| \documentclass{howto} |
| % $Id$ |
| |
| \title{What's New in Python 2.3} |
| \release{0.04} |
| \author{A.M. Kuchling} |
| \authoraddress{\email{amk@amk.ca}} |
| |
| \begin{document} |
| \maketitle |
| \tableofcontents |
| |
| % MacOS framework-related changes (section of its own, probably) |
| |
| %\section{Introduction \label{intro}} |
| |
| {\large This article is a draft, and is currently up to date for some |
| random version of the CVS tree from early November 2002. Please send any |
| additions, comments or errata to the author.} |
| |
| This article explains the new features in Python 2.3. The tentative |
| release date of Python 2.3 is currently scheduled for some undefined |
| time before the end of 2002. |
| |
| This article doesn't attempt to provide a complete specification of |
| the new features, but instead provides a convenient overview. For |
| full details, you should refer to the documentation for Python 2.3, |
| such as the |
| \citetitle[http://www.python.org/doc/2.3/lib/lib.html]{Python Library |
| Reference} and the |
| \citetitle[http://www.python.org/doc/2.3/ref/ref.html]{Python |
| Reference Manual}. If you want to understand the complete |
| implementation and design rationale for a change, refer to the PEP for |
| a particular new feature. |
| |
| |
| %====================================================================== |
| \section{PEP 218: A Standard Set Datatype} |
| |
| The new \module{sets} module contains an implementation of a set |
| datatype. The \class{Set} class is for mutable sets, sets that can |
| have members added and removed. The \class{ImmutableSet} class is for |
| sets that can't be modified, and can be used as dictionary keys. Sets |
| are built on top of dictionaries, so the elements within a set must be |
| hashable. |
| |
| As a simple example, |
| |
| \begin{verbatim} |
| >>> import sets |
| >>> S = sets.Set([1,2,3]) |
| >>> S |
| Set([1, 2, 3]) |
| >>> 1 in S |
| True |
| >>> 0 in S |
| False |
| >>> S.add(5) |
| >>> S.remove(3) |
| >>> S |
| Set([1, 2, 5]) |
| >>> |
| \end{verbatim} |
| |
| The union and intersection of sets can be computed with the |
| \method{union()} and \method{intersection()} methods, or, |
| alternatively, using the bitwise operators \code{\&} and \code{|}. |
| Mutable sets also have in-place versions of these methods, |
| \method{union_update()} and \method{intersection_update()}. |
| |
| \begin{verbatim} |
| >>> S1 = sets.Set([1,2,3]) |
| >>> S2 = sets.Set([4,5,6]) |
| >>> S1.union(S2) |
| Set([1, 2, 3, 4, 5, 6]) |
| >>> S1 | S2 # Alternative notation |
| Set([1, 2, 3, 4, 5, 6]) |
| >>> S1.intersection(S2) |
| Set([]) |
| >>> S1 & S2 # Alternative notation |
| Set([]) |
| >>> S1.union_update(S2) |
| Set([1, 2, 3, 4, 5, 6]) |
| >>> S1 |
| Set([1, 2, 3, 4, 5, 6]) |
| >>> |
| \end{verbatim} |
| |
| It's also possible to take the symmetric difference of two sets. This |
| is the set of all elements in the union that aren't in the |
| intersection. An alternative way of expressing the symmetric |
| difference is that it contains all elements that are in exactly one |
| set. Again, there's an in-place version, with the ungainly name |
| \method{symmetric_difference_update()}. |
| |
| \begin{verbatim} |
| >>> S1 = sets.Set([1,2,3,4]) |
| >>> S2 = sets.Set([3,4,5,6]) |
| >>> S1.symmetric_difference(S2) |
| Set([1, 2, 5, 6]) |
| >>> S1 ^ S2 |
| Set([1, 2, 5, 6]) |
| >>> |
| \end{verbatim} |
| |
| There are also methods, \method{issubset()} and \method{issuperset()}, |
| for checking whether one set is a strict subset or superset of |
| another: |
| |
| \begin{verbatim} |
| >>> S1 = sets.Set([1,2,3]) |
| >>> S2 = sets.Set([2,3]) |
| >>> S2.issubset(S1) |
| True |
| >>> S1.issubset(S2) |
| False |
| >>> S1.issuperset(S2) |
| True |
| >>> |
| \end{verbatim} |
| |
| |
| \begin{seealso} |
| |
| \seepep{218}{Adding a Built-In Set Object Type}{PEP written by Greg V. Wilson. |
| Implemented by Greg V. Wilson, Alex Martelli, and GvR.} |
| |
| \end{seealso} |
| |
| |
| |
| %====================================================================== |
| \section{PEP 255: Simple Generators\label{section-generators}} |
| |
| In Python 2.2, generators were added as an optional feature, to be |
| enabled by a \code{from __future__ import generators} directive. In |
| 2.3 generators no longer need to be specially enabled, and are now |
| always present; this means that \keyword{yield} is now always a |
| keyword. The rest of this section is a copy of the description of |
| generators from the ``What's New in Python 2.2'' document; if you read |
| it when 2.2 came out, you can skip the rest of this section. |
| |
| You're doubtless familiar with how function calls work in Python or C. |
| When you call a function, it gets a private namespace where its local |
| variables are created. When the function reaches a \keyword{return} |
| statement, the local variables are destroyed and the resulting value |
| is returned to the caller. A later call to the same function will get |
| a fresh new set of local variables. But, what if the local variables |
| weren't thrown away on exiting a function? What if you could later |
| resume the function where it left off? This is what generators |
| provide; they can be thought of as resumable functions. |
| |
| Here's the simplest example of a generator function: |
| |
| \begin{verbatim} |
| def generate_ints(N): |
| for i in range(N): |
| yield i |
| \end{verbatim} |
| |
| A new keyword, \keyword{yield}, was introduced for generators. Any |
| function containing a \keyword{yield} statement is a generator |
| function; this is detected by Python's bytecode compiler which |
| compiles the function specially as a result. |
| |
| When you call a generator function, it doesn't return a single value; |
| instead it returns a generator object that supports the iterator |
| protocol. On executing the \keyword{yield} statement, the generator |
| outputs the value of \code{i}, similar to a \keyword{return} |
| statement. The big difference between \keyword{yield} and a |
| \keyword{return} statement is that on reaching a \keyword{yield} the |
| generator's state of execution is suspended and local variables are |
| preserved. On the next call to the generator's \code{.next()} method, |
| the function will resume executing immediately after the |
| \keyword{yield} statement. (For complicated reasons, the |
| \keyword{yield} statement isn't allowed inside the \keyword{try} block |
| of a \code{try...finally} statement; read \pep{255} for a full |
| explanation of the interaction between \keyword{yield} and |
| exceptions.) |
| |
| Here's a sample usage of the \function{generate_ints} generator: |
| |
| \begin{verbatim} |
| >>> gen = generate_ints(3) |
| >>> gen |
| <generator object at 0x8117f90> |
| >>> gen.next() |
| 0 |
| >>> gen.next() |
| 1 |
| >>> gen.next() |
| 2 |
| >>> gen.next() |
| Traceback (most recent call last): |
| File "stdin", line 1, in ? |
| File "stdin", line 2, in generate_ints |
| StopIteration |
| \end{verbatim} |
| |
| You could equally write \code{for i in generate_ints(5)}, or |
| \code{a,b,c = generate_ints(3)}. |
| |
| Inside a generator function, the \keyword{return} statement can only |
| be used without a value, and signals the end of the procession of |
| values; afterwards the generator cannot return any further values. |
| \keyword{return} with a value, such as \code{return 5}, is a syntax |
| error inside a generator function. The end of the generator's results |
| can also be indicated by raising \exception{StopIteration} manually, |
| or by just letting the flow of execution fall off the bottom of the |
| function. |
| |
| You could achieve the effect of generators manually by writing your |
| own class and storing all the local variables of the generator as |
| instance variables. For example, returning a list of integers could |
| be done by setting \code{self.count} to 0, and having the |
| \method{next()} method increment \code{self.count} and return it. |
| However, for a moderately complicated generator, writing a |
| corresponding class would be much messier. |
