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@ -2,6 +2,9 @@
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|
@ -67,12 +70,367 @@ new feature.
.. % 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)')``
@ -87,6 +445,15 @@ Here are all of the changes that Python 2.6 makes to the core Python language.
.. % 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
@ -160,6 +527,11 @@ complete list of changes, or look through the CVS logs for all the details.
(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*.
@ -176,6 +548,15 @@ complete list of changes, or look through the CVS logs for all the details.
.. % 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.
@ -191,6 +572,12 @@ complete list of changes, or look through the CVS logs for all the details.
.. % 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
@ -217,10 +604,37 @@ complete list of changes, or look through the CVS logs for all the details.
.. % Patch #957003
* 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
* 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. (Contributed by Brett Cannon.)
@ -235,6 +649,20 @@ complete list of changes, or look through the CVS logs for all the details.
.. % 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: <urlopen error timed out>
>>>
(Added by Facundo Batista.)
.. % ======================================================================
.. % whole new modules get described in \subsections here