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****************************
What's New in Python 2.7
****************************
:Author: A.M. Kuchling (amk at amk.ca)
:Release: |release|
:Date: |today|
.. Fix accents on Kristjan Valur Jonsson, Fuerstenau, Tarek Ziade.
.. $Id$
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 in a parenthetical comment.
XXX Describe the transmogrify() function added to the socket
module.
(Contributed by P.Y. Developer; :issue:`12345`.)
This saves the maintainer some effort going through the SVN logs
when researching a change.
This article explains the new features in Python 2.7. The final
release of 2.7 is currently scheduled for June 2010; the detailed
schedule is described in :pep:`373`.
.. Compare with previous release in 2 - 3 sentences here.
add hyperlink when the documentation becomes available online.
.. _whatsnew27-python31:
Python 3.1 Features
=======================
Much as Python 2.6 incorporated features from Python 3.0,
version 2.7 incorporates some of the new features
in Python 3.1. The 2.x series continues to provide tools
for migrating to the 3.x series.
A partial list of 3.1 features that were backported to 2.7:
* A version of the :mod:`io` library, rewritten in C for performance.
* The ordered-dictionary type described in :ref:`pep-0372`.
* The new format specifier described in :ref:`pep-0378`.
* The :class:`memoryview` object.
* A small subset of the :mod:`importlib` module `described below <#importlib-section>`__.
* Float-to-string and string-to-float conversions now round their
results more correctly. And :func:`repr` of a floating-point
number *x* returns a result that's guaranteed to round back to the
same number when converted back to a string.
* The :cfunc:`PyLong_AsLongAndOverflow` C API function.
One porting change: the :option:`-3` switch now automatically
enables the :option:`-Qwarn` switch that causes warnings
about using classic division with integers and long integers.
Other new Python3-mode warnings include:
* :func:`operator.isCallable` and :func:`operator.sequenceIncludes`,
which are not supported in 3.x.
.. ========================================================================
.. Large, PEP-level features and changes should be described here.
.. ========================================================================
.. _pep-0372:
PEP 372: Adding an ordered dictionary to collections
====================================================
Regular Python dictionaries iterate over key/value pairs in arbitrary order.
Over the years, a number of authors have written alternative implementations
that remember the order that the keys were originally inserted. Based on
the experiences from those implementations, a new
:class:`collections.OrderedDict` class has been introduced.
The :class:`OrderedDict` API is substantially the same as regular dictionaries
but will iterate over keys and values in a guaranteed order depending on
when a key was first inserted::
>>> from collections import OrderedDict
>>> d = OrderedDict([('first', 1), ('second', 2),
... ('third', 3)])
>>> d.items()
[('first', 1), ('second', 2), ('third', 3)]
If a new entry overwrites an existing entry, the original insertion
position is left unchanged::
>>> d['second'] = 4
>>> d.items()
[('first', 1), ('second', 4), ('third', 3)]
Deleting an entry and reinserting it will move it to the end::
>>> del d['second']
>>> d['second'] = 5
>>> d.items()
[('first', 1), ('third', 3), ('second', 5)]
The :meth:`popitem` method has an optional *last* argument
that defaults to True. If *last* is True, the most recently
added key is returned and removed; if it's False, the
oldest key is selected::
>>> od = OrderedDict([(x,0) for x in range(20)])
>>> od.popitem()
(19, 0)
>>> od.popitem()
(18, 0)
>>> od.popitem(False)
(0, 0)
>>> od.popitem(False)
(1, 0)
Comparing two ordered dictionaries checks both the keys and values,
and requires that the insertion order was the same::
>>> od1 = OrderedDict([('first', 1), ('second', 2),
... ('third', 3)])
>>> od2 = OrderedDict([('third', 3), ('first', 1),
... ('second', 2)])
>>> od1==od2
False
>>> # Move 'third' key to the end
>>> del od2['third'] ; od2['third'] = 3
>>> od1==od2
True
Comparing an :class:`OrderedDict` with a regular dictionary
ignores the insertion order and just compares the keys and values.