| \file{Lib/test/test_generators.py} contains a number of more |
| interesting examples. The simplest one implements an in-order |
| traversal of a tree using generators recursively. |
| |
| \begin{verbatim} |
| # A recursive generator that generates Tree leaves in in-order. |
| def inorder(t): |
| if t: |
| for x in inorder(t.left): |
| yield x |
| yield t.label |
| for x in inorder(t.right): |
| yield x |
| \end{verbatim} |
| |
| Two other examples in \file{Lib/test/test_generators.py} produce |
| solutions for the N-Queens problem (placing $N$ queens on an $NxN$ |
| chess board so that no queen threatens another) and the Knight's Tour |
| (a route that takes a knight to every square of an $NxN$ chessboard |
| without visiting any square twice). |
| |
| The idea of generators comes from other programming languages, |
| especially Icon (\url{http://www.cs.arizona.edu/icon/}), where the |
| idea of generators is central. In Icon, every |
| expression and function call behaves like a generator. One example |
| from ``An Overview of the Icon Programming Language'' at |
| \url{http://www.cs.arizona.edu/icon/docs/ipd266.htm} gives an idea of |
| what this looks like: |
| |
| \begin{verbatim} |
| sentence := "Store it in the neighboring harbor" |
| if (i := find("or", sentence)) > 5 then write(i) |
| \end{verbatim} |
| |
| In Icon the \function{find()} function returns the indexes at which the |
| substring ``or'' is found: 3, 23, 33. In the \keyword{if} statement, |
| \code{i} is first assigned a value of 3, but 3 is less than 5, so the |
| comparison fails, and Icon retries it with the second value of 23. 23 |
| is greater than 5, so the comparison now succeeds, and the code prints |
| the value 23 to the screen. |
| |
| Python doesn't go nearly as far as Icon in adopting generators as a |
| central concept. Generators are considered a new part of the core |
| Python language, but learning or using them isn't compulsory; if they |
| don't solve any problems that you have, feel free to ignore them. |
| One novel feature of Python's interface as compared to |
| Icon's is that a generator's state is represented as a concrete object |
| (the iterator) that can be passed around to other functions or stored |
| in a data structure. |
| |
| \begin{seealso} |
| |
| \seepep{255}{Simple Generators}{Written by Neil Schemenauer, Tim |
| Peters, Magnus Lie Hetland. Implemented mostly by Neil Schemenauer |
| and Tim Peters, with other fixes from the Python Labs crew.} |
| |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{PEP 263: Source Code Encodings \label{section-encodings}} |
| |
| Python source files can now be declared as being in different |
| character set encodings. Encodings are declared by including a |
| specially formatted comment in the first or second line of the source |
| file. For example, a UTF-8 file can be declared with: |
| |
| \begin{verbatim} |
| #!/usr/bin/env python |
| # -*- coding: UTF-8 -*- |
| \end{verbatim} |
| |
| Without such an encoding declaration, the default encoding used is |
| ISO-8859-1, also known as Latin1. |
| |
| The encoding declaration only affects Unicode string literals; the |
| text in the source code will be converted to Unicode using the |
| specified encoding. Note that Python identifiers are still restricted |
| to ASCII characters, so you can't have variable names that use |
| characters outside of the usual alphanumerics. |
| |
| \begin{seealso} |
| |
| \seepep{263}{Defining Python Source Code Encodings}{Written by |
| Marc-Andr\'e Lemburg and Martin von L\"owis; implemented by SUZUKI |
| Hisao and Martin von L\"owis.} |
| |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{PEP 277: Unicode file name support for Windows NT} |
| |
| On Windows NT, 2000, and XP, the system stores file names as Unicode |
| strings. Traditionally, Python has represented file names as byte |
| strings, which is inadequate because it renders some file names |
| inaccessible. |
| |
| Python now allows using arbitrary Unicode strings (within the |
| limitations of the file system) for all functions that expect file |
| names, in particular the \function{open()} built-in. If a Unicode |
| string is passed to \function{os.listdir}, Python now returns a list |
| of Unicode strings. A new function, \function{os.getcwdu()}, returns |
| the current directory as a Unicode string. |
| |
| Byte strings still work as file names, and Python will transparently |
| convert them to Unicode using the \code{mbcs} encoding. |
| |
| Other systems also allow Unicode strings as file names, but convert |
| them to byte strings before passing them to the system which may cause |
| a \exception{UnicodeError} to be raised. Applications can test whether |
| arbitrary Unicode strings are supported as file names by checking |
| \member{os.path.unicode_file_names}, a Boolean value. |
| |
| \begin{seealso} |
| |
| \seepep{277}{Unicode file name support for Windows NT}{Written by Neil |
| Hodgson; implemented by Neil Hodgson, Martin von L\"owis, and Mark |
| Hammond.} |
| |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{PEP 278: Universal Newline Support} |
| |
| The three major operating systems used today are Microsoft Windows, |
| Apple's Macintosh OS, and the various \UNIX\ derivatives. A minor |
| irritation is that these three platforms all use different characters |
| to mark the ends of lines in text files. \UNIX\ uses character 10, |
| the ASCII linefeed, while MacOS uses character 13, the ASCII carriage |
| return, and Windows uses a two-character sequence of a carriage return |
| plus a newline. |
| |
| Python's file objects can now support end of line conventions other |
| than the one followed by the platform on which Python is running. |
| Opening a file with the mode \code{'U'} or \code{'rU'} will open a file |
| for reading in universal newline mode. All three line ending |
| conventions will be translated to a \character{\e n} in the strings |
| returned by the various file methods such as \method{read()} and |
| \method{readline()}. |
| |
| Universal newline support is also used when importing modules and when |
| executing a file with the \function{execfile()} function. This means |
| that Python modules can be shared between all three operating systems |
| without needing to convert the line-endings. |
| |
| This feature can be disabled at compile-time by specifying |
| \longprogramopt{without-universal-newlines} when running Python's |
| \program{configure} script. |
| |
| \begin{seealso} |
| |
| \seepep{278}{Universal Newline Support}{Written |
| and implemented by Jack Jansen.} |
| |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{PEP 279: The \function{enumerate()} Built-in Function\label{section-enumerate}} |
| |
| A new built-in function, \function{enumerate()}, will make |
| certain loops a bit clearer. \code{enumerate(thing)}, where |
| \var{thing} is either an iterator or a sequence, returns a iterator |
| that will return \code{(0, \var{thing[0]})}, \code{(1, |
| \var{thing[1]})}, \code{(2, \var{thing[2]})}, and so forth. Fairly |
| often you'll see code to change every element of a list that looks |
| like this: |
| |
| \begin{verbatim} |
| for i in range(len(L)): |
| item = L[i] |
| # ... compute some result based on item ... |
| L[i] = result |
| \end{verbatim} |
| |
| This can be rewritten using \function{enumerate()} as: |
| |
| \begin{verbatim} |
| for i, item in enumerate(L): |
| # ... compute some result based on item ... |
| L[i] = result |
| \end{verbatim} |
| |
| |
| \begin{seealso} |
| |
| \seepep{279}{The enumerate() built-in function}{Written |
| by Raymond D. Hettinger.} |
| |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{PEP 282: The \module{logging} Package} |
| |
| A standard package for writing logs called \module{logging} has been |
| added to Python 2.3. It provides a powerful and flexible way for |
| components to generate logging output which can then be filtered and |
| processed in various ways. A standard configuration file format can |
| be used to control the logging behaviour of a program. Python comes |
