+
+In another far-reaching change to the build mechanism, Neil Schemenauer
+restructured things so Python now uses a single makefile that isn't recursive,
+instead of makefiles in the top directory and in each of the :file:`Python/`,
+:file:`Parser/`, :file:`Objects/`, and :file:`Modules/` subdirectories. This
+makes building Python faster and also makes hacking the Makefiles clearer and
+simpler.
+
+
+.. seealso::
+
+ :pep:`229` - Using Distutils to Build Python
+ Written and implemented by A.M. Kuchling.
+
+.. ======================================================================
+
+
+PEP 205: Weak References
+========================
+
+Weak references, available through the :mod:`weakref` module, are a minor but
+useful new data type in the Python programmer's toolbox.
+
+Storing a reference to an object (say, in a dictionary or a list) has the side
+effect of keeping that object alive forever. There are a few specific cases
+where this behaviour is undesirable, object caches being the most common one,
+and another being circular references in data structures such as trees.
+
+For example, consider a memoizing function that caches the results of another
+function :func:`f(x)` by storing the function's argument and its result in a
+dictionary::
+
+ _cache = {}
+ def memoize(x):
+ if _cache.has_key(x):
+ return _cache[x]
+
+ retval = f(x)
+
+ # Cache the returned object
+ _cache[x] = retval
+
+ return retval
+
+This version works for simple things such as integers, but it has a side effect;
+the ``_cache`` dictionary holds a reference to the return values, so they'll
+never be deallocated until the Python process exits and cleans up This isn't
+very noticeable for integers, but if :func:`f` returns an object, or a data
+structure that takes up a lot of memory, this can be a problem.
+
+Weak references provide a way to implement a cache that won't keep objects alive
+beyond their time. If an object is only accessible through weak references, the
+object will be deallocated and the weak references will now indicate that the
+object it referred to no longer exists. A weak reference to an object *obj* is
+created by calling ``wr = weakref.ref(obj)``. The object being referred to is
+returned by calling the weak reference as if it were a function: ``wr()``. It
+will return the referenced object, or ``None`` if the object no longer exists.
+
+This makes it possible to write a :func:`memoize` function whose cache doesn't
+keep objects alive, by storing weak references in the cache. ::
+
+ _cache = {}
+ def memoize(x):
+ if _cache.has_key(x):
+ obj = _cache[x]()
+ # If weak reference object still exists,
+ # return it
+ if obj is not None: return obj
+
+ retval = f(x)
+
+ # Cache a weak reference
+ _cache[x] = weakref.ref(retval)
+
+ return retval
+
+The :mod:`weakref` module also allows creating proxy objects which behave like
+weak references --- an object referenced only by proxy objects is deallocated --
+but instead of requiring an explicit call to retrieve the object, the proxy
+transparently forwards all operations to the object as long as the object still
+exists. If the object is deallocated, attempting to use a proxy will cause a
+:exc:`weakref.ReferenceError` exception to be raised. ::
+
+ proxy = weakref.proxy(obj)
+ proxy.attr # Equivalent to obj.attr
+ proxy.meth() # Equivalent to obj.meth()
+ del obj
+ proxy.attr # raises weakref.ReferenceError
+
+
+.. seealso::
+
+ :pep:`205` - Weak References
+ Written and implemented by Fred L. Drake, Jr.
+
+.. ======================================================================
+
+
+PEP 232: Function Attributes
+============================
+
+In Python 2.1, functions can now have arbitrary information attached to them.
+People were often using docstrings to hold information about functions and
+methods, because the ``__doc__`` attribute was the only way of attaching any
+information to a function. For example, in the Zope Web application server,
+functions are marked as safe for public access by having a docstring, and in
+John Aycock's SPARK parsing framework, docstrings hold parts of the BNF grammar
+to be parsed. This overloading is unfortunate, since docstrings are really
+intended to hold a function's documentation; for example, it means you can't
+properly document functions intended for private use in Zope.