How does the :class:`OrderedDict` work? It maintains a doubly-linked
list of keys, appending new keys to the list as they're inserted. A
secondary dictionary maps keys to their corresponding list node, so
deletion doesn't have to traverse the entire linked list and therefore
remains O(1).
.. XXX check O(1)-ness with Raymond
The standard library now supports use of ordered dictionaries in several
modules. The :mod:`configparser` module uses them by default. This lets
configuration files be read, modified, and then written back in their original
order. The *_asdict()* method for :func:`collections.namedtuple` now
returns an ordered dictionary with the values appearing in the same order as
the underlying tuple indicies. The :mod:`json` module is being built-out with
an *object_pairs_hook* to allow OrderedDicts to be built by the decoder.
Support was also added for third-party tools like `PyYAML <http://pyyaml.org/>`_.
.. seealso::
:pep:`372` - Adding an ordered dictionary to collections
PEP written by Armin Ronacher and Raymond Hettinger;
implemented by Raymond Hettinger.
.. _pep-0378:
PEP 378: Format Specifier for Thousands Separator
====================================================
To make program output more readable, it can be useful to add
separators to large numbers and render them as
18,446,744,073,709,551,616 instead of 18446744073709551616.
The fully general solution for doing this is the :mod:`locale` module,
which can use different separators ("," in North America, "." in
Europe) and different grouping sizes, but :mod:`locale` is complicated
to use and unsuitable for multi-threaded applications where different
threads are producing output for different locales.
Therefore, a simple comma-grouping mechanism has been added to the
mini-language used by the string :meth:`format` method. When
formatting a floating-point number, simply include a comma between the
width and the precision::
>>> '{:20,.2}'.format(f)
'18,446,744,073,709,551,616.00'
This mechanism is not adaptable at all; commas are always used as the
separator and the grouping is always into three-digit groups. The
comma-formatting mechanism isn't as general as the :mod:`locale`
module, but it's easier to use.
.. XXX "Format String Syntax" in string.rst could use many more examples.
.. seealso::
:pep:`378` - Format Specifier for Thousands Separator
PEP written by Raymond Hettinger; implemented by Eric Smith.
Other Language Changes
======================
Some smaller changes made to the core Python language are:
* The :keyword:`with` statement can now use multiple context managers
in one statement. Context managers are processed from left to right
and each one is treated as beginning a new :keyword:`with` statement.
This means that::
with A() as a, B() as b:
... suite of statements ...
is equivalent to::
with A() as a:
with B() as b:
... suite of statements ...
The :func:`contextlib.nested` function provides a very similar
function, so it's no longer necessary and has been deprecated.
(Proposed in http://codereview.appspot.com/53094; implemented by
Georg Brandl.)
* Conversions between floating-point numbers and strings are
now correctly rounded on most platforms. These conversions occur
in many different places: :func:`str` on
floats and complex numbers; the :class:`float` and :class:`complex`
constructors;
numeric formatting; serialization and
deserialization of floats and complex numbers using the
:mod:`marshal`, :mod:`pickle`
and :mod:`json` modules;
parsing of float and imaginary literals in Python code;
and :class:`Decimal`-to-float conversion.
Related to this, the :func:`repr` of a floating-point number *x*
now returns a result based on the shortest decimal string that's
guaranteed to round back to *x* under correct rounding (with
round-half-to-even rounding mode). Previously it gave a string
based on rounding x to 17 decimal digits.
The rounding library responsible for this improvement works on
Windows, and on Unix platforms using the gcc, icc, or suncc
compilers. There may be a small number of platforms where correct
operation of this code cannot be guaranteed, so the code is not
used on such systems. You can find out which code is being used
by checking :data:`sys.float_repr_style`, which will be ``short``
if the new code is in use and ``legacy`` if it isn't.
Implemented by Eric Smith and Mark Dickinson, using David Gay's
:file:`dtoa.c` library; :issue:`7117`.
