**************************** What's New in Python 2.6 **************************** .. XXX add trademark info for Apple, Microsoft, SourceForge. :Author: A.M. Kuchling :Release: |release| :Date: |today| .. $Id: whatsnew26.tex 55746 2007-06-02 18:33:53Z neal.norwitz $ Rules for maintenance: * Anyone can add text to this document. Do not spend very much time on the wording of your changes, because your text will probably get rewritten to some degree. * The maintainer will go through Misc/NEWS periodically and add changes; it's therefore more important to add your changes to Misc/NEWS than to this file. * This is not a complete list of every single change; completeness is the purpose of Misc/NEWS. Some changes I consider too small or esoteric to include. If such a change is added to the text, I'll just remove it. (This is another reason you shouldn't spend too much time on writing your addition.) * If you want to draw your new text to the attention of the maintainer, add 'XXX' to the beginning of the paragraph or section. * It's OK to just add a fragmentary note about a change. For example: "XXX Describe the transmogrify() function added to the socket module." The maintainer will research the change and write the necessary text. * You can comment out your additions if you like, but it's not necessary (especially when a final release is some months away). * Credit the author of a patch or bugfix. Just the name is sufficient; the e-mail address isn't necessary. * It's helpful to add the bug/patch number as a comment: .. Patch 12345 XXX Describe the transmogrify() function added to the socket module. (Contributed by P.Y. Developer.) This saves the maintainer the effort of going through the SVN logs when researching a change. This article explains the new features in Python 2.6. No release date for Python 2.6 has been set; it will probably be released in mid 2008. 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 2.6. If you want to understand the complete implementation and design rationale, refer to the PEP for a particular new feature. .. Compare with previous release in 2 - 3 sentences here. add hyperlink when the documentation becomes available online. .. ======================================================================== .. Large, PEP-level features and changes should be described here. .. Should there be a new section here for 3k migration? .. Or perhaps a more general section describing module changes/deprecation? .. ======================================================================== Python 3.0 ================ The development cycle for Python 2.6 also saw the release of the first alphas of Python 3.0, and the development of 3.0 has influenced a number of features in 2.6. Python 3.0 is a far-ranging redesign of Python that breaks compatibility with the 2.x series. This means that existing Python code will need a certain amount of conversion in order to run on Python 3.0. However, not all the changes in 3.0 necessarily break compatibility. In cases where new features won't cause existing code to break, they've been backported to 2.6 and are described in this document in the appropriate place. Some of the 3.0-derived features are: * A :meth:`__complex__` method for converting objects to a complex number. * Alternate syntax for catching exceptions: ``except TypeError as exc``. * The addition of :func:`functools.reduce` as a synonym for the built-in :func:`reduce` function. A new command-line switch, :option:`-3`, enables warnings about features that will be removed in Python 3.0. You can run code with this switch to see how much work will be necessary to port code to 3.0. The value of this switch is available to Python code as the boolean variable :data:`sys.py3kwarning`, and to C extension code as :cdata:`Py_Py3kWarningFlag`. Python 3.0 adds several new built-in functions and change the semantics of some existing built-ins. Entirely new functions such as :func:`bin` have simply been added to Python 2.6, but existing built-ins haven't been changed; instead, the :mod:`future_builtins` module has versions with the new 3.0 semantics. Code written to be compatible with 3.0 can do ``from future_builtins import hex, map`` as necessary. .. seealso:: The 3xxx series of PEPs, which describes the development process for Python 3.0 and various features that have been accepted, rejected, or are still under consideration. Development Changes ================================================== While 2.6 was being developed, the Python development process underwent two significant changes: the developer group switched from SourceForge's issue tracker to a customized Roundup installation, and the documentation was converted from LaTeX to reStructured Text. New Issue Tracker: Roundup -------------------------------------------------- For a long time, the Python developers have been growing increasingly annoyed by SourceForge's bug tracker. SourceForge's hosted solution doesn't permit much customization; for example, it wasn't possible to customize the life cycle of issues. The infrastructure committee of the Python Software Foundation therefore posted a call for issue trackers, asking volunteers to set up different products and import some of the bugs and patches from SourceForge. Four different trackers were examined: Atlassian's `Jira `__, `Launchpad `__, ` `Roundup `__, and Trac `__. The committee eventually settled on Jira and Roundup as the two candidates. Jira is a commercial product that offers a no-cost hosted instance to free-software projects; Roundup is an open-source project that requires volunteers to administer it and a server to host it. After posting a call for volunteers, a new Roundup installation was set up at http://bugs.python.org. One installation of Roundup can host multiple trackers, and this server now also hosts issue trackers for Jython and for the Python web site. It will surely find other uses in the future. Hosting is kindly provided by `Upfront `__ of XXX. Martin von Loewis put a lot of effort into importing existing bugs and patches from SourceForge; his scripts for this import are at XXX. .. seealso:: XXX Roundup web site. bugs.python.org bugs.jython.org Python web site bug tracker New Documentation Format: ReStructured Text -------------------------------------------------- Since the Python project's inception around 1989, the documentation had been written using LaTeX. At that time, most documentation was printed out for later study, not viewed online. LaTeX was widely used because it provided attractive printed output while remaining straightforward to write, once the basic rules of the markup have been learned. LaTeX is still used today for writing technical publications destined for printing, but the landscape for programming tools has shifted. We no longer print out reams of documentation; instead, we browse through it online and HTML has become the most important format to support. Unfortunately, converting LaTeX to HTML is fairly complicated, and Fred L. Drake Jr., the Python documentation editor for many years, spent a lot of time wrestling the conversion process into shape. Occasionally people would suggest converting the documentation into SGML or, later, XML, but performing a good conversion is a major task and no one pursued the task to completion. During the 2.6 development cycle, Georg Brandl put a substantial effort into building a new toolchain called Sphinx for processing the documentation. The input format is reStructured Text, a markup commonly used in the Python community that supports custom extensions and directives. Sphinx concentrates on HTML output, producing attractively styled and modern HTML, but printed output is still supported through conversion to LaTeX as an output format. .. seealso:: `Docutils `__: The fundamental reStructured Text parser and toolset. :ref:`documenting-index`: Describes how to write for Python's documentation. PEP 343: The 'with' statement ============================= The previous version, Python 2.5, added the ':keyword:`with`' statement an optional feature, to be enabled by a ``from __future__ import with_statement`` directive. In 2.6 the statement no longer needs to be specially enabled; this means that :keyword:`with` is now always a keyword. The rest of this section is a copy of the corresponding section from "What's New in Python 2.5" document; if you read it back when Python 2.5 came out, you can skip the rest of this section. The ':keyword:`with`' statement clarifies code that previously would use ``try...finally`` blocks to ensure that clean-up code is executed. In this section, I'll discuss the statement as it will commonly be used. In the next section, I'll examine the implementation details and show how to write objects for use with this statement. The ':keyword:`with`' statement is a new control-flow structure whose basic structure is:: with expression [as variable]: with-block The expression is evaluated, and it should result in an object that supports the context management protocol (that is, has :meth:`__enter__` and :meth:`__exit__` methods. The object's :meth:`__enter__` is called before *with-block* is executed and therefore can run set-up code. It also may return a value that is bound to the name *variable*, if given. (Note carefully that *variable* is *not* assigned the result of *expression*.) After execution of the *with-block* is finished, the object's :meth:`__exit__` method is called, even if the block raised an exception, and can therefore run clean-up code. Some standard Python objects now support the context management protocol and can be used with the ':keyword:`with`' statement. File objects are one example:: with open('/etc/passwd', 'r') as f: for line in f: print line ... more processing code ... After this statement has executed, the file object in *f* will have been automatically closed, even if the :keyword:`for` loop raised an exception part- way through the block. .. note:: In this case, *f* is the same object created by :func:`open`, because :meth:`file.