| with handlers that will write log records to standard error or to a |
| file or socket, send them to the system log, or even e-mail them to a |
| particular address, and of course it's also possible to write your own |
| handler classes. |
| |
| Most application code will deal with one or more \class{Logger} |
| objects, each one used by a particular subsystem of the application. |
| Each \class{Logger} is identified by a name, and names are organized |
| into a hierarchy using \samp{.} as the component separator. For |
| example, you might have \class{Logger} instances named \samp{server}, |
| \samp{server.auth} and \samp{server.network}. The latter two |
| instances fall under the \samp{server} \class{Logger} in the |
| hierarchy. This means that if you turn up the verbosity for |
| \samp{server} or direct \samp{server} messages to a different handler, |
| the changes will also apply to records logged to \samp{server.auth} |
| and \samp{server.network}. There's also a root \class{Logger} with |
| the name \samp{root} that's the parent of all other loggers. |
| |
| For simple uses, the \module{logging} package contains some |
| convenience functions that always use the root log: |
| |
| \begin{verbatim} |
| import logging |
| |
| logging.debug('Debugging information') |
| logging.info('Informational message') |
| logging.warn('Warning: config file %s not found', 'server.conf') |
| logging.error('Error occurred') |
| logging.critical('Critical error -- shutting down') |
| \end{verbatim} |
| |
| This produces the following output: |
| |
| \begin{verbatim} |
| WARN:root:Warning: config file not found |
| ERROR:root:Error occurred |
| CRITICAL:root:Critical error -- shutting down |
| \end{verbatim} |
| |
| In the default configuration, informational and debugging messages are |
| suppressed and the output is sent to standard error; you can change |
| this by calling the \method{setLevel()} method on the root logger. |
| |
| Notice the \function{warn()} call's use of string formatting |
| operators; all of the functions for logging messages take the |
| arguments \code{(\var{msg}, \var{arg1}, \var{arg2}, ...)} and log the |
| string resulting from \code{\var{msg} \% (\var{arg1}, \var{arg2}, |
| ...)}. |
| |
| There's also an \function{exception()} function that records the most |
| recent traceback. Any of the other functions will also record the |
| traceback if you specify a true value for the keyword argument |
| \code{exc_info}. |
| |
| \begin{verbatim} |
| def f(): |
| try: 1/0 |
| except: logging.exception('Problem recorded') |
| |
| f() |
| \end{verbatim} |
| |
| This produces the following output: |
| |
| \begin{verbatim} |
| ERROR:root:Problem recorded |
| Traceback (most recent call last): |
| File "t.py", line 6, in f |
| 1/0 |
| ZeroDivisionError: integer division or modulo by zero |
| \end{verbatim} |
| |
| Slightly more advanced programs will use a logger other than the root |
| logger. The \function{getLogger(\var{name})} is used to get a |
| particular log, creating it if it doesn't exist yet. |
| |
| \begin{verbatim} |
| log = logging.getLogger('server') |
| ... |
| log.info('Listening on port %i', port) |
| ... |
| log.critical('Disk full') |
| ... |
| \end{verbatim} |
| |
| There are more classes that can be customized. When a \class{Logger} |
| instance is told to log a message, it creates a \class{LogRecord} |
| instance that is sent to any number of different \class{Handler} |
| instances. Loggers and handlers can also have an attached list of |
| filters, and each filter can cause the \class{LogRecord} to be ignored |
| or can modify the record before passing it along. \class{LogRecord} |
| instances are converted to text by a \class{Formatter} class. |
| |
| Log records are usually propagated up the hierarchy, so a message |
| logged to \samp{server.auth} is also seen by \samp{server} and |
| \samp{root}, but a handler can prevent this by setting its |
| \member{propagate} attribute to \code{True}. |
| |
| With all of these features the \module{logging} package should provide |
| enough flexibility for even the most complicated applications. This |
| is only a partial overview of the \module{logging} package's features, |
| so please see the |
| \ulink{\module{logging} package's reference documentation}{http://www.python.org/dev/doc/devel/lib/module-logging.html} |
| for all of the details. Reading |
| \pep{282} will also be helpful. |
| |
| |
| \begin{seealso} |
| |
| \seepep{282}{A Logging System}{Written by Vinay Sajip and Trent Mick; |
| implemented by Vinay Sajip.} |
| |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{PEP 285: The \class{bool} Type\label{section-bool}} |
| |
| A Boolean type was added to Python 2.3. Two new constants were added |
| to the \module{__builtin__} module, \constant{True} and |
| \constant{False}. The type object for this new type is named |
| \class{bool}; the constructor for it takes any Python value and |
| converts it to \constant{True} or \constant{False}. |
| |
| \begin{verbatim} |
| >>> bool(1) |
| True |
| >>> bool(0) |
| False |
| >>> bool([]) |
| False |
| >>> bool( (1,) ) |
| True |
| \end{verbatim} |
| |
| Most of the standard library modules and built-in functions have been |
| changed to return Booleans. |
| |
| \begin{verbatim} |
| >>> obj = [] |
| >>> hasattr(obj, 'append') |
| True |
| >>> isinstance(obj, list) |
| True |
| >>> isinstance(obj, tuple) |
| False |
| \end{verbatim} |
| |
| Python's Booleans were added with the primary goal of making code |
| clearer. For example, if you're reading a function and encounter the |
| statement \code{return 1}, you might wonder whether the \code{1} |
| represents a truth value, or whether it's an index, or whether it's a |
| coefficient that multiplies some other quantity. If the statement is |
| \code{return True}, however, the meaning of the return value is quite |
| clearly a truth value. |
| |
| Python's Booleans were not added for the sake of strict type-checking. |
| A very strict language such as Pascal would also prevent you |
| performing arithmetic with Booleans, and would require that the |
| expression in an \keyword{if} statement always evaluate to a Boolean. |
| Python is not this strict, and it never will be. (\pep{285} |
| explicitly says so.) So you can still use any expression in an |
| \keyword{if}, even ones that evaluate to a list or tuple or some |
| random object, and the Boolean type is a subclass of the |
| \class{int} class, so arithmetic using a Boolean still works. |
| |
| \begin{verbatim} |
| >>> True + 1 |
| 2 |
| >>> False + 1 |
| 1 |
| >>> False * 75 |
| 0 |
| >>> True * 75 |
| 75 |
| \end{verbatim} |
| |
| To sum up \constant{True} and \constant{False} in a sentence: they're |
| alternative ways to spell the integer values 1 and 0, with the single |
| difference that \function{str()} and \function{repr()} return the |
| strings \code{'True'} and \code{'False'} instead of \code{'1'} and |
| \code{'0'}. |
| |
| \begin{seealso} |
| |
| \seepep{285}{Adding a bool type}{Written and implemented by GvR.} |
| |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{PEP 293: Codec Error Handling Callbacks} |
| |
| When encoding a Unicode string into a byte string, unencodable |
| characters may be encountered. So far, Python has allowed specifying |
| the error processing as either ``strict'' (raising |
| \exception{UnicodeError}), ``ignore'' (skip the character), or |
| ``replace'' (with question mark), defaulting to ``strict''. It may be |
| desirable to specify an alternative processing of the error, e.g. by |
| inserting an XML character reference or HTML entity reference into the |
| converted string. |
| |
| Python now has a flexible framework to add additional processing |
| strategies. New error handlers can be added with |
| \function{codecs.register_error}. Codecs then can access the error |
| handler with \function{codecs.lookup_error}. An equivalent C API has |
| been added for codecs written in C. The error handler gets the |
| necessary state information, such as the string being converted, the |
| position in the string where the error was detected, and the target |