* The :meth:`str.format` method now supports automatic numbering of the replacement
fields. This makes using :meth:`str.format` more closely resemble using
``%s`` formatting::
>>> '{}:{}:{}'.format(2009, 04, 'Sunday')
'2009:4:Sunday'
>>> '{}:{}:{day}'.format(2009, 4, day='Sunday')
'2009:4:Sunday'
The auto-numbering takes the fields from left to right, so the first ``{...}``
specifier will use the first argument to :meth:`str.format`, the next
specifier will use the next argument, and so on. You can't mix auto-numbering
and explicit numbering -- either number all of your specifier fields or none
of them -- but you can mix auto-numbering and named fields, as in the second
example above. (Contributed by Eric Smith; :issue:`5237`.)
Complex numbers now correctly support usage with :func:`format`.
Specifying a precision or comma-separation applies to both the real
and imaginary parts of the number, but a specified field width and
alignment is applied to the whole of the resulting ``1.5+3j``
output. (Contributed by Eric Smith; :issue:`1588`.)
The 'F' format code now always formats its output using uppercase characters,
so it will now produce 'INF' and 'NAN'.
(Contributed by Eric Smith; :issue:`3382`.)
* The :func:`int` and :func:`long` types gained a ``bit_length``
method that returns the number of bits necessary to represent
its argument in binary::
>>> n = 37
>>> bin(37)
'0b100101'
>>> n.bit_length()
6
>>> n = 2**123-1
>>> n.bit_length()
123
>>> (n+1).bit_length()
124
(Contributed by Fredrik Johansson and Victor Stinner; :issue:`3439`.)
* Conversions from long integers and regular integers to floating
point now round differently, returning the floating-point number
closest to the number. This doesn't matter for small integers that
can be converted exactly, but for large numbers that will
unavoidably lose precision, Python 2.7 now approximates more
closely. For example, Python 2.6 computed the following::
>>> n = 295147905179352891391
>>> float(n)
2.9514790517935283e+20
>>> n - long(float(n))
65535L
Python 2.7's floating-point result is larger, but much closer to the
true value::
>>> n = 295147905179352891391
>>> float(n)
2.9514790517935289e+20
>>> n-long(float(n)
... )
-1L
(Implemented by Mark Dickinson; :issue:`3166`.)
Integer division is also more accurate in its rounding behaviours. (Also
implemented by Mark Dickinson; :issue:`1811`.)
* The :class:`bytearray` type's :meth:`translate` method now accepts
``None`` as its first argument. (Fixed by Georg Brandl;
:issue:`4759`.)
* When using ``@classmethod`` and ``@staticmethod`` to wrap
methods as class or static methods, the wrapper object now
exposes the wrapped function as their :attr:`__func__` attribute.
(Contributed by Amaury Forgeot d'Arc, after a suggestion by
George Sakkis; :issue:`5982`.)
* A new encoding named "cp720", used primarily for Arabic text, is now
supported. (Contributed by Alexander Belchenko and Amaury Forgeot
d'Arc; :issue:`1616979`.)
* The :class:`file` object will now set the :attr:`filename` attribute
on the :exc:`IOError` exception when trying to open a directory
on POSIX platforms. (Noted by Jan Kaliszewski; :issue:`4764`.)
* The Python tokenizer now translates line endings itself, so the
:func:`compile` built-in function can now accept code using any
line-ending convention. Additionally, it no longer requires that the
code end in a newline.
* Extra parentheses in function definitions are illegal in Python 3.x,
meaning that you get a syntax error from ``def f((x)): pass``. In
Python3-warning mode, Python 2.7 will now warn about this odd usage.
(Noted by James Lingard; :issue:`7362`.)
.. ======================================================================
Optimizations
-------------
Several performance enhancements have been added:
.. * A new :program:`configure` option, :option:`--with-computed-gotos`,
compiles the main bytecode interpreter loop using a new dispatch
mechanism that gives speedups of up to 20%, depending on the system
and benchmark. The new mechanism is only supported on certain
compilers, such as gcc, SunPro, and icc.