__enter__` returns *self*. The :mod:`threading` module's locks and condition variables also support the ':keyword:`with`' statement:: lock = threading.Lock() with lock: # Critical section of code ... The lock is acquired before the block is executed and always released once the block is complete. The new :func:`localcontext` function in the :mod:`decimal` module makes it easy to save and restore the current decimal context, which encapsulates the desired precision and rounding characteristics for computations:: from decimal import Decimal, Context, localcontext # Displays with default precision of 28 digits v = Decimal('578') print v.sqrt() with localcontext(Context(prec=16)): # All code in this block uses a precision of 16 digits. # The original context is restored on exiting the block. print v.sqrt() .. _new-26-context-managers: Writing Context Managers ------------------------ Under the hood, the ':keyword:`with`' statement is fairly complicated. Most people will only use ':keyword:`with`' in company with existing objects and don't need to know these details, so you can skip the rest of this section if you like. Authors of new objects will need to understand the details of the underlying implementation and should keep reading. A high-level explanation of the context management protocol is: * The expression is evaluated and should result in an object called a "context manager". The context manager must have :meth:`__enter__` and :meth:`__exit__` methods. * The context manager's :meth:`__enter__` method is called. The value returned is assigned to *VAR*. If no ``as VAR`` clause is present, the value is simply discarded. * The code in *BLOCK* is executed. * If *BLOCK* raises an exception, the :meth:`__exit__(type, value, traceback)` is called with the exception details, the same values returned by :func:`sys.exc_info`. The method's return value controls whether the exception is re-raised: any false value re-raises the exception, and ``True`` will result in suppressing it. You'll only rarely want to suppress the exception, because if you do the author of the code containing the ':keyword:`with`' statement will never realize anything went wrong. * If *BLOCK* didn't raise an exception, the :meth:`__exit__` method is still called, but *type*, *value*, and *traceback* are all ``None``. Let's think through an example. I won't present detailed code but will only sketch the methods necessary for a database that supports transactions. (For people unfamiliar with database terminology: a set of changes to the database are grouped into a transaction. Transactions can be either committed, meaning that all the changes are written into the database, or rolled back, meaning that the changes are all discarded and the database is unchanged. See any database textbook for more information.) Let's assume there's an object representing a database connection. Our goal will be to let the user write code like this:: db_connection = DatabaseConnection() with db_connection as cursor: cursor.execute('insert into ...') cursor.execute('delete from ...') # ... more operations ... The transaction should be committed if the code in the block runs flawlessly or rolled back if there's an exception. Here's the basic interface for :class:`DatabaseConnection` that I'll assume:: class DatabaseConnection: # Database interface def cursor(self): "Returns a cursor object and starts a new transaction" def commit(self): "Commits current transaction" def rollback(self): "Rolls back current transaction" The :meth:`__enter__` method is pretty easy, having only to start a new transaction. For this application the resulting cursor object would be a useful result, so the method will return it. The user can then add ``as cursor`` to their ':keyword:`with`' statement to bind the cursor to a variable name. :: class DatabaseConnection: ... def __enter__(self): # Code to start a new transaction cursor = self.cursor() return cursor The :meth:`__exit__` method is the most complicated because it's where most of the work has to be done. The method has to check if an exception occurred. If there was no exception, the transaction is committed. The transaction is rolled back if there was an exception. In the code below, execution will just fall off the end of the function, returning the default value of ``None``. ``None`` is false, so the exception will be re-raised automatically. If you wished, you could be more explicit and add a :keyword:`return` statement at the marked location. :: class DatabaseConnection: ... def __exit__(self, type, value, tb): if tb is None: # No exception, so commit self.commit() else: # Exception occurred, so rollback. self.rollback() # return False .. _module-contextlib: The contextlib module --------------------- The new :mod:`contextlib` module provides some functions and a decorator that are useful for writing objects for use with the ':keyword:`with`' statement. The decorator is called :func:`contextmanager`, and lets you write a single generator function instead of defining a new class. The generator should yield exactly one value. The code up to the :keyword:`yield` will be executed as the :meth:`__enter__` method, and the value yielded will be the method's return value that will get bound to the variable in the ':keyword:`with`' statement's :keyword:`as` clause, if any. The code after the :keyword:`yield` will be executed in the :meth:`__exit__` method. Any exception raised in the block will be raised by the :keyword:`yield` statement. Our database example from the previous section could be written using this decorator as:: from contextlib import contextmanager @contextmanager def db_transaction(connection): cursor = connection.cursor() try: yield cursor except: connection.rollback() raise else: connection.commit() db = DatabaseConnection() with db_transaction(db) as cursor: ... The :mod:`contextlib` module also has a :func:`nested(mgr1, mgr2, ...)` function that combines a number of context managers so you don't need to write nested ':keyword:`with`' statements. In this example, the single ':keyword:`with`' statement both starts a database transaction and acquires a thread lock:: lock = threading.Lock() with nested (db_transaction(db), lock) as (cursor, locked): ... Finally, the :func:`closing(object)` function returns *object* so that it can be bound to a variable, and calls ``object.close`` at the end of the block. :: import urllib, sys from contextlib import closing with closing(urllib.urlopen('http://www.yahoo.com')) as f: for line in f: sys.stdout.write(line) .. seealso:: :pep:`343` - The "with" statement PEP written by Guido van Rossum and Nick Coghlan; implemented by Mike Bland, Guido van Rossum, and Neal Norwitz. The PEP shows the code generated for a ':keyword:`with`' statement, which can be helpful in learning how the statement works. The documentation for the :mod:`contextlib` module. .. ====================================================================== .. _pep-0366: PEP 366: Explicit Relative Imports From a Main Module ============================================================ Python's :option:`-m` switch allows running a module as a script. When you ran a module that was located inside a package, relative imports didn't work correctly. The fix in Python 2.6 adds a :attr:`__package__` attribute to modules. When present, relative imports will be relative to the value of this attribute instead of the :attr:`__name__` attribute. PEP 302-style importers can then set :attr:`__package__`. The :mod:`runpy` module that implements the :option:`-m` switch now does this, so relative imports can now be used in scripts running from inside a package. .. ====================================================================== .. :: .. _pep-0370: PEP 370: XXX ===================================================== When you run Python, the module search page ``sys.modules`` usually includes a directory whose path ends in ``"site-packages"``. This directory is intended to hold locally-installed packages available to all users on a machine or using a particular site installation. Python 2.6 introduces a convention for user-specific site directories. .. seealso:: :pep:`370` - XXX PEP written by XXX; implemented by Christian