524 lines
17 KiB
TeX
524 lines
17 KiB
TeX
\documentclass{howto}
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\usepackage{distutils}
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% $Id$
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\title{What's New in Python 2.5}
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\release{0.0}
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\author{A.M. Kuchling}
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\authoraddress{\email{amk@amk.ca}}
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\begin{document}
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\maketitle
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\tableofcontents
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This article explains the new features in Python 2.5. No release date
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for Python 2.5 has been set; it will probably be released in late 2005.
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% Compare with previous release in 2 - 3 sentences here.
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This article doesn't attempt to provide a complete specification of
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the new features, but instead provides a convenient overview. For
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full details, you should refer to the documentation for Python 2.5.
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% add hyperlink when the documentation becomes available online.
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If you want to understand the complete implementation and design
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rationale, refer to the PEP for a particular new feature.
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%======================================================================
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\section{PEP 309: Partial Function Application}
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The \module{functional} module is intended to contain tools for
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functional-style programming. Currently it only contains
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\class{partial}, but new functions will probably be added in future
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versions of Python.
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For programs written in a functional style, it can be useful to
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construct variants of existing functions that have some of the
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parameters filled in. Consider a Python function \code{f(a, b, c)};
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you could create a new function \code{g(b, c)} that was equivalent to
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\code{f(1, b, c)}. This is called ``partial function application'',
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and is provided by the \class{partial} class in the new
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\module{functional} module.
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The constructor for \class{partial} takes the arguments
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\code{(\var{function}, \var{arg1}, \var{arg2}, ...
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\var{kwarg1}=\var{value1}, \var{kwarg2}=\var{value2})}. The resulting
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object is callable, so you can just call it to invoke \var{function}
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with the filled-in arguments.
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Here's a small but realistic example:
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\begin{verbatim}
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import functional
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def log (message, subsystem):
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"Write the contents of 'message' to the specified subsystem."
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print '%s: %s' % (subsystem, message)
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...
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server_log = functional.partial(log, subsystem='server')
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\end{verbatim}
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Here's another example, from a program that uses PyGTk. Here a
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context-sensitive pop-up menu is being constructed dynamically. The
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callback provided for the menu option is a partially applied version
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of the \method{open_item()} method, where the first argument has been
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provided.
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\begin{verbatim}
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...
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class Application:
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def open_item(self, path):
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...
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def init (self):
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open_func = functional.partial(self.open_item, item_path)
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popup_menu.append( ("Open", open_func, 1) )
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\end{verbatim}
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\begin{seealso}
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\seepep{309}{Partial Function Application}{PEP proposed and written by
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Peter Harris; implemented by Hye-Shik Chang, with adaptations by
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Raymond Hettinger.}
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\end{seealso}
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%======================================================================
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\section{PEP 314: Metadata for Python Software Packages v1.1}
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Some simple dependency support was added to Distutils. The
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\function{setup()} function now has \code{requires},\code{provides},
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and \code{obsoletes}. When you build a source distribution using the
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\code{sdist} command, the dependency information will be recorded in
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the \file{PKG-INFO} file.
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Another new keyword is \code{download_url}, which should be set to a
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URL for the package's source code. This means it's now possible to
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look up an entry in the package index, determine the dependencies for
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a package, and download the required packages.
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% XXX put example here
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\begin{seealso}
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\seepep{314}{Metadata for Python Software Packages v1.1}{PEP proposed
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and written by A.M. Kuchling, Richard Jones, and Fred Drake;
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implemented by Richard Jones and Fred Drake.}
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\end{seealso}
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%======================================================================
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\section{PEP 342: New Generator Features}
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As introduced in Python 2.3, generators only produce output; once a
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generator's code was invoked to create an iterator, there's no way to
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pass new parameters into the function when its execution is resumed.
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Hackish solutions to this include making the generator's code look at
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a global variable and then changing the global variable's value, or
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passing in some mutable object that callers then modify. Python
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2.5 adds the ability to pass values \emph{into} a generator.