* A new opcode was added to perform the initial setup for
:keyword:`with` statements, looking up the :meth:`__enter__` and
:meth:`__exit__` methods. (Contributed by Benjamin Peterson.)
* The garbage collector now performs better for one common usage
pattern: when many objects are being allocated without deallocating
any of them. This would previously take quadratic
time for garbage collection, but now the number of full garbage collections
is reduced as the number of objects on the heap grows.
The new logic is to only perform a full garbage collection pass when
the middle generation has been collected 10 times and when the
number of survivor objects from the middle generation exceeds 10% of
the number of objects in the oldest generation. (Suggested by Martin
von Loewis and implemented by Antoine Pitrou; :issue:`4074`.)
* The garbage collector tries to avoid tracking simple containers
which can't be part of a cycle. In Python 2.7, this is now true for
tuples and dicts containing atomic types (such as ints, strings,
etc.). Transitively, a dict containing tuples of atomic types won't
be tracked either. This helps reduce the cost of each
garbage collection by decreasing the number of objects to be
considered and traversed by the collector.
(Contributed by Antoine Pitrou; :issue:`4688`.)
* Long integers are now stored internally either in base 2**15 or in base
2**30, the base being determined at build time. Previously, they
were always stored in base 2**15. Using base 2**30 gives
significant performance improvements on 64-bit machines, but
benchmark results on 32-bit machines have been mixed. Therefore,
the default is to use base 2**30 on 64-bit machines and base 2**15
on 32-bit machines; on Unix, there's a new configure option
:option:`--enable-big-digits` that can be used to override this default.
Apart from the performance improvements this change should be
invisible to end users, with one exception: for testing and
debugging purposes there's a new structseq ``sys.long_info`` that
provides information about the internal format, giving the number of
bits per digit and the size in bytes of the C type used to store
each digit::
>>> import sys
>>> sys.long_info
sys.long_info(bits_per_digit=30, sizeof_digit=4)
(Contributed by Mark Dickinson; :issue:`4258`.)
Another set of changes made long objects a few bytes smaller: 2 bytes
smaller on 32-bit systems and 6 bytes on 64-bit.
(Contributed by Mark Dickinson; :issue:`5260`.)
* The division algorithm for long integers has been made faster
by tightening the inner loop, doing shifts instead of multiplications,
and fixing an unnecessary extra iteration.
Various benchmarks show speedups of between 50% and 150% for long
integer divisions and modulo operations.
(Contributed by Mark Dickinson; :issue:`5512`.)
Bitwise operations are also significantly faster (initial patch by
Gregory Smith; :issue:`1087418`).
* The implementation of ``%`` checks for the left-side operand being
a Python string and special-cases it; this results in a 1-3%
performance increase for applications that frequently use ``%``
with strings, such as templating libraries.
(Implemented by Collin Winter; :issue:`5176`.)
* List comprehensions with an ``if`` condition are compiled into
faster bytecode. (Patch by Antoine Pitrou, back-ported to 2.7
by Jeffrey Yasskin; :issue:`4715`.)
* Converting an integer or long integer to a decimal string was made
faster by special-casing base 10 instead of using a generalized
conversion function that supports arbitrary bases.
(Patch by Gawain Bolton; :issue:`6713`.)
* The :meth:`rindex`, :meth:`rpartition`, and :meth:`rsplit` methods
of string objects now uses a fast reverse-search algorithm instead of
a character-by-character scan. This is often faster by a factor of 10.
(Added by Florent Xicluna; :issue:`7462`.)
* The :mod:`pickle` and :mod:`cPickle` modules now automatically
intern the strings used for attribute names, reducing memory usage
of the objects resulting from unpickling. (Contributed by Jake
McGuire; :issue:`5084`.)
* The :mod:`cPickle` module now special-cases dictionaries,
nearly halving the time required to pickle them.
(Contributed by Collin Winter; :issue:`5670`.)
.. ======================================================================
New and Improved Modules
========================
As in every release, 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 Subversion logs for all the details.