Heimes. .. ====================================================================== .. _pep-3101: PEP 3101: Advanced String Formatting ===================================================== In Python 3.0, the `%` operator is supplemented by a more powerful string formatting method, :meth:`format`. Support for the :meth:`format` method has been backported to Python 2.6. In 2.6, both 8-bit and Unicode strings have a `.format()` method that treats the string as a template and takes the arguments to be formatted. The formatting template uses curly brackets (`{`, `}`) as special characters:: # Substitute positional argument 0 into the string. "User ID: {0}".format("root") -> "User ID: root" # Use the named keyword arguments uid = 'root' 'User ID: {uid} Last seen: {last_login}'.format(uid='root', last_login = '5 Mar 2008 07:20') -> 'User ID: root Last seen: 5 Mar 2008 07:20' Curly brackets can be escaped by doubling them:: format("Empty dict: {{}}") -> "Empty dict: {}" Field names can be integers indicating positional arguments, such as ``{0}``, ``{1}``, etc. or names of keyword arguments. You can also supply compound field names that read attributes or access dictionary keys:: import sys 'Platform: {0.platform}\nPython version: {0.version}'.format(sys) -> 'Platform: darwin\n Python version: 2.6a1+ (trunk:61261M, Mar 5 2008, 20:29:41) \n [GCC 4.0.1 (Apple Computer, Inc. build 5367)]' import mimetypes 'Content-type: {0[.mp4]}'.format(mimetypes.types_map) -> 'Content-type: video/mp4' Note that when using dictionary-style notation such as ``[.mp4]``, you don't need to put any quotation marks around the string; it will look up the value using ``.mp4`` as the key. Strings beginning with a number will be converted to an integer. You can't write more complicated expressions inside a format string. So far we've shown how to specify which field to substitute into the resulting string. The precise formatting used is also controllable by adding a colon followed by a format specifier. For example:: # Field 0: left justify, pad to 15 characters # Field 1: right justify, pad to 6 characters fmt = '{0:15} ${1:>6}' fmt.format('Registration', 35) -> 'Registration $ 35' fmt.format('Tutorial', 50) -> 'Tutorial $ 50' fmt.format('Banquet', 125) -> 'Banquet $ 125' Format specifiers can reference other fields through nesting:: fmt = '{0:{1}}' fmt.format('Invoice #1234', 15) -> 'Invoice #1234 ' width = 35 fmt.format('Invoice #1234', width) -> 'Invoice #1234 ' The alignment of a field within the desired width can be specified: ================ ============================================ Character Effect ================ ============================================ < (default) Left-align > Right-align ^ Center = (For numeric types only) Pad after the sign. ================ ============================================ Format specifiers can also include a presentation type, which controls how the value is formatted. For example, floating-point numbers can be formatted as a general number or in exponential notation: >>> '{0:g}'.format(3.75) '3.75' >>> '{0:e}'.format(3.75) '3.750000e+00' A variety of presentation types are available. Consult the 2.6 documentation for a complete list (XXX add link, once it's in the 2.6 docs), but here's a sample:: 'b' - Binary. Outputs the number in base 2. 'c' - Character. Converts the integer to the corresponding Unicode character before printing. 'd' - Decimal Integer. Outputs the number in base 10. 'o' - Octal format. Outputs the number in base 8. 'x' - Hex format. Outputs the number in base 16, using lower- case letters for the digits above 9. 'e' - Exponent notation. Prints the number in scientific notation using the letter 'e' to indicate the exponent. 'g' - General format. This prints the number as a fixed-point number, unless the number is too large, in which case it switches to 'e' exponent notation. 'n' - Number. This is the same as 'g', except that it uses the current locale setting to insert the appropriate number separator characters. '%' - Percentage. Multiplies the number by 100 and displays in fixed ('f') format, followed by a percent sign. Classes and types can define a __format__ method to control how they're formatted. It receives a single argument, the format specifier:: def __format__(self, format_spec): if isinstance(format_spec, unicode): return unicode(str(self)) else: return str(self) There's also a format() built-in that will format a single value. It calls the type's :meth:`__format__` method with the provided specifier:: >>> format(75.6564, '.2f') '75.66' .. seealso:: :pep:`3101` - Advanced String Formatting PEP written by Talin. .. ====================================================================== .. _pep-3105: PEP 3105: ``print`` As a Function ===================================================== The ``print`` statement becomes the :func:`print` function in Python 3.0. Making :func:`print` a function makes it easier to replace within a module by doing 'def print(...)' or importing a new function from somewhere else. Python 2.6 has a ``__future__`` import that removes ``print`` as language syntax, letting you use the functional form instead. For example:: from __future__ import print_function print('# of entries', len(dictionary), file=sys.stderr) The signature of the new function is:: def print(*args, sep=' ', end='\n', file=None) The parameters are: * **args**: positional arguments whose values will be printed out. * **sep**: the separator, which will be printed between arguments. * **end**: the ending text, which will be printed after all of the arguments have been output. * **file**: the file object to which the output will be sent. .. seealso:: :pep:`3105` - Make print a function PEP written by Georg Brandl. .. ====================================================================== .. _pep-3110: PEP 3110: Exception-Handling Changes ===================================================== One error that Python programmers occasionally make is the following:: try: ... except TypeError, ValueError: ... The author is probably trying to catch both :exc:`TypeError` and :exc:`ValueError` exceptions, but this code actually does something different: it will catch :exc:`TypeError` and bind the resulting exception object to the local name ``"ValueError"``. The correct code would have specified a tuple:: try: ... except (TypeError, ValueError): ... This error is possible because the use of the comma here is ambiguous: does it indicate two different nodes in the parse tree, or a single node that's a tuple. Python 3.0 changes the syntax to make this unambiguous by replacing the comma with the word "as". To catch an exception and store the exception object in the variable ``exc``, you must write:: try: ... except TypeError as exc: ... Python 3.0 will only support the use of "as", and therefore interprets the first example as catching two different exceptions. Python 2.6 supports both the comma and "as", so existing code will continue to work. .. seealso:: :pep:`3110` - Catching Exceptions in Python 3000 PEP written and implemented by Collin Winter. .. ====================================================================== .. _pep-3112: PEP 3112: Byte Literals ===================================================== Python 3.0 adopts Unicode as the language's fundamental string type, and denotes 8-bit literals differently, either as ``b'string'`` or using a :class:`bytes` constructor. For future compatibility, Python 2.6 adds :class:`bytes` as a synonym for the :class:`str` type, and it also supports the ``b''`` notation. .. seealso:: :pep:`3112` - Bytes literals in Python 3000 PEP written by Jason Orendorff; backported to 2.6 by Christian Heimes. .. ====================================================================== .. _pep-3118: PEP 3118: Revised Buffer Protocol ===================================================== The buffer protocol is a C-level API that lets Python extensions XXX .. seealso:: :pep:`3118` - Revising the buffer protocol PEP written by Travis Oliphant and Carl Banks. .. ====================================================================== .. _pep-3119: PEP 3119: Abstract Base Classes ===================================================== XXX write this -- this section is currently just brief notes. How to identify a file object? ABCs are a collection of classes describing various interfaces. Classes can derive from an ABC to indicate they support that ABC's interface. Concrete classes should obey the semantics specified by an ABC, but Python can't check this; it's up to the implementor. A metaclass lets you declare that an existing class or type derives from a particular ABC. You can even class AppendableSequence: __metaclass__ = ABCMeta AppendableSequence.register(list) assert issubclass(list, AppendableSequence) assert isinstance([], AppendableSequence) @abstractmethod decorator -- you can't instantiate classes w/ an abstract method. :: @abstractproperty decorator @abstractproperty def readonly(self): return self._x .. seealso:: :pep:`3119` - Introducing Abstract Base Classes PEP written by Guido van Rossum and Talin. Implemented by XXX. Backported to 2.6 by Benjamin Aranguren, with