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To refresh your memory of basic generators, here's a simple example:
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\begin{verbatim}
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def counter (maximum):
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i = 0
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while i < maximum:
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yield i
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i += 1
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\end{verbatim}
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When you call \code{counter(10)}, the result is an iterator that
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returns the values from 0 up to 9. On encountering the
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\keyword{yield} statement, the iterator returns the provided value and
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suspends the function's execution, preserving the local variables.
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Execution resumes on the following call to the iterator's
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\method{next()} method, picking up after the \keyword{yield}.
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In Python 2.3, \keyword{yield} was a statement; it didn't return any
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value. In 2.5, \keyword{yield} is now an expression, returning a
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value that can be assigned to a variable or otherwise operated on:
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\begin{verbatim}
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val = (yield i)
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\end{verbatim}
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I recommend that you always put parentheses around a \keyword{yield}
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expression when you're doing something with the returned value, as in
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the above example. The parentheses aren't always necessary, but it's
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easier to always add them instead of having to remember when they're
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needed. The exact rules are that a \keyword{yield}-expression must
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always be parenthesized except when it occurs at the top-level
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expression on the right-hand side of an assignment, meaning
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you can to write \code{val = yield i} but \code{val = (yield i) + 12}.
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Values are sent into a generator by calling its
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\method{send(\var{value})} method. The generator's code is then
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resumed and the \keyword{yield} expression produces \var{value}.
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If the regular \method{next()} method is called, the \keyword{yield}
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returns \constant{None}.
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Here's the previous example, modified to allow changing the value of
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the internal counter.
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\begin{verbatim}
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def counter (maximum):
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i = 0
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while i < maximum:
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val = (yield i)
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# If value provided, change counter
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if val is not None:
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i = val
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else:
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i += 1
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\end{verbatim}
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And here's an example of changing the counter:
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\begin{verbatim}
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>>> it = counter(10)
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>>> print it.next()
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0
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>>> print it.next()
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1
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>>> print it.send(8)
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8
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>>> print it.next()
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9
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>>> print it.next()
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Traceback (most recent call last):
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File ``t.py'', line 15, in ?
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print it.next()
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StopIteration
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\end{verbatim}
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Because \keyword{yield} will often be returning \constant{None},
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you shouldn't just use its value in expressions unless you're sure
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that only the \method{send()} method will be used.
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There are two other new methods on generators in addition to
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\method{send()}:
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\begin{itemize}
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\item \method{throw(\var{type}, \var{value}=None,
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\var{traceback}=None)} is used to raise an exception inside the
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generator; the exception is raised by the \keyword{yield} expression
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where the generator's execution is paused.
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\item \method{close()} raises a new \exception{GeneratorExit}
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exception inside the generator to terminate the iteration.
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On receiving this
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exception, the generator's code must either raise
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\exception{GeneratorExit} or \exception{StopIteration}; catching the
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exception and doing anything else is illegal and will trigger
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a \exception{RuntimeError}. \method{close()} will also be called by
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Python's garbage collection when the generator is garbage-collected.
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If you need to run cleanup code in case of a \exception{GeneratorExit},
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I suggest using a \code{try: ... finally:} suite instead of
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catching \exception{GeneratorExit}.
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\end{itemize}
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The cumulative effect of these changes is to turn generators from
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one-way producers of information into both producers and consumers.
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Generators also become \emph{coroutines}, a more generalized form of
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subroutines; subroutines are entered at one point and exited at
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another point (the top of the function, and a \keyword{return
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statement}), but coroutines can be entered, exited, and resumed at
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many different points (the \keyword{yield} statements).science term
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\begin{seealso}
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\seepep{342}{Coroutines via Enhanced Generators}{PEP written by
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Guido van Rossum and Phillip J. Eby;
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implemented by Phillip J. Eby. Includes examples of
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some fancier uses of generators as coroutines.}
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\seeurl{http://en.wikipedia.org/wiki/Coroutine}{The Wikipedia entry for
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coroutines.}
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\seeurl{http://www.sidhe.org/~dan/blog/archives/000178.html}{An
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explanation of coroutines from a Perl point of view, written by Dan
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Sugalski.}
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\end{seealso}
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%======================================================================
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\section{Other Language Changes}
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Here are all of the changes that Python 2.5 makes to the core Python
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language.