* The :mod:`bdb` module's base debugging class :class:`Bdb`
gained a feature for skipping modules. The constructor
now takes an iterable containing glob-style patterns such as
``django.*``; the debugger will not step into stack frames
from a module that matches one of these patterns.
(Contributed by Maru Newby after a suggestion by
Senthil Kumaran; :issue:`5142`.)
* The :mod:`bz2` module's :class:`BZ2File` now supports the context
management protocol, so you can write ``with bz2.BZ2File(...) as f: ...``.
(Contributed by Hagen Fuerstenau; :issue:`3860`.)
* New class: the :class:`Counter` class in the :mod:`collections` module is
useful for tallying data. :class:`Counter` instances behave mostly
like dictionaries but return zero for missing keys instead of
raising a :exc:`KeyError`:
.. doctest::
:options: +NORMALIZE_WHITESPACE
>>> from collections import Counter
>>> c = Counter()
>>> for letter in 'here is a sample of english text':
... c[letter] += 1
...
>>> c
Counter({' ': 6, 'e': 5, 's': 3, 'a': 2, 'i': 2, 'h': 2,
'l': 2, 't': 2, 'g': 1, 'f': 1, 'm': 1, 'o': 1, 'n': 1,
'p': 1, 'r': 1, 'x': 1})
>>> c['e']
5
>>> c['z']
0
There are two additional :class:`Counter` methods: :meth:`most_common`
returns the N most common elements and their counts, and :meth:`elements`
returns an iterator over the contained element, repeating each element
as many times as its count::
>>> c.most_common(5)
[(' ', 6), ('e', 5), ('s', 3), ('a', 2), ('i', 2)]
>>> c.elements() ->
'a', 'a', ' ', ' ', ' ', ' ', ' ', ' ',
'e', 'e', 'e', 'e', 'e', 'g', 'f', 'i', 'i',
'h', 'h', 'm', 'l', 'l', 'o', 'n', 'p', 's',
's', 's', 'r', 't', 't', 'x'
Contributed by Raymond Hettinger; :issue:`1696199`.
The new `OrderedDict` class is described in the earlier section
:ref:`pep-0372`.
The :class:`namedtuple` class now has an optional *rename* parameter.
If *rename* is true, field names that are invalid because they've
been repeated or that aren't legal Python identifiers will be
renamed to legal names that are derived from the field's
position within the list of fields:
>>> from collections import namedtuple
>>> T = namedtuple('T', ['field1', '$illegal', 'for', 'field2'], rename=True)
>>> T._fields
('field1', '_1', '_2', 'field2')
(Added by Raymond Hettinger; :issue:`1818`.)
The :class:`deque` data type now exposes its maximum length as the
read-only :attr:`maxlen` attribute, and has a
:meth:`reverse` method that reverses the elements of the deque in-place.
(Added by Raymond Hettinger.)
* The :mod:`copy` module's :func:`deepcopy` function will now
correctly copy bound instance methods. (Implemented by
Robert Collins; :issue:`1515`.)
* The :mod:`ctypes` module now always converts ``None`` to a C NULL
pointer for arguments declared as pointers. (Changed by Thomas
Heller; :issue:`4606`.)
* New method: the :mod:`datetime` module's :class:`timedelta` class
gained a :meth:`total_seconds` method that returns the number of seconds
in the duration. (Contributed by Brian Quinlan; :issue:`5788`.)
* New method: the :class:`Decimal` class gained a
:meth:`from_float` class method that performs an exact conversion
of a floating-point number to a :class:`Decimal`.
Note that this is an **exact** conversion that strives for the
closest decimal approximation to the floating-point representation's value;
the resulting decimal value will therefore still include the inaccuracy,
if any.
For example, ``Decimal.from_float(0.1)`` returns
``Decimal('0.1000000000000000055511151231257827021181583404541015625')``.
(Implemented by Raymond Hettinger; :issue:`4796`.)