| encoding. The handler can then either raise an exception, or return a |
| replacement string. |
| |
| Two additional error handlers have been implemented using this |
| framework: ``backslashreplace'' uses Python backslash quoting to |
| represent the unencodable character, and ``xmlcharrefreplace'' emits |
| XML character references. |
| |
| \begin{seealso} |
| |
| \seepep{293}{Codec Error Handling Callbacks}{Written and implemented by |
| Walter D\"orwald.} |
| |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{Extended Slices\label{section-slices}} |
| |
| Ever since Python 1.4, the slicing syntax has supported an optional |
| third ``step'' or ``stride'' argument. For example, these are all |
| legal Python syntax: \code{L[1:10:2]}, \code{L[:-1:1]}, |
| \code{L[::-1]}. This was added to Python included at the request of |
| the developers of Numerical Python. However, the built-in sequence |
| types of lists, tuples, and strings have never supported this feature, |
| and you got a \exception{TypeError} if you tried it. Michael Hudson |
| contributed a patch that was applied to Python 2.3 and fixed this |
| shortcoming. |
| |
| For example, you can now easily extract the elements of a list that |
| have even indexes: |
| |
| \begin{verbatim} |
| >>> L = range(10) |
| >>> L[::2] |
| [0, 2, 4, 6, 8] |
| \end{verbatim} |
| |
| Negative values also work, so you can make a copy of the same list in |
| reverse order: |
| |
| \begin{verbatim} |
| >>> L[::-1] |
| [9, 8, 7, 6, 5, 4, 3, 2, 1, 0] |
| \end{verbatim} |
| |
| This also works for strings: |
| |
| \begin{verbatim} |
| >>> s='abcd' |
| >>> s[::2] |
| 'ac' |
| >>> s[::-1] |
| 'dcba' |
| \end{verbatim} |
| |
| as well as tuples and arrays. |
| |
| If you have a mutable sequence (i.e. a list or an array) you can |
| assign to or delete an extended slice, but there are some differences |
| in assignment to extended and regular slices. Assignment to a regular |
| slice can be used to change the length of the sequence: |
| |
| \begin{verbatim} |
| >>> a = range(3) |
| >>> a |
| [0, 1, 2] |
| >>> a[1:3] = [4, 5, 6] |
| >>> a |
| [0, 4, 5, 6] |
| \end{verbatim} |
| |
| but when assigning to an extended slice the list on the right hand |
| side of the statement must contain the same number of items as the |
| slice it is replacing: |
| |
| \begin{verbatim} |
| >>> a = range(4) |
| >>> a |
| [0, 1, 2, 3] |
| >>> a[::2] |
| [0, 2] |
| >>> a[::2] = range(0, -2, -1) |
| >>> a |
| [0, 1, -1, 3] |
| >>> a[::2] = range(3) |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| ValueError: attempt to assign list of size 3 to extended slice of size 2 |
| \end{verbatim} |
| |
| Deletion is more straightforward: |
| |
| \begin{verbatim} |
| >>> a = range(4) |
| >>> a[::2] |
| [0, 2] |
| >>> del a[::2] |
| >>> a |
| [1, 3] |
| \end{verbatim} |
| |
| One can also now pass slice objects to builtin sequences |
| \method{__getitem__} methods: |
| |
| \begin{verbatim} |
| >>> range(10).__getitem__(slice(0, 5, 2)) |
| [0, 2, 4] |
| \end{verbatim} |
| |
| or use them directly in subscripts: |
| |
| \begin{verbatim} |
| >>> range(10)[slice(0, 5, 2)] |
| [0, 2, 4] |
| \end{verbatim} |
| |
| To make implementing sequences that support extended slicing in Python |
| easier, slice ojects now have a method \method{indices} which given |
| the length of a sequence returns \code{(start, stop, step)} handling |
| omitted and out-of-bounds indices in a manner consistent with regular |
| slices (and this innocuous phrase hides a welter of confusing |
| details!). The method is intended to be used like this: |
| |
| \begin{verbatim} |
| class FakeSeq: |
| ... |
| def calc_item(self, i): |
| ... |
| def __getitem__(self, item): |
| if isinstance(item, slice): |
| return FakeSeq([self.calc_item(i) |
| in range(*item.indices(len(self)))]) |
| else: |
| return self.calc_item(i) |
| \end{verbatim} |
| |
| From this example you can also see that the builtin ``\class{slice}'' |
| object is now the type object for the slice type, and is no longer a |
| function. This is consistent with Python 2.2, where \class{int}, |
| \class{str}, etc., underwent the same change. |
| |
| |
| %====================================================================== |
| \section{Other Language Changes} |
| |
| Here are all of the changes that Python 2.3 makes to the core Python |
| language. |
| |
| \begin{itemize} |
| \item The \keyword{yield} statement is now always a keyword, as |
| described in section~\ref{section-generators} of this document. |
| |
| \item A new built-in function \function{enumerate()} |
| was added, as described in section~\ref{section-enumerate} of this |
| document. |
| |
| \item Two new constants, \constant{True} and \constant{False} were |
| added along with the built-in \class{bool} type, as described in |
| section~\ref{section-bool} of this document. |
| |
| \item The \function{int()} type constructor will now return a long |
| integer instead of raising an \exception{OverflowError} when a string |
| or floating-point number is too large to fit into an integer. This |
| can lead to the paradoxical result that |
| \code{isinstance(int(\var{expression}), int)} is false, but that seems unlikely to cause problems in practice. |
| |
| \item Built-in types now support the extended slicing syntax, |
| as described in section~\ref{section-slices} of this document. |
| |
| \item Dictionaries have a new method, \method{pop(\var{key})}, that |
| returns the value corresponding to \var{key} and removes that |
| key/value pair from the dictionary. \method{pop()} will raise a |
| \exception{KeyError} if the requested key isn't present in the |
| dictionary: |
| |
| \begin{verbatim} |
| >>> d = {1:2} |
| >>> d |
| {1: 2} |
| >>> d.pop(4) |
| Traceback (most recent call last): |
| File "stdin", line 1, in ? |
| KeyError: 4 |
| >>> d.pop(1) |
| 2 |
| >>> d.pop(1) |
| Traceback (most recent call last): |
| File "stdin", line 1, in ? |
| KeyError: pop(): dictionary is empty |
| >>> d |
| {} |
| >>> |
| \end{verbatim} |
| |
| (Patch contributed by Raymond Hettinger.) |
| |
| \item The \keyword{assert} statement no longer checks the \code{__debug__} |
| flag, so you can no longer disable assertions by assigning to \code{__debug__}. |
| Running Python with the \programopt{-O} switch will still generate |
| code that doesn't execute any assertions. |
| |
| \item Most type objects are now callable, so you can use them |
| to create new objects such as functions, classes, and modules. (This |
| means that the \module{new} module can be deprecated in a future |
| Python version, because you can now use the type objects available |
| in the \module{types} module.) |
| % XXX should new.py use PendingDeprecationWarning? |
| For example, you can create a new module object with the following code: |
| |
| \begin{verbatim} |
| >>> import types |
| >>> m = types.ModuleType('abc','docstring') |
| >>> m |
| <module 'abc' (built-in)> |
| >>> m.__doc__ |
| 'docstring' |
| \end{verbatim} |
| |
| \item |
| A new warning, \exception{PendingDeprecationWarning} was added to |
| indicate features which are in the process of being |
| deprecated. The warning will \emph{not} be printed by default. To |
| check for use of features that will be deprecated in the future, |
| supply \programopt{-Walways::PendingDeprecationWarning::} on the |
| command line or use \function{warnings.filterwarnings()}. |
| |
| \item Using \code{None} as a variable name will now result in a |
| \exception{SyntaxWarning} warning. In a future version of Python, |
| \code{None} may finally become a keyword. |
| |
| \item The method resolution order used by new-style classes has |
| changed, though you'll only notice the difference if you have a really |
| complicated inheritance hierarchy. (Classic classes are unaffected by |
| this change.) Python 2.2 originally used a topological sort of a |
| class's ancestors, but 2.3 now uses the C3 algorithm as described in |
| the paper \ulink{``A Monotonic Superclass Linearization for |
| Dylan''}{http://www.webcom.com/haahr/dylan/linearization-oopsla96.html}. |