+
+Arbitrary attributes can now be set and retrieved on functions using the regular
+Python syntax::
+
+ def f(): pass
+
+ f.publish = 1
+ f.secure = 1
+ f.grammar = "A ::= B (C D)*"
+
+The dictionary containing attributes can be accessed as the function's
+:attr:`__dict__`. Unlike the :attr:`__dict__` attribute of class instances, in
+functions you can actually assign a new dictionary to :attr:`__dict__`, though
+the new value is restricted to a regular Python dictionary; you *can't* be
+tricky and set it to a :class:`UserDict` instance, or any other random object
+that behaves like a mapping.
+
+
+.. seealso::
+
+ :pep:`232` - Function Attributes
+ Written and implemented by Barry Warsaw.
+
+.. ======================================================================
+
+
+PEP 235: Importing Modules on Case-Insensitive Platforms
+========================================================
+
+Some operating systems have filesystems that are case-insensitive, MacOS and
+Windows being the primary examples; on these systems, it's impossible to
+distinguish the filenames ``FILE.PY`` and ``file.py``, even though they do store
+the file's name in its original case (they're case-preserving, too).
+
+In Python 2.1, the :keyword:`import` statement will work to simulate case-
+sensitivity on case-insensitive platforms. Python will now search for the first
+case-sensitive match by default, raising an :exc:`ImportError` if no such file
+is found, so ``import file`` will not import a module named ``FILE.PY``. Case-
+insensitive matching can be requested by setting the :envvar:`PYTHONCASEOK`
+environment variable before starting the Python interpreter.
+
+.. ======================================================================
+
+
+PEP 217: Interactive Display Hook
+=================================
+
+When using the Python interpreter interactively, the output of commands is
+displayed using the built-in :func:`repr` function. In Python 2.1, the variable
+:func:`sys.displayhook` can be set to a callable object which will be called
+instead of :func:`repr`. For example, you can set it to a special pretty-
+printing function::
+
+ >>> # Create a recursive data structure
+ ... L = [1,2,3]
+ >>> L.append(L)
+ >>> L # Show Python's default output
+ [1, 2, 3, [...]]
+ >>> # Use pprint.pprint() as the display function
+ ... import sys, pprint
+ >>> sys.displayhook = pprint.pprint
+ >>> L
+ [1, 2, 3, <Recursion on list with id=135143996>]
+ >>>
+
+
+.. seealso::
+
+ :pep:`217` - Display Hook for Interactive Use
+ Written and implemented by Moshe Zadka.
+
+.. ======================================================================
+
+
+PEP 208: New Coercion Model
+===========================
+
+How numeric coercion is done at the C level was significantly modified. This
+will only affect the authors of C extensions to Python, allowing them more
+flexibility in writing extension types that support numeric operations.
+
+Extension types can now set the type flag ``Py_TPFLAGS_CHECKTYPES`` in their
+``PyTypeObject`` structure to indicate that they support the new coercion model.
+In such extension types, the numeric slot functions can no longer assume that
+they'll be passed two arguments of the same type; instead they may be passed two
+arguments of differing types, and can then perform their own internal coercion.
+If the slot function is passed a type it can't handle, it can indicate the
+failure by returning a reference to the ``Py_NotImplemented`` singleton value.
+The numeric functions of the other type will then be tried, and perhaps they can
+handle the operation; if the other type also returns ``Py_NotImplemented``, then
+a :exc:`TypeError` will be raised. Numeric methods written in Python can also
+return ``Py_NotImplemented``, causing the interpreter to act as if the method
+did not exist (perhaps raising a :exc:`TypeError`, perhaps trying another
+object's numeric methods).
+
+
+.. seealso::
+
+ :pep:`208` - Reworking the Coercion Model
+ Written and implemented by Neil Schemenauer, heavily based upon earlier work by
+ Marc-André Lemburg. Read this to understand the fine points of how numeric
+ operations will now be processed at the C level.
+
+.. ======================================================================
+
+
+PEP 241: Metadata in Python Packages
+====================================
+
+A common complaint from Python users is that there's no single catalog of all
+the Python modules in existence. T. Middleton's Vaults of Parnassus at
+http://www.vex.net/parnassus/ are the largest catalog of Python modules, but
+registering software at the Vaults is optional, and many people don't bother.