The constructor for :class:`Decimal` now accepts non-European
Unicode characters, such as Arabic-Indic digits. (Contributed by
Mark Dickinson; :issue:`6595`.)
When using :class:`Decimal` instances with a string's
:meth:`format` method, the default alignment was previously
left-alignment. This has been changed to right-alignment, which seems
more sensible for numeric types. (Changed by Mark Dickinson; :issue:`6857`.)
* Distutils is being more actively developed, thanks to Tarek Ziade
who has taken over maintenance of the package. A new
:file:`setup.py` subcommand, ``check``, will
check that the arguments being passed to the :func:`setup` function
are complete and correct (:issue:`5732`).
:func:`distutils.sdist.add_defaults` now uses
*package_dir* and *data_files* to create the MANIFEST file.
:mod:`distutils.sysconfig` now reads the :envvar:`AR` and
:envvar:`ARFLAGS` environment variables.
.. ARFLAGS done in #5941
It is no longer mandatory to store clear-text passwords in the
:file:`.pypirc` file when registering and uploading packages to PyPI. As long
as the username is present in that file, the :mod:`distutils` package will
prompt for the password if not present. (Added by Tarek Ziade,
based on an initial contribution by Nathan Van Gheem; :issue:`4394`.)
A Distutils setup can now specify that a C extension is optional by
setting the *optional* option setting to true. If this optional is
supplied, failure to build the extension will not abort the build
process, but instead simply not install the failing extension.
(Contributed by Georg Brandl; :issue:`5583`.)
The :class:`distutils.dist.DistributionMetadata` class'
:meth:`read_pkg_file` method will read the contents of a package's
:file:`PKG-INFO` metadata file. For an example of its use, see
:ref:`reading-metadata`.
(Contributed by Tarek Ziade; :issue:`7457`.)
:file:`setup.py` files will now accept a :option:`--no-user-cfg` switch
to skip reading the :file:`~/.pydistutils.cfg` file. (Suggested by
by Michael Hoffman, and implemented by Paul Winkler; :issue:`1180`.)
* The :class:`Fraction` class now accepts two rational numbers
as arguments to its constructor.
(Implemented by Mark Dickinson; :issue:`5812`.)
* The :mod:`ftplib` module gained the ability to establish secure FTP
connections using TLS encapsulation of authentication as well as
subsequent control and data transfers. This is provided by the new
:class:`ftplib.FTP_TLS` class.
(Contributed by Giampaolo Rodola', :issue:`2054`.) The :meth:`storbinary`
method for binary uploads can now restart uploads thanks to an added
*rest* parameter (patch by Pablo Mouzo; :issue:`6845`.)
* New function: the :mod:`gc` module's :func:`is_tracked` returns
true if a given instance is tracked by the garbage collector, false
otherwise. (Contributed by Antoine Pitrou; :issue:`4688`.)
* The :mod:`gzip` module's :class:`GzipFile` now supports the context
management protocol, so you can write ``with gzip.GzipFile(...) as f: ...``.
(Contributed by Hagen Fuerstenau; :issue:`3860`.)
It's now possible to override the modification time
recorded in a gzipped file by providing an optional timestamp to
the constructor. (Contributed by Jacques Frechet; :issue:`4272`.)
* The default :class:`HTTPResponse` class used by the :mod:`httplib` module now
supports buffering, resulting in much faster reading of HTTP responses.
(Contributed by Kristjan Valur Jonsson; :issue:`4879`.)
* The :mod:`imaplib` module now supports IPv6 addresses.
(Contributed by Derek Morr; :issue:`1655`.)
* The :mod:`io` library has been upgraded to the version shipped with
Python 3.1. For 3.1, the I/O library was entirely rewritten in C
and is 2 to 20 times faster depending on the task at hand. The
original Python version was renamed to the :mod:`_pyio` module.
One minor resulting change: the :class:`io.TextIOBase` class now
has an :attr:`errors` attribute giving the error setting
used for encoding and decoding errors (one of ``'strict'``, ``'replace'``,
``'ignore'``).