| To understand the motivation for this change, read the thread on |
| python-dev starting with the message at |
| \url{http://mail.python.org/pipermail/python-dev/2002-October/029035.html}. |
| Samuele Pedroni first pointed out the problem and also implemented the |
| fix by coding the C3 algorithm. |
| |
| \item Python runs multithreaded programs by switching between threads |
| after executing N bytecodes. The default value for N has been |
| increased from 10 to 100 bytecodes, speeding up single-threaded |
| applications by reducing the switching overhead. Some multithreaded |
| applications may suffer slower response time, but that's easily fixed |
| by setting the limit back to a lower number by calling |
| \function{sys.setcheckinterval(\var{N})}. |
| |
| \item One minor but far-reaching change is that the names of extension |
| types defined by the modules included with Python now contain the |
| module and a \character{.} in front of the type name. For example, in |
| Python 2.2, if you created a socket and printed its |
| \member{__class__}, you'd get this output: |
| |
| \begin{verbatim} |
| >>> s = socket.socket() |
| >>> s.__class__ |
| <type 'socket'> |
| \end{verbatim} |
| |
| In 2.3, you get this: |
| \begin{verbatim} |
| >>> s.__class__ |
| <type '_socket.socket'> |
| \end{verbatim} |
| |
| \end{itemize} |
| |
| |
| %====================================================================== |
| \subsection{String Changes} |
| |
| \begin{itemize} |
| |
| \item The \code{in} operator now works differently for strings. |
| Previously, when evaluating \code{\var{X} in \var{Y}} where \var{X} |
| and \var{Y} are strings, \var{X} could only be a single character. |
| That's now changed; \var{X} can be a string of any length, and |
| \code{\var{X} in \var{Y}} will return \constant{True} if \var{X} is a |
| substring of \var{Y}. If \var{X} is the empty string, the result is |
| always \constant{True}. |
| |
| \begin{verbatim} |
| >>> 'ab' in 'abcd' |
| True |
| >>> 'ad' in 'abcd' |
| False |
| >>> '' in 'abcd' |
| True |
| \end{verbatim} |
| |
| Note that this doesn't tell you where the substring starts; the |
| \method{find()} method is still necessary to figure that out. |
| |
| \item The \method{strip()}, \method{lstrip()}, and \method{rstrip()} |
| string methods now have an optional argument for specifying the |
| characters to strip. The default is still to remove all whitespace |
| characters: |
| |
| \begin{verbatim} |
| >>> ' abc '.strip() |
| 'abc' |
| >>> '><><abc<><><>'.strip('<>') |
| 'abc' |
| >>> '><><abc<><><>\n'.strip('<>') |
| 'abc<><><>\n' |
| >>> u'\u4000\u4001abc\u4000'.strip(u'\u4000') |
| u'\u4001abc' |
| >>> |
| \end{verbatim} |
| |
| (Suggested by Simon Brunning, and implemented by Walter D\"orwald.) |
| |
| \item The \method{startswith()} and \method{endswith()} |
| string methods now accept negative numbers for the start and end |
| parameters. |
| |
| \item Another new string method is \method{zfill()}, originally a |
| function in the \module{string} module. \method{zfill()} pads a |
| numeric string with zeros on the left until it's the specified width. |
| Note that the \code{\%} operator is still more flexible and powerful |
| than \method{zfill()}. |
| |
| \begin{verbatim} |
| >>> '45'.zfill(4) |
| '0045' |
| >>> '12345'.zfill(4) |
| '12345' |
| >>> 'goofy'.zfill(6) |
| '0goofy' |
| \end{verbatim} |
| |
| (Contributed by Walter D\"orwald.) |
| |
| \item A new type object, \class{basestring}, has been added. |
| Both 8-bit strings and Unicode strings inherit from this type, so |
| \code{isinstance(obj, basestring)} will return \constant{True} for |
| either kind of string. It's a completely abstract type, so you |
| can't create \class{basestring} instances. |
| |
| \item Interned strings are no longer immortal. Interned will now be |
| garbage-collected in the usual way when the only reference to them is |
| from the internal dictionary of interned strings. (Implemented by |
| Oren Tirosh.) |
| |
| \end{itemize} |
| |
| |
| %====================================================================== |
| \subsection{Optimizations} |
| |
| \begin{itemize} |
| |
| \item The \method{sort()} method of list objects has been extensively |
| rewritten by Tim Peters, and the implementation is significantly |
| faster. |
| |
| \item Multiplication of large long integers is now much faster thanks |
| to an implementation of Karatsuba multiplication, an algorithm that |
| scales better than the O(n*n) required for the grade-school |
| multiplication algorithm. (Original patch by Christopher A. Craig, |
| and significantly reworked by Tim Peters.) |
| |
| \item The \code{SET_LINENO} opcode is now gone. This may provide a |
| small speed increase, subject to your compiler's idiosyncrasies. |
| (Removed by Michael Hudson.) |
| |
| \item A number of small rearrangements have been made in various |
| hotspots to improve performance, inlining a function here, removing |
| some code there. (Implemented mostly by GvR, but lots of people have |
| contributed to one change or another.) |
| |
| \end{itemize} |
| |
| |
| %====================================================================== |
| \section{New and Improved Modules} |
| |
| As usual, Python's standard modules had a number of enhancements and |
| bug fixes. Here's a partial list of the most notable changes, sorted |
| alphabetically by module name. Consult the |
| \file{Misc/NEWS} file in the source tree for a more |
| complete list of changes, or look through the CVS logs for all the |
| details. |
| |
| \begin{itemize} |
| |
| \item The \module{array} module now supports arrays of Unicode |
| characters using the \character{u} format character. Arrays also now |
| support using the \code{+=} assignment operator to add another array's |
| contents, and the \code{*=} assignment operator to repeat an array. |
| (Contributed by Jason Orendorff.) |
| |
| \item The \module{bsddb} module has been updated to version 3.4.0 |
| of the \ulink{PyBSDDB}{http://pybsddb.sourceforge.net} package, |
| providing a more complete interface to the transactional features of |
| the BerkeleyDB library. |
| The old version of the module has been renamed to |
| \module{bsddb185} and is no longer built automatically; you'll |
| have to edit \file{Modules/Setup} to enable it. Note that the new |
| \module{bsddb} package is intended to be compatible with the |
| old module, so be sure to file bugs if you discover any |
| incompatibilities. |
| |
| \item The Distutils \class{Extension} class now supports |
| an extra constructor argument named \var{depends} for listing |
| additional source files that an extension depends on. This lets |
| Distutils recompile the module if any of the dependency files are |
| modified. For example, if \file{sampmodule.c} includes the header |
| file \file{sample.h}, you would create the \class{Extension} object like |
| this: |
| |
| \begin{verbatim} |
| ext = Extension("samp", |
| sources=["sampmodule.c"], |
| depends=["sample.h"]) |
| \end{verbatim} |
| |
| Modifying \file{sample.h} would then cause the module to be recompiled. |
| (Contributed by Jeremy Hylton.) |
| |
| \item Other minor changes to Distutils: |
| it now checks for the \envvar{CC}, \envvar{CFLAGS}, \envvar{CPP}, |
| \envvar{LDFLAGS}, and \envvar{CPPFLAGS} environment variables, using |
| them to override the settings in Python's configuration (contributed |
| by Robert Weber); the \function{get_distutils_option()} method lists |
| recently-added extensions to Distutils. |
| |
| \item The \module{getopt} module gained a new function, |
| \function{gnu_getopt()}, that supports the same arguments as the existing |
| \function{getopt()} function but uses GNU-style scanning mode. |
| The existing \function{getopt()} stops processing options as soon as a |
| non-option argument is encountered, but in GNU-style mode processing |
| continues, meaning that options and arguments can be mixed. For |
| example: |
| |
| \begin{verbatim} |
| >>> getopt.getopt(['-f', 'filename', 'output', '-v'], 'f:v') |
| ([('-f', 'filename')], ['output', '-v']) |
| >>> getopt.gnu_getopt(['-f', 'filename', 'output', '-v'], 'f:v') |