Alex Martelli. .. ====================================================================== .. _pep-3127: PEP 3127: Integer Literal Support and Syntax ===================================================== XXX write this -- this section is currently just brief notes. Python 3.0 changes the syntax for octal integer literals, and adds supports for binary integers: 0o instad of 0, and 0b for binary. Python 2.6 doesn't support this, but a bin() builtin was added. XXX changes to the hex/oct builtins New bin() built-in returns the binary form of a number. .. seealso:: :pep:`3127` - Integer Literal Support and Syntax PEP written by Patrick Maupin. .. ====================================================================== .. _pep-3129: PEP 3129: Class Decorators ===================================================== Decorators have been extended from functions to classes. It's now legal to write:: @foo @bar class A: pass This is equivalent to:: class A: pass A = foo(bar(A)) XXX need to find a good motivating example. .. seealso:: :pep:`3129` - Class Decorators PEP written by Collin Winter. .. ====================================================================== .. _pep-3141: PEP 3141: A Type Hierarchy for Numbers ===================================================== In Python 3.0, several abstract base classes for numeric types, inspired by Scheme's numeric tower, are being added. This change was backported to 2.6 as the :mod:`numbers` module. The most general ABC is :class:`Number`. It defines no operations at all, and only exists to allow checking if an object is a number by doing ``isinstance(obj, Number)``. Numbers are further divided into :class:`Exact` and :class:`Inexact`. Exact numbers can represent values precisely and operations never round off the results or introduce tiny errors that may break the commutativity and associativity properties; inexact numbers may perform such rounding or introduce small errors. Integers, long integers, and rational numbers are exact, while floating-point and complex numbers are inexact. :class:`Complex` is a subclass of :class:`Number`. Complex numbers can undergo the basic operations of addition, subtraction, multiplication, division, and exponentiation, and you can retrieve the real and imaginary parts and obtain a number's conjugate. Python's built-in complex type is an implementation of :class:`Complex`. :class:`Real` further derives from :class:`Complex`, and adds operations that only work on real numbers: :func:`floor`, :func:`trunc`, rounding, taking the remainder mod N, floor division, and comparisons. :class:`Rational` numbers derive from :class:`Real`, have :attr:`numerator` and :attr:`denominator` properties, and can be converted to floats. Python 2.6 adds a simple rational-number class, :class:`Fraction`, in the :mod:`fractions` module. (It's called :class:`Fraction` instead of :class:`Rational` to avoid a name clash with :class:`numbers.Rational`.) :class:`Integral` numbers derive from :class:`Rational`, and can be shifted left and right with ``<<`` and ``>>``, combined using bitwise operations such as ``&`` and ``|``, and can be used as array indexes and slice boundaries. In Python 3.0, the PEP slightly redefines the existing built-ins :func:`round`, :func:`math.floor`, :func:`math.ceil`, and adds a new one, :func:`math.trunc`, that's been backported to Python 2.6. :func:`math.trunc` rounds toward zero, returning the closest :class:`Integral` that's between the function's argument and zero. .. seealso:: :pep:`3141` - A Type Hierarchy for Numbers PEP written by Jeffrey Yasskin. XXX link: Discusses Scheme's numeric tower. The :mod:`fractions` Module -------------------------------------------------- To fill out the hierarchy of numeric types, a rational-number class has been added as the :mod:`fractions` module. Rational numbers are represented as a fraction, and can exactly represent numbers such as two-thirds that floating-point numbers can only approximate. The :class:`Fraction` constructor takes two :class:`Integral` values that will be the numerator and denominator of the resulting fraction. :: >>> from fractions import Fraction >>> a = Fraction(2, 3) >>> b = Fraction(2, 5) >>> float(a), float(b) (0.66666666666666663, 0.40000000000000002) >>> a+b Fraction(16, 15) >>> a/b Fraction(5, 3) To help in converting floating-point numbers to rationals, the float type now has a :meth:`as_integer_ratio()` method that returns the numerator and denominator for a fraction that evaluates to the same floating-point value:: >>> (2.5) .as_integer_ratio() (5, 2) >>> (3.1415) .as_integer_ratio() (7074029114692207L, 2251799813685248L) >>> (1./3) .as_integer_ratio() (6004799503160661L, 18014398509481984L) Note that values that can only be approximated by floating-point numbers, such as 1./3, are not simplified to the number being approximated; the fraction attempts to match the floating-point value **exactly**. The :mod:`fractions` module is based upon an implementation by Sjoerd Mullender that was in Python's :file:`Demo/classes/` directory for a long time. This implementation was significantly updated by Jeffrey Yasskin. Other Language Changes ====================== Here are all of the changes that Python 2.6 makes to the core Python language. * When calling a function using the ``**`` syntax to provide keyword arguments, you are no longer required to use a Python dictionary; any mapping will now work:: >>> def f(**kw): ... print sorted(kw) ... >>> ud=UserDict.UserDict() >>> ud['a'] = 1 >>> ud['b'] = 'string' >>> f(**ud) ['a', 'b'] .. Patch 1686487 * The built-in types now have improved support for extended slicing syntax, where various combinations of ``(start, stop, step)`` are supplied. Previously, the support was partial and certain corner cases wouldn't work. (Implemented by Thomas Wouters.) .. Revision 57619 * Properties now have three attributes, :attr:`getter`, :attr:`setter` and :attr:`deleter`, that are useful shortcuts for adding or modifying a getter, setter or deleter function to an existing property. You would use them like this:: class C(object): @property def x(self): return self._x @x.setter def x(self, value): self._x = value @x.deleter def x(self): del self._x class D(C): @C.x.getter def x(self): return self._x * 2 @x.setter def x(self, value): self._x = value / 2 * C functions and methods that use :cfunc:`PyComplex_AsCComplex` will now accept arguments that have a :meth:`__complex__` method. In particular, the functions in the :mod:`cmath` module will now accept objects with this method. This is a backport of a Python 3.0 change. (Contributed by Mark Dickinson.) .. Patch #1675423 A numerical nicety: when creating a complex number from two floats on systems that support signed zeros (-0 and +0), the :func:`complex` constructor will now preserve the sign of the zero. .. Patch 1507 * More floating-point features were also added. The :func:`float` function will now turn the strings ``+nan`` and ``-nan`` into the corresponding IEEE 754 Not A Number values, and ``+inf`` and ``-inf`` into positive or negative infinity. This works on any platform with IEEE 754 semantics. (Contributed by Christian Heimes.) .. Patch 1635 Other functions in the :mod:`math` module, :func:`isinf` and :func:`isnan`, return true if their floating-point argument is infinite or Not A Number. .. Patch 1640 The ``math.copysign(x, y)`` function copies the sign bit of an IEEE 754 number, returning the absolute value of *x* combined with the sign bit of *y*. For example, ``math.copysign(1, -0.0)`` returns -1.0. (Contributed by Christian Heimes.) * Changes to the :class:`Exception` interface as dictated by :pep:`352` continue to be made. For 2.6, the :attr:`message` attribute is being deprecated in favor of the :attr:`args` attribute. * The :exc:`GeneratorExit` exception now subclasses :exc:`BaseException` instead of :exc:`Exception`. This means that an exception handler that does ``except Exception:`` will not inadvertently catch :exc:`GeneratorExit`. (Contributed by Chad Austin.) .. Patch #1537 * Generator objects now have a :attr:`gi_code` attribute that refers to the original code object backing the generator. (Contributed by Collin Winter.) .. Patch #1473257 * The :func:`compile` built-in function now accepts keyword arguments as well as positional parameters. (Contributed by Thomas Wouters.) .. Patch 1444529 * The :func:`complex` constructor now accepts strings containing parenthesized complex numbers, letting ``complex(repr(cmplx))`` will now round-trip values. For example, ``complex('(3+4j)')`` now returns the value (3+4j). .. Patch 1491866 * The string :meth:`translate` method now accepts ``None`` as the translation table parameter, which is treated as the identity transformation. This makes it easier to carry out operations that only delete characters. (Contributed by Bengt Richter.) .. Patch 1193128 * The built-in :func:`dir` function now checks for a :meth:`__dir__` method on the objects it receives. This method must return a list of strings containing the names of valid attributes for the object, and lets the object control the value that :func:`dir` produces. Objects