The :class:`io.FileIO` class now raises an :exc:`OSError` when passed
an invalid file descriptor. (Implemented by Benjamin Peterson;
:issue:`4991`.)
* New function: ``itertools.compress(data, selectors)`` takes two
iterators. Elements of *data* are returned if the corresponding
value in *selectors* is true::
itertools.compress('ABCDEF', [1,0,1,0,1,1]) =>
A, C, E, F
New function: ``itertools.combinations_with_replacement(iter, r)``
returns all the possible *r*-length combinations of elements from the
iterable *iter*. Unlike :func:`combinations`, individual elements
can be repeated in the generated combinations::
itertools.combinations_with_replacement('abc', 2) =>
('a', 'a'), ('a', 'b'), ('a', 'c'),
('b', 'b'), ('b', 'c'), ('c', 'c')
Note that elements are treated as unique depending on their position
in the input, not their actual values.
The :class:`itertools.count` function now has a *step* argument that
allows incrementing by values other than 1. :func:`count` also
now allows keyword arguments, and using non-integer values such as
floats or :class:`Decimal` instances. (Implemented by Raymond
Hettinger; :issue:`5032`.)
:func:`itertools.combinations` and :func:`itertools.product` were
previously raising :exc:`ValueError` for values of *r* larger than
the input iterable. This was deemed a specification error, so they
now return an empty iterator. (Fixed by Raymond Hettinger; :issue:`4816`.)
* The :mod:`json` module was upgraded to version 2.0.9 of the
simplejson package, which includes a C extension that makes
encoding and decoding faster.
(Contributed by Bob Ippolito; :issue:`4136`.)
To support the new :class:`OrderedDict` type, :func:`json.load`
now has an optional *object_pairs_hook* parameter that will be called
with any object literal that decodes to a list of pairs.
(Contributed by Raymond Hettinger; :issue:`5381`.)
* New functions: the :mod:`math` module gained
:func:`erf` and :func:`erfc` for the error function and the complementary error function,
:func:`expm1` which computes ``e**x - 1`` with more precision than
using :func:`exp` and subtracting 1,
:func:`gamma` for the Gamma function, and
:func:`lgamma` for the natural log of the Gamma function.
(Contributed by Mark Dickinson and nirinA raseliarison; :issue:`3366`.)
* The :mod:`multiprocessing` module's :class:`Manager*` classes
can now be passed a callable that will be called whenever
a subprocess is started, along with a set of arguments that will be
passed to the callable.
(Contributed by lekma; :issue:`5585`.)
* The :mod:`nntplib` module now supports IPv6 addresses.
(Contributed by Derek Morr; :issue:`1664`.)
* New functions: the :mod:`os` module wraps the following POSIX system
calls: :func:`getresgid` and :func:`getresuid`, which return the
real, effective, and saved GIDs and UIDs;
:func:`setresgid` and :func:`setresuid`, which set
real, effective, and saved GIDs and UIDs to new values;
:func:`initgroups`. (GID/UID functions
contributed by Travis H.; :issue:`6508`. Support for initgroups added
by Jean-Paul Calderone; :issue:`7333`.)
* The :mod:`pydoc` module now has help for the various symbols that Python
uses. You can now do ``help('<<')`` or ``help('@')``, for example.
(Contributed by David Laban; :issue:`4739`.)
* The :mod:`re` module's :func:`split`, :func:`sub`, and :func:`subn`
now accept an optional *flags* argument, for consistency with the
other functions in the module. (Added by Gregory P. Smith.)
* The :mod:`shutil` module's :func:`copyfile` and :func:`copytree`
functions now raises a :exc:`SpecialFileError` exception when
asked to copy a named pipe. Previously the code would treat
named pipes like a regular file by opening them for reading, and
this would block indefinitely. (Fixed by Antoine Pitrou; :issue:`3002`.)
* New functions: in the :mod:`site` module, three new functions
return various site- and user-specific paths.
:func:`getsitepackages` returns a list containing all
global site-packages directories, and
:func:`getusersitepackages` returns the path of the user's
site-packages directory.