| ([('-f', 'filename'), ('-v', '')], ['output']) |
| \end{verbatim} |
| |
| (Contributed by Peter \AA{strand}.) |
| |
| \item The \module{grp}, \module{pwd}, and \module{resource} modules |
| now return enhanced tuples: |
| |
| \begin{verbatim} |
| >>> import grp |
| >>> g = grp.getgrnam('amk') |
| >>> g.gr_name, g.gr_gid |
| ('amk', 500) |
| \end{verbatim} |
| |
| \item The new \module{heapq} module contains an implementation of a |
| heap queue algorithm. A heap is an array-like data structure that |
| keeps items in a sorted order such that, for every index k, heap[k] <= |
| heap[2*k+1] and heap[k] <= heap[2*k+2]. This makes it quick to remove |
| the smallest item, and inserting a new item while maintaining the heap |
| property is O(lg~n). (See |
| \url{http://www.nist.gov/dads/HTML/priorityque.html} for more |
| information about the priority queue data structure.) |
| |
| The \module{heapq} module provides \function{heappush()} and |
| \function{heappop()} functions for adding and removing items while |
| maintaining the heap property on top of some other mutable Python |
| sequence type. For example: |
| |
| \begin{verbatim} |
| >>> import heapq |
| >>> heap = [] |
| >>> for item in [3, 7, 5, 11, 1]: |
| ... heapq.heappush(heap, item) |
| ... |
| >>> heap |
| [1, 3, 5, 11, 7] |
| >>> heapq.heappop(heap) |
| 1 |
| >>> heapq.heappop(heap) |
| 3 |
| >>> heap |
| [5, 7, 11] |
| >>> |
| >>> heapq.heappush(heap, 5) |
| >>> heap = [] |
| >>> for item in [3, 7, 5, 11, 1]: |
| ... heapq.heappush(heap, item) |
| ... |
| >>> heap |
| [1, 3, 5, 11, 7] |
| >>> heapq.heappop(heap) |
| 1 |
| >>> heapq.heappop(heap) |
| 3 |
| >>> heap |
| [5, 7, 11] |
| >>> |
| \end{verbatim} |
| |
| (Contributed by Kevin O'Connor.) |
| |
| \item Two new functions in the \module{math} module, |
| \function{degrees(\var{rads})} and \function{radians(\var{degs})}, |
| convert between radians and degrees. Other functions in the |
| \module{math} module such as |
| \function{math.sin()} and \function{math.cos()} have always required |
| input values measured in radians. (Contributed by Raymond Hettinger.) |
| |
| \item Seven new functions, \function{getpgid()}, \function{killpg()}, |
| \function{lchown()}, \function{major()}, \function{makedev()}, |
| \function{minor()}, and \function{mknod()}, were added to the |
| \module{posix} module that underlies the \module{os} module. |
| (Contributed by Gustavo Niemeyer and Geert Jansen.) |
| |
| \item The parser objects provided by the \module{pyexpat} module |
| can now optionally buffer character data, resulting in fewer calls to |
| your character data handler and therefore faster performance. Setting |
| the parser object's \member{buffer_text} attribute to \constant{True} |
| will enable buffering. |
| |
| \item The \function{sample(\var{population}, \var{k})} function was |
| added to the \module{random} module. \var{population} is a sequence |
| containing the elements of a population, and \function{sample()} |
| chooses \var{k} elements from the population without replacing chosen |
| elements. \var{k} can be any value up to \code{len(\var{population})}. |
| For example: |
| |
| \begin{verbatim} |
| >>> pop = range(6) ; pop |
| [0, 1, 2, 3, 4, 5] |
| >>> random.sample(pop, 3) # Choose three elements |
| [0, 4, 3] |
| >>> random.sample(pop, 6) # Choose all six elements |
| [4, 5, 0, 3, 2, 1] |
| >>> random.sample(pop, 6) # Choose six again |
| [4, 2, 3, 0, 5, 1] |
| >>> random.sample(pop, 7) # Can't choose more than six |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| File "random.py", line 396, in sample |
| raise ValueError, "sample larger than population" |
| ValueError: sample larger than population |
| >>> |
| \end{verbatim} |
| |
| \item The \module{readline} module also gained a number of new |
| functions: \function{get_history_item()}, |
| \function{get_current_history_length()}, and \function{redisplay()}. |
| |
| \item Support for more advanced POSIX signal handling was added |
| to the \module{signal} module by adding the \function{sigpending}, |
| \function{sigprocmask} and \function{sigsuspend} functions, where supported |
| by the platform. These functions make it possible to avoid some previously |
| unavoidable race conditions. |
| |
| \item The \module{socket} module now supports timeouts. You |
| can call the \method{settimeout(\var{t})} method on a socket object to |
| set a timeout of \var{t} seconds. Subsequent socket operations that |
| take longer than \var{t} seconds to complete will abort and raise a |
| \exception{socket.error} exception. |
| |
| The original timeout implementation was by Tim O'Malley. Michael |
| Gilfix integrated it into the Python \module{socket} module, after the |
| patch had undergone a lengthy review. After it was checked in, Guido |
| van~Rossum rewrote parts of it. This is a good example of the free |
| software development process in action. |
| |
| \item The value of the C \constant{PYTHON_API_VERSION} macro is now exposed |
| at the Python level as \code{sys.api_version}. |
| |
| \item The new \module{textwrap} module contains functions for wrapping |
| strings containing paragraphs of text. The \function{wrap(\var{text}, |
| \var{width})} function takes a string and returns a list containing |
| the text split into lines of no more than the chosen width. The |
| \function{fill(\var{text}, \var{width})} function returns a single |
| string, reformatted to fit into lines no longer than the chosen width. |
| (As you can guess, \function{fill()} is built on top of |
| \function{wrap()}. For example: |
| |
| \begin{verbatim} |
| >>> import textwrap |
| >>> paragraph = "Not a whit, we defy augury: ... more text ..." |
| >>> textwrap.wrap(paragraph, 60) |
| ["Not a whit, we defy augury: there's a special providence in", |
| "the fall of a sparrow. If it be now, 'tis not to come; if it", |
| ...] |
| >>> print textwrap.fill(paragraph, 35) |
| Not a whit, we defy augury: there's |
| a special providence in the fall of |
| a sparrow. If it be now, 'tis not |
| to come; if it be not to come, it |
| will be now; if it be not now, yet |
| it will come: the readiness is all. |
| >>> |
| \end{verbatim} |
| |
| The module also contains a \class{TextWrapper} class that actually |
| implements the text wrapping strategy. Both the |
| \class{TextWrapper} class and the \function{wrap()} and |
| \function{fill()} functions support a number of additional keyword |
| arguments for fine-tuning the formatting; consult the module's |
| documentation for details. |
| %XXX add a link to the module docs? |
| (Contributed by Greg Ward.) |
| |
| \item The \module{time} module's \function{strptime()} function has |
| long been an annoyance because it uses the platform C library's |
| \function{strptime()} implementation, and different platforms |
| sometimes have odd bugs. Brett Cannon contributed a portable |
| implementation that's written in pure Python, which should behave |
| identically on all platforms. |
| |
| \item The DOM implementation |
| in \module{xml.dom.minidom} can now generate XML output in a |
| particular encoding, by specifying an optional encoding argument to |
| the \method{toxml()} and \method{toprettyxml()} methods of DOM nodes. |
| |
| \item The \function{*stat()} family of functions can now report |
| fractions of a second in a timestamp. Such time stamps are |
| represented as floats, similar to \function{time.time()}. |
| |
| During testing, it was found that some applications will break if time |
| stamps are floats. For compatibility, when using the tuple interface |
| of the \class{stat_result}, time stamps are represented as integers. |
| When using named fields (a feature first introduced in Python 2.2), |
| time stamps are still represented as ints, unless |
| \function{os.stat_float_times()} is invoked to enable float return |
| values: |
| |
| \begin{verbatim} |
| >>> os.stat("/tmp").st_mtime |
| 1034791200 |
| >>> os.stat_float_times(True) |
| >>> os.stat("/tmp").st_mtime |
| 1034791200.6335014 |
| \end{verbatim} |
| |
| In Python 2.4, the default will change to always returning floats. |