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\begin{itemize}
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\item The \function{min()} and \function{max()} built-in functions
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gained a \code{key} keyword argument analogous to the \code{key}
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argument for \method{sort()}. This argument supplies a function
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that takes a single argument and is called for every value in the list;
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\function{min()}/\function{max()} will return the element with the
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smallest/largest return value from this function.
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For example, to find the longest string in a list, you can do:
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\begin{verbatim}
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L = ['medium', 'longest', 'short']
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# Prints 'longest'
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print max(L, key=len)
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# Prints 'short', because lexicographically 'short' has the largest value
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print max(L)
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\end{verbatim}
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(Contributed by Steven Bethard and Raymond Hettinger.)
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\item Two new built-in functions, \function{any()} and
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\function{all()}, evaluate whether an iterator contains any true or
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false values. \function{any()} returns \constant{True} if any value
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returned by the iterator is true; otherwise it will return
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\constant{False}. \function{all()} returns \constant{True} only if
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all of the values returned by the iterator evaluate as being true.
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% XXX who added?
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\item The list of base classes in a class definition can now be empty.
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As an example, this is now legal:
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\begin{verbatim}
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class C():
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pass
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\end{verbatim}
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(Implemented by Brett Cannon.)
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\end{itemize}
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%======================================================================
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\subsection{Optimizations}
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\begin{itemize}
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\item When they were introduced
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in Python 2.4, the built-in \class{set} and \class{frozenset} types
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were built on top of Python's dictionary type.
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In 2.5 the internal data structure has been customized for implementing sets,
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and as a result sets will use a third less memory and are somewhat faster.
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(Implemented by Raymond Hettinger.)
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\end{itemize}
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The net result of the 2.5 optimizations is that Python 2.5 runs the
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pystone benchmark around XX\% faster than Python 2.4.
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%======================================================================
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\section{New, Improved, and Deprecated Modules}
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As usual, Python's standard library received a number of enhancements and
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bug fixes. Here's a partial list of the most notable changes, sorted
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alphabetically by module name. Consult the
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\file{Misc/NEWS} file in the source tree for a more
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complete list of changes, or look through the CVS logs for all the
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details.
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\begin{itemize}
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% collections.deque now has .remove()
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% the cPickle module no longer accepts the deprecated None option in the
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% args tuple returned by __reduce__().
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% csv module improvements
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% datetime.datetime() now has a strptime class method which can be used to
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% create datetime object using a string and format.
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\item A new \module{hashlib} module has been added to replace the
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\module{md5} and \module{sha} modules. \module{hashlib} adds support
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for additional secure hashes (SHA-224, SHA-256, SHA-384, and SHA-512).
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When available, the module uses OpenSSL for fast platform optimized
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implementations of algorithms. The old \module{md5} and \module{sha}
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modules still exist as wrappers around hashlib to preserve backwards
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compatibility. (Contributed by Gregory P. Smith.)
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\item The \function{nsmallest()} and
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\function{nlargest()} functions in the \module{heapq} module
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now support a \code{key} keyword argument similar to the one
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provided by the \function{min()}/\function{max()} functions
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and the \method{sort()} methods. For example:
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Example:
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\begin{verbatim}
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>>> import heapq
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>>> L = ["short", 'medium', 'longest', 'longer still']
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>>> heapq.nsmallest(2, L) # Return two lowest elements, lexicographically
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['longer still', 'longest']
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>>> heapq.nsmallest(2, L, key=len) # Return two shortest elements
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['short', 'medium']
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\end{verbatim}
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(Contributed by Raymond Hettinger.)
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\item The \function{itertools.islice()} function now accepts
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\code{None} for the start and step arguments. This makes it more
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compatible with the attributes of slice objects, so that you can now write
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the following:
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\begin{verbatim}
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s = slice(5) # Create slice object
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itertools.islice(iterable, s.start, s.stop, s.step)
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\end{verbatim}
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(Contributed by Raymond Hettinger.)
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\item The \module{operator} module's \function{itemgetter()}
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and \function{attrgetter()} functions now support multiple fields.