that have :meth:`__getattr__` or :meth:`__getattribute__` methods can use this to advertise pseudo-attributes they will honor. .. Patch 1591665 * Instance method objects have new attributes for the object and function comprising the method; the new synonym for :attr:`im_self` is :attr:`__self__`, and :attr:`im_func` is also available as :attr:`__func__`. The old names are still supported in Python 2.6; they're gone in 3.0. * An obscure change: when you use the the :func:`locals` function inside a :keyword:`class` statement, the resulting dictionary no longer returns free variables. (Free variables, in this case, are variables referred to in the :keyword:`class` statement that aren't attributes of the class.) .. ====================================================================== Optimizations ------------- * Type objects now have a cache of methods that can reduce the amount of work required to find the correct method implementation for a particular class; once cached, the interpreter doesn't need to traverse base classes to figure out the right method to call. The cache is cleared if a base class or the class itself is modified, so the cache should remain correct even in the face of Python's dynamic nature. (Original optimization implemented by Armin Rigo, updated for Python 2.6 by Kevin Jacobs.) .. Patch 1700288 * All of the functions in the :mod:`struct` module have been rewritten in C, thanks to work at the Need For Speed sprint. (Contributed by Raymond Hettinger.) * Internally, a bit is now set in type objects to indicate some of the standard built-in types. This speeds up checking if an object is a subclass of one of these types. (Contributed by Neal Norwitz.) * Unicode strings now uses faster code for detecting whitespace and line breaks; this speeds up the :meth:`split` method by about 25% and :meth:`splitlines` by 35%. (Contributed by Antoine Pitrou.) Memory usage is reduced by using pymalloc for the Unicode string's data. * The ``with`` statement now stores the :meth:`__exit__` method on the stack, producing a small speedup. (Implemented by Jeffrey Yasskin.) * To reduce memory usage, the garbage collector will now clear internal free lists when garbage-collecting the highest generation of objects. This may return memory to the OS sooner. The net result of the 2.6 optimizations is that Python 2.6 runs the pystone benchmark around XX% faster than Python 2.5. .. ====================================================================== New, Improved, and Deprecated Modules ===================================== As usual, Python's standard library received 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. * The :mod:`bsddb.dbshelve` module now uses the highest pickling protocol available, instead of restricting itself to protocol 1. (Contributed by W. Barnes.) .. Patch 1551443 * A new data type in the :mod:`collections` module: :class:`namedtuple(typename, fieldnames)` is a factory function that creates subclasses of the standard tuple whose fields are accessible by name as well as index. For example:: >>> var_type = collections.namedtuple('variable', ... 'id name type size') # Names are separated by spaces or commas. # 'id, name, type, size' would also work. >>> var_type._fields ('id', 'name', 'type', 'size') >>> var = var_type(1, 'frequency', 'int', 4) >>> print var[0], var.id # Equivalent 1 1 >>> print var[2], var.type # Equivalent int int >>> var._asdict() {'size': 4, 'type': 'int', 'id': 1, 'name': 'frequency'} >>> v2 = var._replace(name='amplitude') >>> v2 variable(id=1, name='amplitude', type='int', size=4) Where the new :class:`namedtuple` type proved suitable, the standard library has been modified to return them. For example, the :meth:`Decimal.as_tuple` method now returns a named tuple with :attr:`sign`, :attr:`digits`, and :attr:`exponent` fields. (Contributed by Raymond Hettinger.) * Another change to the :mod:`collections` module is that the :class:`deque` type now supports an optional *maxlen* parameter; if supplied, the deque's size will be restricted to no more than *maxlen* items. Adding more items to a full deque causes old items to be discarded. :: >>> from collections import deque >>> dq=deque(maxlen=3) >>> dq deque([], maxlen=3) >>> dq.append(1) ; dq.append(2) ; dq.append(3) >>> dq deque([1, 2, 3], maxlen=3) >>> dq.append(4) >>> dq deque([2, 3, 4], maxlen=3) (Contributed by Raymond Hettinger.) * The :mod:`ctypes` module now supports a :class:`c_bool` datatype that represents the C99 ``bool`` type. (Contributed by David Remahl.) .. Patch 1649190 The :mod:`ctypes` string, buffer and array types also have improved support for extended slicing syntax, where various combinations of ``(start, stop, step)`` are supplied. (Implemented by Thomas Wouters.) .. Revision 57769 * A new method in the :mod:`curses` module: for a window, :meth:`chgat` changes the display characters for a certain number of characters on a single line. (Contributed by Fabian Kreutz.) :: # Boldface text starting at y=0,x=21 # and affecting the rest of the line. stdscr.chgat(0,21, curses.A_BOLD) The :class:`Textbox` class in the :mod:`curses.textpad` module now supports editing in insert mode as well as overwrite mode. Insert mode is enabled by supplying a true value for the *insert_mode* parameter when creating the :class:`Textbox` instance. * The :mod:`datetime` module's :meth:`strftime` methods now support a ``%f`` format code that expands to the number of microseconds in the object, zero-padded on the left to six places. (Contributed by XXX.) .. Patch 1158 * The :mod:`decimal` module was updated to version 1.66 of `the General Decimal Specification `__. New features include some methods for some basic mathematical functions such as :meth:`exp` and :meth:`log10`:: >>> Decimal(1).exp() Decimal("2.718281828459045235360287471") >>> Decimal("2.7182818").ln() Decimal("0.9999999895305022877376682436") >>> Decimal(1000).log10() Decimal("3") The :meth:`as_tuple` method of :class:`Decimal` objects now returns a named tuple with :attr:`sign`, :attr:`digits`, and :attr:`exponent` fields. (Implemented by Facundo Batista and Mark Dickinson. Named tuple support added by Raymond Hettinger.) * The :mod:`difflib` module's :class:`SequenceMatcher` class now returns named tuples representing matches. In addition to behaving like tuples, the returned values also have :attr:`a`, :attr:`b`, and :attr:`size` attributes. (Contributed by Raymond Hettinger.) * An optional ``timeout`` parameter was added to the :class:`ftplib.FTP` class constructor as well as the :meth:`connect` method, specifying a timeout measured in seconds. (Added by Facundo Batista.) Also, the :class:`FTP` class's :meth:`storbinary` and :meth:`storlines` now take an optional *callback* parameter that will be called with each block of data after the data has been sent. (Contributed by Phil Schwartz.) .. Patch 1221598 * The :func:`reduce` built-in function is also available in the :mod:`functools` module. In Python 3.0, the built-in is dropped and it's only available from :mod:`functools`; currently there are no plans to drop the built-in in the 2.x series. (Patched by Christian Heimes.) .. Patch 1739906 * The :func:`glob.glob` function can now return Unicode filenames if a Unicode path was used and Unicode filenames are matched within the directory. .. Patch #1001604 * The :mod:`gopherlib` module has been removed. * A new function in the :mod:`heapq` module: ``merge(iter1, iter2, ...)`` takes any number of iterables that return data *in sorted order*, and returns a new iterator that returns the contents of all the iterators, also in sorted order. For example:: heapq.merge([1, 3, 5, 9], [2, 8, 16]) -> [1, 2, 3, 5, 8, 9, 16] Another new function, ``heappushpop(heap, item)``, pushes *item* onto *heap*, then pops off and returns the smallest item. This is more efficient than making a call to :func:`heappush` and then :func:`heappop`. (Contributed by Raymond Hettinger.) * An optional ``timeout`` parameter was added to the :class:`httplib.HTTPConnection` and :class:`HTTPSConnection` class constructors, specifying a timeout measured in seconds. (Added by Facundo Batista.) * Most of the :mod:`inspect` module's functions, such as :func:`getmoduleinfo` and :func:`getargs`, now return named tuples. In addition to behaving like tuples, the elements of the return value can also be accessed as attributes. (Contributed by Raymond Hettinger.) Some new functions in the module include :func:`isgenerator`, :func:`isgeneratorfunction`, and :func:`isabstract`. * The :mod:`itertools` module gained several new functions. ``izip_longest(iter1, iter2, ...