| |
| Application developers should use this feature only if all their |
| libraries work properly when confronted with floating point time |
| stamps, or if they use the tuple API. If used, the feature should be |
| activated on an application level instead of trying to enable it on a |
| per-use basis. |
| |
| \end{itemize} |
| |
| |
| %====================================================================== |
| \subsection{The \module{optparse} Module} |
| |
| The \module{getopt} module provides simple parsing of command-line |
| arguments. The new \module{optparse} module (originally named Optik) |
| provides more elaborate command-line parsing that follows the Unix |
| conventions, automatically creates the output for \longprogramopt{help}, |
| and can perform different actions |
| |
| You start by creating an instance of \class{OptionParser} and telling |
| it what your program's options are. |
| |
| \begin{verbatim} |
| from optparse import OptionParser |
| |
| op = OptionParser() |
| op.add_option('-i', '--input', |
| action='store', type='string', dest='input', |
| help='set input filename') |
| op.add_option('-l', '--length', |
| action='store', type='int', dest='length', |
| help='set maximum length of output') |
| \end{verbatim} |
| |
| Parsing a command line is then done by calling the \method{parse_args()} |
| method. |
| |
| \begin{verbatim} |
| options, args = op.parse_args(sys.argv[1:]) |
| print options |
| print args |
| \end{verbatim} |
| |
| This returns an object containing all of the option values, |
| and a list of strings containing the remaining arguments. |
| |
| Invoking the script with the various arguments now works as you'd |
| expect it to. Note that the length argument is automatically |
| converted to an integer. |
| |
| \begin{verbatim} |
| $ ./python opt.py -i data arg1 |
| <Values at 0x400cad4c: {'input': 'data', 'length': None}> |
| ['arg1'] |
| $ ./python opt.py --input=data --length=4 |
| <Values at 0x400cad2c: {'input': 'data', 'length': 4}> |
| ['arg1'] |
| $ |
| \end{verbatim} |
| |
| The help message is automatically generated for you: |
| |
| \begin{verbatim} |
| $ ./python opt.py --help |
| usage: opt.py [options] |
| |
| options: |
| -h, --help show this help message and exit |
| -iINPUT, --input=INPUT |
| set input filename |
| -lLENGTH, --length=LENGTH |
| set maximum length of output |
| $ |
| \end{verbatim} |
| % $ prevent Emacs tex-mode from getting confused |
| |
| Optik was written by Greg Ward, with suggestions from the readers of |
| the Getopt SIG. |
| |
| \begin{seealso} |
| \seeurl{http://optik.sourceforge.net} |
| {The Optik site has tutorial and reference documentation for |
| \module{optparse}. |
| % XXX change to point to Python docs, when those docs get written. |
| } |
| \end{seealso} |
| |
| |
| %====================================================================== |
| \section{Specialized Object Allocator (pymalloc)\label{section-pymalloc}} |
| |
| An experimental feature added to Python 2.1 was a specialized object |
| allocator called pymalloc, written by Vladimir Marangozov. Pymalloc |
| was intended to be faster than the system \cfunction{malloc()} and have |
| less memory overhead for typical allocation patterns of Python |
| programs. The allocator uses C's \cfunction{malloc()} function to get |
| large pools of memory, and then fulfills smaller memory requests from |
| these pools. |
| |
| In 2.1 and 2.2, pymalloc was an experimental feature and wasn't |
| enabled by default; you had to explicitly turn it on by providing the |
| \longprogramopt{with-pymalloc} option to the \program{configure} |
| script. In 2.3, pymalloc has had further enhancements and is now |
| enabled by default; you'll have to supply |
| \longprogramopt{without-pymalloc} to disable it. |
| |
| This change is transparent to code written in Python; however, |
| pymalloc may expose bugs in C extensions. Authors of C extension |
| modules should test their code with the object allocator enabled, |
| because some incorrect code may cause 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 \cfunction{malloc()} |
| and \cfunction{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 |
| \cfunction{malloc()} and \cfunction{free()} any more, and calling the |
| wrong function to free memory may get you a core dump. For example, |
| if memory was allocated using \cfunction{PyObject_Malloc()}, it has to |
| be freed using \cfunction{PyObject_Free()}, not \cfunction{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. |
| |
| As part of this change, the confusing multiple interfaces for |
| allocating memory have been consolidated down into two API families. |
| Memory allocated with one family must not be manipulated with |
| functions from the other family. |
| |
| There is another family of functions specifically for allocating |
| Python \emph{objects} (as opposed to memory). |
| |
| \begin{itemize} |
| \item To allocate and free an undistinguished chunk of memory use |
| the ``raw memory'' family: \cfunction{PyMem_Malloc()}, |
| \cfunction{PyMem_Realloc()}, and \cfunction{PyMem_Free()}. |
| |
| \item The ``object memory'' family is the interface to the pymalloc |
| facility described above and is biased towards a large number of |
| ``small'' allocations: \cfunction{PyObject_Malloc}, |
| \cfunction{PyObject_Realloc}, and \cfunction{PyObject_Free}. |
| |
| \item To allocate and free Python objects, use the ``object'' family |
| \cfunction{PyObject_New()}, \cfunction{PyObject_NewVar()}, and |
| \cfunction{PyObject_Del()}. |
| \end{itemize} |
| |
| Thanks to lots of work by Tim Peters, pymalloc in 2.3 also provides |
| debugging features to catch memory overwrites and doubled frees in |
| both extension modules and in the interpreter itself. To enable this |
| support, turn on the Python interpreter's debugging code by running |
| \program{configure} with \longprogramopt{with-pydebug}. |
| |
| To aid extension writers, a header file \file{Misc/pymemcompat.h} is |
| distributed with the source to Python 2.3 that allows Python |
| extensions to use the 2.3 interfaces to memory allocation and compile |
| against any version of Python since 1.5.2. You would copy the file |
| from Python's source distribution and bundle it with the source of |
| your extension. |
| |
| \begin{seealso} |
| |
| \seeurl{http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/python/python/dist/src/Objects/obmalloc.c} |
| {For the full details of the pymalloc implementation, see |
| the comments at the top of the file \file{Objects/obmalloc.c} in the |
| Python source code. The above link points to the file within the |
| SourceForge CVS browser.} |
| |
| \end{seealso} |
| |
| |
| % ====================================================================== |
| \section{Build and C API Changes} |
| |
| Changes to Python's build process and to the C API include: |
| |
| \begin{itemize} |
| |
| \item The C-level interface to the garbage collector has been changed, |
| to make it easier to write extension types that support garbage |
| collection, and to make it easier to debug misuses of the functions. |
| Various functions have slightly different semantics, so a bunch of |
| functions had to be renamed. Extensions that use the old API will |
| still compile but will \emph{not} participate in garbage collection, |
| so updating them for 2.3 should be considered fairly high priority. |
| |
| To upgrade an extension module to the new API, perform the following |
| steps: |
| |
| \begin{itemize} |
| |
| \item Rename \cfunction{Py_TPFLAGS_GC} to \cfunction{PyTPFLAGS_HAVE_GC}. |
| |
| \item Use \cfunction{PyObject_GC_New} or \cfunction{PyObject_GC_NewVar} to |
| allocate objects, and \cfunction{PyObject_GC_Del} to deallocate them. |
| |
| \item Rename \cfunction{PyObject_GC_Init} to \cfunction{PyObject_GC_Track} and |
| \cfunction{PyObject_GC_Fini} to \cfunction{PyObject_GC_UnTrack}. |
| |
| \item Remove \cfunction{PyGC_HEAD_SIZE} from object size calculations. |
| |
| \item Remove calls to \cfunction{PyObject_AS_GC} and \cfunction{PyObject_FROM_GC}. |
| |
| \end{itemize} |