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A call such as \code{operator.attrgetter('a', 'b')}
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will return a function
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that retrieves the \member{a} and \member{b} attributes. Combining
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this new feature with the \method{sort()} method's \code{key} parameter
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lets you easily sort lists using multiple fields.
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% XXX who added?
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\item The \module{os} module underwent a number of changes. The
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\member{stat_float_times} variable now defaults to true, meaning that
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\function{os.stat()} will now return time values as floats. (This
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doesn't necessarily mean that \function{os.stat()} will return times
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that are precise to fractions of a second; not all systems support
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such precision.)
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Constants named \member{os.SEEK_SET}, \member{os.SEEK_CUR}, and
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\member{os.SEEK_END} have been added; these are the parameters to the
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\function{os.lseek()} function. Two new constants for locking are
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\member{os.O_SHLOCK} and \member{os.O_EXLOCK}.
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On FreeBSD, the \function{os.stat()} function now returns
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times with nanosecond resolution, and the returned object
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now has \member{st_gen} and \member{st_birthtime}.
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The \member{st_flags} member is also available, if the platform supports it.
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% XXX patch 1180695, 1212117
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\item New module: \module{spwd} provides functions for accessing the
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shadow password database on systems that support it.
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% XXX give example
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\item The \class{TarFile} class in the \module{tarfile} module now has
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an \method{extractall()} method that extracts all members from the
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archive into the current working directory. It's also possible to set
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a different directory as the extraction target, and to unpack only a
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subset of the archive's members.
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A tarfile's compression can be autodetected by
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using the mode \code{'r|*'}.
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% patch 918101
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(Contributed by Lars Gust\"abel.)
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\item The \module{xmlrpclib} module now supports returning
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\class{datetime} objects for the XML-RPC date type. Supply
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\code{use_datetime=True} to the \function{loads()} function
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or the \class{Unmarshaller} class to enable this feature.
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% XXX patch 1120353
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\end{itemize}
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%======================================================================
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% whole new modules get described in \subsections here
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% XXX new distutils features: upload
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% ======================================================================
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\section{Build and C API Changes}
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Changes to Python's build process and to the C API include:
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\begin{itemize}
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\item The built-in set types now have an official C API. Call
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\cfunction{PySet_New()} and \cfunction{PyFrozenSet_New()} to create a
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new set, \cfunction{PySet_Add()} and \cfunction{PySet_Discard()} to
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add and remove elements, and \cfunction{PySet_Contains} and
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\cfunction{PySet_Size} to examine the set's state.
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\item The \cfunction{PyRange_New()} function was removed. It was
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never documented, never used in the core code, and had dangerously lax
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error checking.
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\end{itemize}
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%======================================================================
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\subsection{Port-Specific Changes}
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Platform-specific changes go here.
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%======================================================================
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\section{Other Changes and Fixes \label{section-other}}
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As usual, there were a bunch of other improvements and bugfixes
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scattered throughout the source tree. A search through the CVS change
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logs finds there were XXX patches applied and YYY bugs fixed between
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Python 2.4 and 2.5. Both figures are likely to be underestimates.
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Some of the more notable changes are:
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\begin{itemize}
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\item Details go here.
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\end{itemize}
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%======================================================================
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\section{Porting to Python 2.5}
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This section lists previously described changes that may require
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changes to your code:
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\begin{itemize}
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\item Some old deprecated modules (\module{statcache}, \module{tzparse},
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\module{whrandom}) have been moved to \file{Lib/lib-old}.
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You can get access to these modules again by adding the directory
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to your \code{sys.path}:
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\begin{verbatim}
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import os
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from distutils import sysconfig
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lib_dir = sysconfig.get_python_lib(standard_lib=True)
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old_dir = os.path.join(lib_dir, 'lib-old')
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sys.path.append(old_dir)
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\end{verbatim}
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Doing so is discouraged, however; it's better to update any code that
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still uses these modules.
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% the pickle module no longer uses the deprecated bin parameter.
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\end{itemize}
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%======================================================================
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\section{Acknowledgements \label{acks}}
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The author would like to thank the following people for offering
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suggestions, corrections and assistance with various drafts of this
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article: .
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\end{document}
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