**************************** What's New in Python 2.6 **************************** .. % XXX mention switch to reST for documentation .. % XXX mention switch to Roundup for bug tracking :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 log .. % 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? .. % sets module deprecated .. % ====================================================================== Python 3.0 ================ .. % XXX add general comment about Python 3.0 features in 2.6 .. % XXX mention -3 switch 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. .. 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. 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 generators`` directive. In 2.6 the statement no longer need 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-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-3119: PEP 3119: Abstract Base Classes ===================================================== XXX .. 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). Other Language Changes ====================== Here are all of the changes that Python 2.6 makes to the core Python language. * 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. * 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 :func:`compile` built-in function now accepts keyword arguments as well as positional parameters. (Contributed by XXX.) .. % 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. .. % Patch 1591665 * 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 ------------- * 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.) 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. * 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') var = var_type(1, 'frequency', 'int', 4) print var[0], var.id # Equivalent print var[2], var.type # Equivalent (Contributed by Raymond Hettinger.) * 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. :: # Boldface text starting at y=0,x=21 # and affecting the rest of the line. stdscr.chgat(0,21, curses.A_BOLD) (Contributed by Fabian Kreutz.) * 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.) * 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] (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.) * A new function in the :mod:`itertools` module: ``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)] (Contributed by Raymond Hettinger.) * 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 * 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 * 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 XXX.) .. % Patch 957650 * 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.) * The :mod:`rgbimg` module has been removed. * 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 * 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 * 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. >>> .. % 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.) .. % ====================================================================== .. % whole new modules get described in \subsections here .. % ====================================================================== Build and C API Changes ======================= Changes to Python's build process and to the C API include: * Detailed changes are listed here. .. % ====================================================================== Port-Specific Changes --------------------- Platform-specific changes go here. .. % ====================================================================== .. _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: * Details go here. .. % ====================================================================== Porting to Python 2.6 ===================== This section lists previously described changes that may require changes to your code: * The :mod:`socket` module exception :exc:`socket.error` now inherits from :exc:`IOError`. .. % ====================================================================== .. _acks: Acknowledgements ================ The author would like to thank the following people for offering suggestions, corrections and assistance with various drafts of this article: .