[, fillvalue])`` makes tuples from each of the elements; if some of the iterables are shorter than others, the missing values are set to *fillvalue*. For example:: itertools.izip_longest([1,2,3], [1,2,3,4,5]) -> [(1, 1), (2, 2), (3, 3), (None, 4), (None, 5)] ``product(iter1, iter2, ..., [repeat=N])`` returns the Cartesian product of the supplied iterables, a set of tuples containing every possible combination of the elements returned from each iterable. :: itertools.product([1,2,3], [4,5,6]) -> [(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)] The optional *repeat* keyword argument is used for taking the product of an iterable or a set of iterables with themselves, repeated *N* times. With a single iterable argument, *N*-tuples are returned:: itertools.product([1,2], repeat=3)) -> [(1, 1, 1), (1, 1, 2), (1, 2, 1), (1, 2, 2), (2, 1, 1), (2, 1, 2), (2, 2, 1), (2, 2, 2)] With two iterables, *2N*-tuples are returned. :: itertools(product([1,2], [3,4], repeat=2) -> [(1, 3, 1, 3), (1, 3, 1, 4), (1, 3, 2, 3), (1, 3, 2, 4), (1, 4, 1, 3), (1, 4, 1, 4), (1, 4, 2, 3), (1, 4, 2, 4), (2, 3, 1, 3), (2, 3, 1, 4), (2, 3, 2, 3), (2, 3, 2, 4), (2, 4, 1, 3), (2, 4, 1, 4), (2, 4, 2, 3), (2, 4, 2, 4)] ``combinations(iterable, r)`` returns sub-sequences of length *r* from the elements of *iterable*. :: itertools.combinations('123', 2) -> [('1', '2'), ('1', '3'), ('2', '3')] itertools.combinations('123', 3) -> [('1', '2', '3')] itertools.combinations('1234', 3) -> [('1', '2', '3'), ('1', '2', '4'), ('1', '3', '4'), ('2', '3', '4')] ``permutations(iter[, r])`` returns all the permutations of length *r* of the iterable's elements. If *r* is not specified, it will default to the number of elements produced by the iterable. itertools.permutations([1,2,3,4], 2) -> [(1, 2), (1, 3), (1, 4), (2, 1), (2, 3), (2, 4), (3, 1), (3, 2), (3, 4), (4, 1), (4, 2), (4, 3)] ``itertools.chain(*iterables)` is an existing function in :mod:`itertools` that gained a new constructor in Python 2.6. ``itertools.chain.from_iterable(iterable)`` takes a single iterable that should return other iterables. :func:`chain` will then return all the elements of the first iterable, then all the elements of the second, and so on. :: chain.from_iterable([[1,2,3], [4,5,6]]) -> [1, 2, 3, 4, 5, 6] (All contributed by Raymond Hettinger.) * The :mod:`logging` module's :class:`FileHandler` class and its subclasses :class:`WatchedFileHandler`, :class:`RotatingFileHandler`, and :class:`TimedRotatingFileHandler` now have an optional *delay* parameter to its constructor. If *delay* is true, opening of the log file is deferred until the first :meth:`emit` call is made. (Contributed by Vinay Sajip.) * The :mod:`macfs` module has been removed. This in turn required the :func:`macostools.touched` function to be removed because it depended on the :mod:`macfs` module. .. Patch #1490190 * :class:`mmap` objects now have a :meth:`rfind` method that finds a substring, beginning at the end of the string and searching backwards. The :meth:`find` method also gained an *end* parameter containing the index at which to stop the forward search. (Contributed by John Lenton.) * (3.0-warning mode) The :mod:`new` module has been removed from Python 3.0. Importing it therefore triggers a warning message. * The :mod:`operator` module gained a :func:`methodcaller` function that takes a name and an optional set of arguments, returning a callable that will call the named function on any arguments passed to it. For example:: >>> # Equivalent to lambda s: s.replace('old', 'new') >>> replacer = operator.methodcaller('replace', 'old', 'new') >>> replacer('old wine in old bottles') 'new wine in new bottles' (Contributed by Georg Brandl, after a suggestion by Gregory Petrosyan.) The :func:`attrgetter` function now accepts dotted names and performs the corresponding attribute lookups:: >>> inst_name = operator.attrgetter('__class__.__name__') >>> inst_name('') 'str' >>> inst_name(help) '_Helper' (Contributed by Georg Brandl, after a suggestion by Barry Warsaw.) * New functions in the :mod:`os` module include ``fchmod(fd, mode)``, ``fchown(fd, uid, gid)``, and ``lchmod(path, mode)``, on operating systems that support these functions. :func:`fchmod` and :func:`fchown` let you change the mode and ownership of an opened file, and :func:`lchmod` changes the mode of a symlink. (Contributed by Georg Brandl and Christian Heimes.) * The :func:`os.walk` function now has a ``followlinks`` parameter. If set to True, it will follow symlinks pointing to directories and visit the directory's contents. For backward compatibility, the parameter's default value is false. Note that the function can fall into an infinite recursion if there's a symlink that points to a parent directory. .. Patch 1273829 * The ``os.environ`` object's :meth:`clear` method will now unset the environment variables using :func:`os.unsetenv` in addition to clearing the object's keys. (Contributed by Martin Horcicka.) .. Patch #1181 * In the :mod:`os.path` module, the :func:`splitext` function has been changed to not split on leading period characters. This produces better results when operating on Unix's dot-files. For example, ``os.path.splitext('.ipython')`` now returns ``('.ipython', '')`` instead of ``('', '.ipython')``. .. Bug #115886 A new function, :func:`relpath(path, start)` returns a relative path from the ``start`` path, if it's supplied, or from the current working directory to the destination ``path``. (Contributed by Richard Barran.) .. Patch 1339796 On Windows, :func:`os.path.expandvars` will now expand environment variables in the form "%var%", and "~user" will be expanded into the user's home directory path. (Contributed by Josiah Carlson.) .. Patch 957650 * The Python debugger provided by the :mod:`pdb` module gained a new command: "run" restarts the Python program being debugged, and can optionally take new command-line arguments for the program. (Contributed by Rocky Bernstein.) .. Patch #1393667 * The :mod:`pickletools` module now has an :func:`optimize` function that takes a string containing a pickle and removes some unused opcodes, returning a shorter pickle that contains the same data structure. (Contributed by Raymond Hettinger.) * New functions in the :mod:`posix` module: :func:`chflags` and :func:`lchflags` are wrappers for the corresponding system calls (where they're available). Constants for the flag values are defined in the :mod:`stat` module; some possible values include :const:`UF_IMMUTABLE` to signal the file may not be changed and :const:`UF_APPEND` to indicate that data can only be appended to the file. (Contributed by M. Levinson.) ``os.closerange(*low*, *high*)`` efficiently closes all file descriptors from *low* to *high*, ignoring any errors and not including *high* itself. This function is now used by the :mod:`subprocess` module to make starting processes faster. (Contributed by Georg Brandl.) .. Patch #1663329 * The :mod:`pyexpat` module's :class:`Parser` objects now allow setting their :attr:`buffer_size` attribute to change the size of the buffer used to hold character data. (Contributed by Achim Gaedke.) .. Patch 1137 * The :mod:`Queue` module now provides queue classes that retrieve entries in different orders. The :class:`PriorityQueue` class stores queued items in a heap and retrieves them in priority order, and :class:`LifoQueue` retrieves the most recently added entries first, meaning that it behaves like a stack. (Contributed by Raymond Hettinger.) * The :mod:`random` module's :class:`Random` objects can now be pickled on a 32-bit system and unpickled on a 64-bit system, and vice versa. Unfortunately, this change also means that Python 2.6's :class:`Random` objects can't be unpickled correctly on earlier versions of Python. (Contributed by Shawn Ligocki.) .. Issue 1727780 The new ``triangular(low, high, mode)`` function returns random numbers following a triangular distribution. The returned values are between *low* and *high*, not including *high* itself, and with *mode* as the mode, the most frequently occurring value in the distribution. (Contributed by Raymond Hettinger. XXX check) .. Patch 1681432 * Long regular expression searches carried out by the :mod:`re` module will now check for signals being delivered, so especially long searches can now be interrupted. (Contributed by Josh Hoyt and Ralf Schmitt.) .. Patch 846388 * The :mod:`rgbimg` module has been removed. * The :mod:`sched` module's :class:`scheduler` instances now have a read-only :attr:`queue` attribute that returns the contents of the scheduler's queue, represented as a list of named tuples with the fields ``(time, priority, action, argument)``. (Contributed by Raymond Hettinger XXX check.) .. Patch 1861 * The :mod:`select` module now has wrapper functions for the Linux :cfunc:`epoll` and BSD :cfunc:`kqueue` system calls. Also, a :meth:`modify` method was added to the existing :class:`poll` objects; ``pollobj.modify(fd, eventmask)`` takes a file descriptor or file object and an event mask, (Contributed by XXX.) .. Patch 1657 * The :mod:`sets` module has been deprecated; it's better to use the built-in :class:`set` and :class:`frozenset` types. * Integrating signal handling with GUI handling event loops like those used by Tkinter or GTk+ has long been a problem; most software ends up polling, waking up every fraction of a second. The :mod:`signal` module can now make this more efficient. Calling ``signal.set_wakeup_fd(fd)`` sets a file descriptor to be used; when a signal is received, a byte is written to that file descriptor. There's also a C-level function, :cfunc:`PySignal_SetWakeupFd`, for setting the descriptor. Event loops will