+
+As a first small step toward fixing the problem, Python software packaged using
+the Distutils :command:`sdist` command will include a file named
+:file:`PKG-INFO` containing information about the package such as its name,
+version, and author (metadata, in cataloguing terminology). PEP 241 contains
+the full list of fields that can be present in the :file:`PKG-INFO` file. As
+people began to package their software using Python 2.1, more and more packages
+will include metadata, making it possible to build automated cataloguing systems
+and experiment with them. With the result experience, perhaps it'll be possible
+to design a really good catalog and then build support for it into Python 2.2.
+For example, the Distutils :command:`sdist` and :command:`bdist_\*` commands
+could support a :option:`upload` option that would automatically upload your
+package to a catalog server.
+
+You can start creating packages containing :file:`PKG-INFO` even if you're not
+using Python 2.1, since a new release of the Distutils will be made for users of
+earlier Python versions. Version 1.0.2 of the Distutils includes the changes
+described in PEP 241, as well as various bugfixes and enhancements. It will be
+available from the Distutils SIG at http://www.python.org/sigs/distutils-sig/.
+
+
+.. seealso::
+
+ :pep:`241` - Metadata for Python Software Packages
+ Written and implemented by A.M. Kuchling.
+
+ :pep:`243` - Module Repository Upload Mechanism
+ Written by Sean Reifschneider, this draft PEP describes a proposed mechanism for
+ uploading Python packages to a central server.
+
+.. ======================================================================
+
+
+New and Improved Modules
+========================
+
+* Ka-Ping Yee contributed two new modules: :mod:`inspect.py`, a module for
+ getting information about live Python code, and :mod:`pydoc.py`, a module for
+ interactively converting docstrings to HTML or text. As a bonus,
+ :file:`Tools/scripts/pydoc`, which is now automatically installed, uses
+ :mod:`pydoc.py` to display documentation given a Python module, package, or
+ class name. For example, ``pydoc xml.dom`` displays the following::
+
+ Python Library Documentation: package xml.dom in xml
+
+ NAME
+ xml.dom - W3C Document Object Model implementation for Python.
+
+ FILE
+ /usr/local/lib/python2.1/xml/dom/__init__.pyc
+
+ DESCRIPTION
+ The Python mapping of the Document Object Model is documented in the
+ Python Library Reference in the section on the xml.dom package.
+
+ This package contains the following modules:
+ ...
+
+ :file:`pydoc` also includes a Tk-based interactive help browser. :file:`pydoc`
+ quickly becomes addictive; try it out!
+
+* Two different modules for unit testing were added to the standard library.
+ The :mod:`doctest` module, contributed by Tim Peters, provides a testing
+ framework based on running embedded examples in docstrings and comparing the
+ results against the expected output. PyUnit, contributed by Steve Purcell, is a
+ unit testing framework inspired by JUnit, which was in turn an adaptation of
+ Kent Beck's Smalltalk testing framework. See http://pyunit.sourceforge.net/ for
+ more information about PyUnit.
+
+* The :mod:`difflib` module contains a class, :class:`SequenceMatcher`, which
+ compares two sequences and computes the changes required to transform one
+ sequence into the other. For example, this module can be used to write a tool
+ similar to the Unix :program:`diff` program, and in fact the sample program
+ :file:`Tools/scripts/ndiff.py` demonstrates how to write such a script.
+
+* :mod:`curses.panel`, a wrapper for the panel library, part of ncurses and of
+ SYSV curses, was contributed by Thomas Gellekum. The panel library provides
+ windows with the additional feature of depth. Windows can be moved higher or
+ lower in the depth ordering, and the panel library figures out where panels
+ overlap and which sections are visible.
+
+* The PyXML package has gone through a few releases since Python 2.0, and Python
+ 2.1 includes an updated version of the :mod:`xml` package. Some of the
+ noteworthy changes include support for Expat 1.2 and later versions, the ability
+ for Expat parsers to handle files in any encoding supported by Python, and
+ various bugfixes for SAX, DOM, and the :mod:`minidom` module.