| |
| \item Python can now optionally be built as a shared library |
| (\file{libpython2.3.so}) by supplying \longprogramopt{enable-shared} |
| when running Python's \program{configure} script. (Contributed by Ondrej |
| Palkovsky.) |
| |
| \item The \csimplemacro{DL_EXPORT} and \csimplemacro{DL_IMPORT} macros |
| are now deprecated. Initialization functions for Python extension |
| modules should now be declared using the new macro |
| \csimplemacro{PyMODINIT_FUNC}, while the Python core will generally |
| use the \csimplemacro{PyAPI_FUNC} and \csimplemacro{PyAPI_DATA} |
| macros. |
| |
| \item The interpreter can be compiled without any docstrings for |
| the built-in functions and modules by supplying |
| \longprogramopt{without-doc-strings} to the \program{configure} script. |
| This makes the Python executable about 10\% smaller, but will also |
| mean that you can't get help for Python's built-ins. (Contributed by |
| Gustavo Niemeyer.) |
| |
| \item The cycle detection implementation used by the garbage collection |
| has proven to be stable, so it's now being made mandatory; you can no |
| longer compile Python without it, and the |
| \longprogramopt{with-cycle-gc} switch to \program{configure} has been removed. |
| |
| \item The \cfunction{PyArg_NoArgs()} macro is now deprecated, and code |
| that uses it should be changed. For Python 2.2 and later, the method |
| definition table can specify the |
| \constant{METH_NOARGS} flag, signalling that there are no arguments, and |
| the argument checking can then be removed. If compatibility with |
| pre-2.2 versions of Python is important, the code could use |
| \code{PyArg_ParseTuple(args, "")} instead, but this will be slower |
| than using \constant{METH_NOARGS}. |
| |
| \item A new function, \cfunction{PyObject_DelItemString(\var{mapping}, |
| char *\var{key})} was added |
| as shorthand for |
| \code{PyObject_DelItem(\var{mapping}, PyString_New(\var{key})}. |
| |
| \item The \method{xreadlines()} method of file objects, introduced in |
| Python 2.1, is no longer necessary because files now behave as their |
| own iterator. \method{xreadlines()} was originally introduced as a |
| faster way to loop over all the lines in a file, but now you can |
| simply write \code{for line in file_obj}. |
| |
| \item File objects now manage their internal string buffer |
| differently by increasing it exponentially when needed. |
| This results in the benchmark tests in \file{Lib/test/test_bufio.py} |
| speeding up from 57 seconds to 1.7 seconds, according to one |
| measurement. |
| |
| \item It's now possible to define class and static methods for a C |
| extension type by setting either the \constant{METH_CLASS} or |
| \constant{METH_STATIC} flags in a method's \ctype{PyMethodDef} |
| structure. |
| |
| \item Python now includes a copy of the Expat XML parser's source code, |
| removing any dependence on a system version or local installation of |
| Expat. |
| |
| \end{itemize} |
| |
| |
| %====================================================================== |
| \subsection{Port-Specific Changes} |
| |
| Support for a port to IBM's OS/2 using the EMX runtime environment was |
| merged into the main Python source tree. EMX is a POSIX emulation |
| layer over the OS/2 system APIs. The Python port for EMX tries to |
| support all the POSIX-like capability exposed by the EMX runtime, and |
| mostly succeeds; \function{fork()} and \function{fcntl()} are |
| restricted by the limitations of the underlying emulation layer. The |
| standard OS/2 port, which uses IBM's Visual Age compiler, also gained |
| support for case-sensitive import semantics as part of the integration |
| of the EMX port into CVS. (Contributed by Andrew MacIntyre.) |
| |
| On MacOS, most toolbox modules have been weaklinked to improve |
| backward compatibility. This means that modules will no longer fail |
| to load if a single routine is missing on the curent OS version. |
| Instead calling the missing routine will raise an exception. |
| (Contributed by Jack Jansen.) |
| |
| The RPM spec files, found in the \file{Misc/RPM/} directory in the |
| Python source distribution, were updated for 2.3. (Contributed by |
| Sean Reifschneider.) |
| |
| Python now supports AtheOS (\url{http://www.atheos.cx}) and GNU/Hurd. |
| |
| |
| %====================================================================== |
| \section{Other Changes and Fixes} |
| |
| As usual, there were a bunch of other improvements and bugfixes |
| scattered throughout the source tree. A search through the CVS change |
| logs finds there were 289 patches applied and 323 bugs fixed between |
| Python 2.2 and 2.3. Both figures are likely to be underestimates. |
| |
| Some of the more notable changes are: |
| |
| \begin{itemize} |
| |
| \item The tools used to build the documentation now work under Cygwin |
| as well as \UNIX. |
| |
| \item The \code{SET_LINENO} opcode has been removed. Back in the |
| mists of time, this opcode was needed to produce line numbers in |
| tracebacks and support trace functions (for, e.g., \module{pdb}). |
| Since Python 1.5, the line numbers in tracebacks have been computed |
| using a different mechanism that works with ``python -O''. For Python |
| 2.3 Michael Hudson implemented a similar scheme to determine when to |
| call the trace function, removing the need for \code{SET_LINENO} |
| entirely. |
| |
| It would be difficult to detect any resulting difference from Python |
| code, apart from a slight speed up when Python is run without |
| \programopt{-O}. |
| |
| C extensions that access the \member{f_lineno} field of frame objects |
| should instead call \code{PyCode_Addr2Line(f->f_code, f->f_lasti)}. |
| This will have the added effect of making the code work as desired |
| under ``python -O'' in earlier versions of Python. |
| |
| \end{itemize} |
| |
| |
| %====================================================================== |
| \section{Porting to Python 2.3} |
| |
| This section lists changes that may actually require changes to your code: |
| |
| \begin{itemize} |
| |
| \item \keyword{yield} is now always a keyword; if it's used as a |
| variable name in your code, a different name must be chosen. |
| |
| \item For strings \var{X} and \var{Y}, \code{\var{X} in \var{Y}} now works |
| if \var{X} is more than one character long. |
| |
| \item The \function{int()} type constructor will now return a long |
| integer instead of raising an \exception{OverflowError} when a string |
| or floating-point number is too large to fit into an integer. |
| |
| \item You can no longer disable assertions by assigning to \code{__debug__}. |
| |
| \item The Distutils \function{setup()} function has gained various new |
| keyword arguments such as \var{depends}. Old versions of the |
| Distutils will abort if passed unknown keywords. The fix is to check |
| for the presence of the new \function{get_distutil_options()} function |
| in your \file{setup.py} if you want to only support the new keywords |
| with a version of the Distutils that supports them: |
| |
| \begin{verbatim} |
| from distutils import core |
| |
| kw = {'sources': 'foo.c', ...} |
| if hasattr(core, 'get_distutil_options'): |
| kw['depends'] = ['foo.h'] |
| ext = Extension(**kw) |
| \end{verbatim} |
| |
| \item Using \code{None} as a variable name will now result in a |
| \exception{SyntaxWarning} warning. |
| |
| \item Names of extension types defined by the modules included with |
| Python now contain the module and a \character{.} in front of the type |
| name. |
| |
| \end{itemize} |
| |
| |
| %====================================================================== |
| \section{Acknowledgements \label{acks}} |
| |
| The author would like to thank the following people for offering |
| suggestions, corrections and assistance with various drafts of this |
| article: Simon Brunning, Michael Chermside, Scott David Daniels, |
| Fred~L. Drake, Jr., Michael Hudson, Detlef Lannert, Martin von |
| L\"owis, Andrew MacIntyre, Lalo Martins, Gustavo Niemeyer, Neal |
| Norwitz, Neil Schemenauer, Jason Tishler. |
| |
| \end{document} |