use this by opening a pipe to create two descriptors, one for reading and one for writing. The writeable descriptor will be passed to :func:`set_wakeup_fd`, and the readable descriptor will be added to the list of descriptors monitored by the event loop via :cfunc:`select` or :cfunc:`poll`. On receiving a signal, a byte will be written and the main event loop will be woken up, without the need to poll. (Contributed by Adam Olsen.) .. Patch 1583 The :func:`siginterrupt` function is now available from Python code, and allows changing whether signals can interrupt system calls or not. (Contributed by Ralf Schmitt.) * The :mod:`smtplib` module now supports SMTP over SSL thanks to the addition of the :class:`SMTP_SSL` class. This class supports an interface identical to the existing :class:`SMTP` class. Both class constructors also have an optional ``timeout`` parameter that specifies a timeout for the initial connection attempt, measured in seconds. An implementation of the LMTP protocol (:rfc:`2033`) was also added to the module. LMTP is used in place of SMTP when transferring e-mail between agents that don't manage a mail queue. (SMTP over SSL contributed by Monty Taylor; timeout parameter added by Facundo Batista; LMTP implemented by Leif Hedstrom.) .. Patch #957003 * In the :mod:`smtplib` module, SMTP.starttls() now complies with :rfc:`3207` and forgets any knowledge obtained from the server not obtained from the TLS negotiation itself. (Patch contributed by Bill Fenner.) .. Issue 829951 * The :mod:`socket` module now supports TIPC (http://tipc.sf.net), a high-performance non-IP-based protocol designed for use in clustered environments. TIPC addresses are 4- or 5-tuples. (Contributed by Alberto Bertogli.) .. Patch #1646 * The base classes in the :mod:`SocketServer` module now support calling a :meth:`handle_timeout` method after a span of inactivity specified by the server's :attr:`timeout` attribute. (Contributed by Michael Pomraning.) .. Patch #742598 * The :mod:`struct` module now supports the C99 :ctype:`_Bool` type, using the format character ``'?'``. (Contributed by David Remahl.) * A new variable in the :mod:`sys` module, :attr:`float_info`, is an object containing information about the platform's floating-point support derived from the :file:`float.h` file. Attributes of this object include :attr:`mant_dig` (number of digits in the mantissa), :attr:`epsilon` (smallest difference between 1.0 and the next largest value representable), and several others. (Contributed by Christian Heimes.) .. Patch 1534 Another new variable, :attr:`dont_write_bytecode`, controls whether Python writes any :file:`.pyc` or :file:`.pyo` files on importing a module. If this variable is true, the compiled files are not written. The variable is initially set on start-up by supplying the :option:`-B` switch to the Python interpreter, or by setting the :envvar:`PYTHONDONTWRITEBYTECODE` environment variable before running the interpreter. Python code can subsequently change the value of this variable to control whether bytecode files are written or not. (Contributed by Neal Norwitz and Georg Brandl.) Information about the command-line arguments supplied to the Python interpreter are available as attributes of a ``sys.flags`` named tuple. For example, the :attr:`verbose` attribute is true if Python was executed in verbose mode, :attr:`debug` is true in debugging mode, etc. These attributes are all read-only. (Contributed by Christian Heimes.) It's now possible to determine the current profiler and tracer functions by calling :func:`sys.getprofile` and :func:`sys.gettrace`. (Contributed by Georg Brandl.) .. Patch #1648 * The :mod:`tarfile` module now supports POSIX.1-2001 (pax) and POSIX.1-1988 (ustar) format tarfiles, in addition to the GNU tar format that was already supported. The default format is GNU tar; specify the ``format`` parameter to open a file using a different format:: tar = tarfile.open("output.tar", "w", format=tarfile.PAX_FORMAT) The new ``errors`` parameter lets you specify an error handling scheme for character conversions: the three standard ways Python can handle errors ``'strict'``, ``'ignore'``, ``'replace'`` , or the special value ``'utf-8'``, which replaces bad characters with their UTF-8 representation. Character conversions occur because the PAX format supports Unicode filenames, defaulting to UTF-8 encoding. The :meth:`TarFile.add` method now accepts a ``exclude`` argument that's a function that can be used to exclude certain filenames from an archive. The function must take a filename and return true if the file should be excluded or false if it should be archived. The function is applied to both the name initially passed to :meth:`add` and to the names of files in recursively-added directories. (All changes contributed by Lars Gustäbel). * An optional ``timeout`` parameter was added to the :class:`telnetlib.Telnet` class constructor, specifying a timeout measured in seconds. (Added by Facundo Batista.) * The :class:`tempfile.NamedTemporaryFile` class usually deletes the temporary file it created when the file is closed. This behaviour can now be changed by passing ``delete=False`` to the constructor. (Contributed by Damien Miller.) .. Patch #1537850 A new class, :class:`SpooledTemporaryFile`, behaves like a temporary file but stores its data in memory until a maximum size is exceeded. On reaching that limit, the contents will be written to an on-disk temporary file. (Contributed by Dustin J. Mitchell.) The :class:`NamedTemporaryFile` and :class:`SpooledTemporaryFile` classes both work as context managers, so you can write ``with tempfile.NamedTemporaryFile() as tmp: ...``. (Contributed by Alexander Belopolsky.) .. Issue #2021 * The :mod:`test.test_support` module now contains a :func:`EnvironmentVarGuard` context manager that supports temporarily changing environment variables and automatically restores them to their old values. Another context manager, :class:`TransientResource`, can surround calls to resources that may or may not be available; it will catch and ignore a specified list of exceptions. For example, a network test may ignore certain failures when connecting to an external web site:: with test_support.TransientResource(IOError, errno=errno.ETIMEDOUT): f = urllib.urlopen('https://sf.net') ... (Contributed by Brett Cannon.) * The :mod:`textwrap` module can now preserve existing whitespace at the beginnings and ends of the newly-created lines by specifying ``drop_whitespace=False`` as an argument:: >>> S = """This sentence has a bunch of extra whitespace.""" >>> print textwrap.fill(S, width=15) This sentence has a bunch of extra whitespace. >>> print textwrap.fill(S, drop_whitespace=False, width=15) This sentence has a bunch of extra whitespace. >>> (Contributed by Dwayne Bailey.) .. Patch #1581073 * The :mod:`timeit` module now accepts callables as well as strings for the statement being timed and for the setup code. Two convenience functions were added for creating :class:`Timer` instances: ``repeat(stmt, setup, time, repeat, number)`` and ``timeit(stmt, setup, time, number)`` create an instance and call the corresponding method. (Contributed by Erik Demaine.) .. Patch #1533909 * An optional ``timeout`` parameter was added to the :func:`urllib.urlopen` function and the :class:`urllib.ftpwrapper` class constructor, as well as the :func:`urllib2.urlopen` function. The parameter specifies a timeout measured in seconds. For example:: >>> u = urllib2.urlopen("http://slow.example.com", timeout=3) Traceback (most recent call last): ... urllib2.URLError: >>> (Added by Facundo Batista.) * The XML-RPC classes :class:`SimpleXMLRPCServer` and :class:`DocXMLRPCServer` classes can now be prevented from immediately opening and binding to their socket by passing True as the ``bind_and_activate`` constructor parameter. This can be used to modify the instance's :attr:`allow_reuse_address` attribute before calling the :meth:`server_bind` and :meth:`server_activate` methods to open the socket and begin listening for connections. (Contributed by Peter Parente.) .. Patch 1599845 :class:`SimpleXMLRPCServer` also has a :attr:`_send_traceback_header` attribute; if true, the exception and formatted traceback are returned as HTTP headers "X-Exception" and "X-Traceback". This feature is for debugging purposes only and should not be used on production servers because the tracebacks could possibly reveal passwords or other sensitive information. (Contributed by Alan McIntyre as part of his project for Google's Summer of Code 2007.) * The :mod:`zipfile` module's :class:`ZipFile` class now has :meth:`extract` and :meth:`extractall` methods that will unpack a single file or all the files in the archive to the current directory, or to a specified directory:: z = zipfile.ZipFile('python-251.zip') # Unpack a single file, writing it relative to the /tmp directory. z.extract('Python/sysmodule.c', '/tmp') # Unpack all the files in the archive. z.extractall() (Contributed by Alan McIntyre.) .. Patch 467924 .. ====================================================================== .. whole new modules get described