+
+* Ping also contributed another hook for handling uncaught exceptions.
+ :func:`sys.excepthook` can be set to a callable object. When an exception isn't
+ caught by any :keyword:`try`...\ :keyword:`except` blocks, the exception will be
+ passed to :func:`sys.excepthook`, which can then do whatever it likes. At the
+ Ninth Python Conference, Ping demonstrated an application for this hook:
+ printing an extended traceback that not only lists the stack frames, but also
+ lists the function arguments and the local variables for each frame.
+
+* Various functions in the :mod:`time` module, such as :func:`asctime` and
+ :func:`localtime`, require a floating point argument containing the time in
+ seconds since the epoch. The most common use of these functions is to work with
+ the current time, so the floating point argument has been made optional; when a
+ value isn't provided, the current time will be used. For example, log file
+ entries usually need a string containing the current time; in Python 2.1,
+ ``time.asctime()`` can be used, instead of the lengthier
+ ``time.asctime(time.localtime(time.time()))`` that was previously required.
+
+ This change was proposed and implemented by Thomas Wouters.
+
+* The :mod:`ftplib` module now defaults to retrieving files in passive mode,
+ because passive mode is more likely to work from behind a firewall. This
+ request came from the Debian bug tracking system, since other Debian packages
+ use :mod:`ftplib` to retrieve files and then don't work from behind a firewall.
+ It's deemed unlikely that this will cause problems for anyone, because Netscape
+ defaults to passive mode and few people complain, but if passive mode is
+ unsuitable for your application or network setup, call :meth:`set_pasv(0)` on
+ FTP objects to disable passive mode.
+
+* Support for raw socket access has been added to the :mod:`socket` module,
+ contributed by Grant Edwards.
+
+* The :mod:`pstats` module now contains a simple interactive statistics browser
+ for displaying timing profiles for Python programs, invoked when the module is
+ run as a script. Contributed by Eric S. Raymond.
+
+* A new implementation-dependent function, :func:`sys._getframe([depth])`, has
+ been added to return a given frame object from the current call stack.
+ :func:`sys._getframe` returns the frame at the top of the call stack; if the
+ optional integer argument *depth* is supplied, the function returns the frame
+ that is *depth* calls below the top of the stack. For example,
+ ``sys._getframe(1)`` returns the caller's frame object.
+
+ This function is only present in CPython, not in Jython or the .NET
+ implementation. Use it for debugging, and resist the temptation to put it into
+ production code.
+
+.. ======================================================================
+
+
+Other Changes and Fixes
+=======================
+
+There were relatively few smaller changes made in Python 2.1 due to the shorter
+release cycle. A search through the CVS change logs turns up 117 patches
+applied, and 136 bugs fixed; both figures are likely to be underestimates. Some
+of the more notable changes are:
+
+* A specialized object allocator is now optionally available, that should be
+ faster than the system :func:`malloc` and have less memory overhead. The
+ allocator uses C's :func:`malloc` function to get large pools of memory, and
+ then fulfills smaller memory requests from these pools. It can be enabled by
+ providing the :option:`--with-pymalloc` option to the :program:`configure`
+ script; see :file:`Objects/obmalloc.c` for the implementation details.
+
+ Authors of C extension modules should test their code with the object allocator
+ enabled, because some incorrect code may break, causing core dumps at runtime.
+ There are a bunch of memory allocation functions in Python's C API that have
+ previously been just aliases for the C library's :func:`malloc` and
+ :func:`free`, meaning that if you accidentally called mismatched functions, the
+ error wouldn't be noticeable. When the object allocator is enabled, these
+ functions aren't aliases of :func:`malloc` and :func:`free` any more, and
+ calling the wrong function to free memory will get you a core dump. For
+ example, if memory was allocated using :func:`PyMem_New`, it has to be freed
+ using :func:`PyMem_Del`, not :func:`free`. A few modules included with Python
+ fell afoul of this and had to be fixed; doubtless there are more third-party
+ modules that will have the same problem.