in subsections here Improved SSL Support -------------------------------------------------- Bill Janssen made extensive improvements to Python 2.6's support for SSL. XXX use ssl.sslsocket - subclass of socket.socket. XXX Can specify if certificate is required, and obtain certificate info by calling getpeercert method. XXX sslwrap() behaves like socket.ssl XXX Certain features require the OpenSSL package to be installed, notably the 'openssl' binary. .. seealso:: SSL module documentation. .. ====================================================================== plistlib: A Property-List Parser -------------------------------------------------- A commonly-used format on MacOS X is the ``.plist`` format, which stores basic data types (numbers, strings, lists, and dictionaries) and serializes them into an XML-based format. (It's a lot like the XML-RPC serialization of data types.) Despite being primarily used on MacOS X, the format has nothing Mac-specific about it and the Python implementation works on any platform that Python supports, so the :mod:`plistlib` module has been promoted to the standard library. Using the module is simple:: import sys import plistlib import datetime # Create data structure data_struct = dict(lastAccessed=datetime.datetime.now(), version=1, categories=('Personal', 'Shared', 'Private')) # Create string containing XML. plist_str = plistlib.writePlistToString(data_struct) new_struct = plistlib.readPlistFromString(plist_str) print data_struct print new_struct # Write data structure to a file and read it back. plistlib.writePlist(data_struct, '/tmp/customizations.plist') new_struct = plistlib.readPlist('/tmp/customizations.plist') # read/writePlist accepts file-like objects as well as paths. plistlib.writePlist(data_struct, sys.stdout) .. ====================================================================== Build and C API Changes ======================= Changes to Python's build process and to the C API include: * Python 2.6 can be built with Microsoft Visual Studio 2008. See the :file:`PCbuild9` directory for the build files. (Implemented by Christian Heimes.) * Python now can only be compiled with C89 compilers (after 19 years!). This means that the Python source tree can now drop its own implementations of :cfunc:`memmove` and :cfunc:`strerror`, which are in the C89 standard library. * The BerkeleyDB module now has a C API object, available as ``bsddb.db.api``. This object can be used by other C extensions that wish to use the :mod:`bsddb` module for their own purposes. (Contributed by Duncan Grisby.) .. Patch 1551895 * Several functions return information about the platform's floating-point support. :cfunc:`PyFloat_GetMax` returns the maximum representable floating point value, and :cfunc:`PyFloat_GetMin` returns the minimum positive value. :cfunc:`PyFloat_GetInfo` returns a dictionary containing more information from the :file:`float.h` file, such as ``"mant_dig"`` (number of digits in the mantissa), ``"epsilon"`` (smallest difference between 1.0 and the next largest value representable), and several others. (Contributed by Christian Heimes.) .. Issue 1534 * Python's C API now includes two functions for case-insensitive string comparisons, ``PyOS_stricmp(char*, char*)`` and ``PyOS_strnicmp(char*, char*, Py_ssize_t)``. (Contributed by Christian Heimes.) .. Issue 1635 * Some macros were renamed in both 3.0 and 2.6 to make it clearer that they are macros, not functions. :cmacro:`Py_Size()` became :cmacro:`Py_SIZE()`, :cmacro:`Py_Type()` became :cmacro:`Py_TYPE()`, and :cmacro:`Py_Refcnt()` became :cmacro:`Py_REFCNT()`. The mixed-case macros are still available in Python 2.6 for backward compatibility. .. Issue 1629 * Distutils now places C extensions it builds in a different directory when running on a debug version of Python. (Contributed by Collin Winter.) .. Patch 1530959 * Several basic data types, such as integers and strings, maintain internal free lists of objects that can be re-used. The data structures for these free lists now follow a naming convention: the variable is always named ``free_list``, the counter is always named ``numfree``, and a macro :cmacro:`Py_MAXFREELIST` is always defined. .. ====================================================================== Port-Specific Changes: Windows ----------------------------------- * The :mod:`msvcrt` module now supports both the normal and wide char variants of the console I/O API. The :func:`getwch` function reads a keypress and returns a Unicode value, as does the :func:`getwche` function. The :func:`putwch` function takes a Unicode character and writes it to the console. (Contributed by Christian Heimes.) * :func:`os.path.expandvars` will now expand environment variables in the form "%var%", and "~user" will be expanded into the user's home directory path. (Contributed by Josiah Carlson.) * The :mod:`socket` module's socket objects now have an :meth:`ioctl` method that provides a limited interface to the :cfunc:`WSAIoctl` system interface. * The :mod:`_winreg` module now has a function, :func:`ExpandEnvironmentStrings`, that expands environment variable references such as ``%NAME%`` in an input string. The handle objects provided by this module now support the context protocol, so they can be used in :keyword:`with` statements. (Contributed by Christian Heimes.) * The new default compiler on Windows is Visual Studio 2008 (VS 9.0). The build directories for Visual Studio 2003 (VS7.1) and 2005 (VS8.0) were moved into the PC/ directory. The new PCbuild directory supports cross compilation for X64, debug builds and Profile Guided Optimization (PGO). PGO builds are roughly 10% faster than normal builds. (Contributed by Christian Heimes with help from Amaury Forgeot d'Arc and Martin von Loewis.) .. ====================================================================== .. _section-other: Other Changes and Fixes ======================= As usual, there were a bunch of other improvements and bugfixes scattered throughout the source tree. A search through the change logs finds there were XXX patches applied and YYY bugs fixed between Python 2.5 and 2.6. Both figures are likely to be underestimates. Some of the more notable changes are: * It's now possible to prevent Python from writing any :file:`.pyc` or :file:`.pyo` files by either supplying the :option:`-B` switch or setting the :envvar:`PYTHONDONTWRITEBYTECODE` environment variable to any non-empty string when running the Python interpreter. These are also used to set the :data:`sys.dont_write_bytecode` attribute; Python code can change this variable to control whether bytecode files are subsequently written. (Contributed by Neal Norwitz and Georg Brandl.) .. ====================================================================== Porting to Python 2.6 ===================== This section lists previously described changes and other bugfixes that may require changes to your code: * The :meth:`__init__` method of :class:`collections.deque` now clears any existing contents of the deque before adding elements from the iterable. This change makes the behavior match that of ``list.__init__()``. * The :class:`Decimal` constructor now accepts leading and trailing whitespace when passed a string. Previously it would raise an :exc:`InvalidOperation` exception. On the other hand, the :meth:`create_decimal` method of :class:`Context` objects now explicitly disallows extra whitespace, raising a :exc:`ConversionSyntax` exception. * Due to an implementation accident, if you passed a file path to the built-in :func:`__import__` function, it would actually import the specified file. This was never intended to work, however, and the implementation now explicitly checks for this case and raises an :exc:`ImportError`. * The :mod:`socket` module exception :exc:`socket.error` now inherits from :exc:`IOError`. Previously it wasn't a subclass of :exc:`StandardError` but now it is, through :exc:`IOError`. (Implemented by Gregory P. Smith.) .. Issue 1706815 * The :mod:`xmlrpclib` module no longer automatically converts :class:`datetime.date` and :class:`datetime.time` to the :class:`xmlrpclib.DateTime` type; the conversion semantics were not necessarily correct for all applications. Code using :mod:`xmlrpclib` should convert :class:`date` and :class:`time` instances. .. Issue 1330538 * (3.0-warning mode) The :class:`Exception` class now warns when accessed using slicing or index access; having :class:`Exception` behave like a tuple is being phased out. * (3.0-warning mode) inequality comparisons between two dictionaries or two objects that don't implement comparison methods are reported as warnings. ``dict1 == dict2`` still works, but ``dict1 < dict2`` is being phased out. Comparisons between cells, which are an implementation detail of Python's scoping rules, also cause warnings because such comparisons are forbidden entirely in 3.0. .. ====================================================================== .. _acks: Acknowledgements ================ The author would like to thank the following people for offering suggestions, corrections and assistance with various drafts of this article: .