+
+ The object allocator was contributed by Vladimir Marangozov.
+
+* The speed of line-oriented file I/O has been improved because people often
+ complain about its lack of speed, and because it's often been used as a naïve
+ benchmark. The :meth:`readline` method of file objects has therefore been
+ rewritten to be much faster. The exact amount of the speedup will vary from
+ platform to platform depending on how slow the C library's :func:`getc` was, but
+ is around 66%, and potentially much faster on some particular operating systems.
+ Tim Peters did much of the benchmarking and coding for this change, motivated by
+ a discussion in comp.lang.python.
+
+ A new module and method for file objects was also added, contributed by Jeff
+ Epler. The new method, :meth:`xreadlines`, is similar to the existing
+ :func:`xrange` built-in. :func:`xreadlines` returns an opaque sequence object
+ that only supports being iterated over, reading a line on every iteration but
+ not reading the entire file into memory as the existing :meth:`readlines` method
+ does. You'd use it like this::
+
+ for line in sys.stdin.xreadlines():
+ # ... do something for each line ...
+ ...
+
+ For a fuller discussion of the line I/O changes, see the python-dev summary for
+ January 1-15, 2001 at http://www.python.org/dev/summary/2001-01-1.html.
+
+* A new method, :meth:`popitem`, was added to dictionaries to enable
+ destructively iterating through the contents of a dictionary; this can be faster
+ for large dictionaries because there's no need to construct a list containing
+ all the keys or values. ``D.popitem()`` removes a random ``(key, value)`` pair
+ from the dictionary ``D`` and returns it as a 2-tuple. This was implemented
+ mostly by Tim Peters and Guido van Rossum, after a suggestion and preliminary
+ patch by Moshe Zadka.
+
+* Modules can now control which names are imported when ``from module import *``
+ is used, by defining an ``__all__`` attribute containing a list of names that
+ will be imported. One common complaint is that if the module imports other
+ modules such as :mod:`sys` or :mod:`string`, ``from module import *`` will add
+ them to the importing module's namespace. To fix this, simply list the public
+ names in ``__all__``::
+
+ # List public names
+ __all__ = ['Database', 'open']
+
+ A stricter version of this patch was first suggested and implemented by Ben
+ Wolfson, but after some python-dev discussion, a weaker final version was
+ checked in.
+
+* Applying :func:`repr` to strings previously used octal escapes for
+ non-printable characters; for example, a newline was ``'\012'``. This was a
+ vestigial trace of Python's C ancestry, but today octal is of very little
+ practical use. Ka-Ping Yee suggested using hex escapes instead of octal ones,
+ and using the ``\n``, ``\t``, ``\r`` escapes for the appropriate characters,
+ and implemented this new formatting.
+
+* Syntax errors detected at compile-time can now raise exceptions containing the
+ filename and line number of the error, a pleasant side effect of the compiler
+ reorganization done by Jeremy Hylton.
+
+* C extensions which import other modules have been changed to use
+ :func:`PyImport_ImportModule`, which means that they will use any import hooks
+ that have been installed. This is also encouraged for third-party extensions
+ that need to import some other module from C code.
+
+* The size of the Unicode character database was shrunk by another 340K thanks
+ to Fredrik Lundh.
+
+* Some new ports were contributed: MacOS X (by Steven Majewski), Cygwin (by
+ Jason Tishler); RISCOS (by Dietmar Schwertberger); Unixware 7 (by Billy G.
+ Allie).
+
+And there's the usual list of minor bugfixes, minor memory leaks, docstring
+edits, and other tweaks, too lengthy to be worth itemizing; see the CVS logs for
+the full details if you want them.
+
+.. ======================================================================
+
+
+Acknowledgements
+================
+
+The author would like to thank the following people for offering suggestions on
+various drafts of this article: Graeme Cross, David Goodger, Jay Graves, Michael
+Hudson, Marc-André Lemburg, Fredrik Lundh, Neil Schemenauer, Thomas Wouters.
+