:func:`getuserbase` returns the value of the :envvar:`USER_BASE`
environment variable, giving the path to a directory that can be used
to store data.
(Contributed by Tarek Ziade; :issue:`6693`.)
* The :mod:`socket` module's :class:`SSL` objects now support the
buffer API, which fixed a test suite failure. (Fixed by Antoine Pitrou;
:issue:`7133`.)
* The :mod:`SocketServer` module's :class:`TCPServer` class now
has a :attr:`disable_nagle_algorithm` class attribute.
The default value is False; if overridden to be True,
new request connections will have the TCP_NODELAY option set to
prevent buffering many small sends into a single TCP packet.
(Contributed by Kristjan Valur Jonsson; :issue:`6192`.)
* The :mod:`struct` module will no longer silently ignore overflow
errors when a value is too large for a particular integer format
code (one of ``bBhHiIlLqQ``); it now always raises a
:exc:`struct.error` exception. (Changed by Mark Dickinson;
:issue:`1523`.)
* New function: the :mod:`subprocess` module's
:func:`check_output` runs a command with a specified set of arguments
and returns the command's output as a string when the command runs without
error, or raises a :exc:`CalledProcessError` exception otherwise.
::
>>> subprocess.check_output(['df', '-h', '.'])
'Filesystem Size Used Avail Capacity Mounted on\n
/dev/disk0s2 52G 49G 3.0G 94% /\n'
>>> subprocess.check_output(['df', '-h', '/bogus'])
...
subprocess.CalledProcessError: Command '['df', '-h', '/bogus']' returned non-zero exit status 1
(Contributed by Gregory P. Smith.)
* New function: :func:`is_declared_global` in the :mod:`symtable` module
returns true for variables that are explicitly declared to be global,
false for ones that are implicitly global.
(Contributed by Jeremy Hylton.)
* The ``sys.version_info`` value is now a named tuple, with attributes
named ``major``, ``minor``, ``micro``, ``releaselevel``, and ``serial``.
(Contributed by Ross Light; :issue:`4285`.)
* The :mod:`tarfile` module now supports filtering the :class:`TarInfo`
objects being added to a tar file. When you call :meth:`TarFile.add`,
instance, you may supply an optional *filter* argument
that's a callable. The *filter* callable will be passed the
:class:`TarInfo` for every file being added, and can modify and return it.
If the callable returns ``None``, the file will be excluded from the
resulting archive. This is more powerful than the existing
*exclude* argument, which has therefore been deprecated.
(Added by Lars Gustaebel; :issue:`6856`.)
* The :mod:`threading` module's :meth:`Event.wait` method now returns
the internal flag on exit. This means the method will usually
return true because :meth:`wait` is supposed to block until the
internal flag becomes true. The return value will only be false if
a timeout was provided and the operation timed out.
(Contributed by Tim Lesher; :issue:`1674032`.)
* The :func:`is_zipfile` function in the :mod:`zipfile` module now
accepts a file object, in addition to the path names accepted in earlier
versions. (Contributed by Gabriel Genellina; :issue:`4756`.)
:mod:`zipfile` now supports archiving empty directories and
extracts them correctly. (Fixed by Kuba Wieczorek; :issue:`4710`.)
.. ======================================================================
.. whole new modules get described in subsections here
Unit Testing Enhancements
---------------------------------
The :mod:`unittest` module was enhanced in several ways.
The progress messages now shows 'x' for expected failures
and 'u' for unexpected successes when run in verbose mode.
(Contributed by Benjamin Peterson.)
Test cases can raise the :exc:`SkipTest` exception to skip a test.
(:issue:`1034053`.)
.. XXX describe test discovery (Contributed by Michael Foord; :issue:`6001`.)
The error messages for :meth:`assertEqual`,
:meth:`assertTrue`, and :meth:`assertFalse`
failures now provide more information. If you set the
:attr:`longMessage` attribute of your :class:`TestCase` classes to
true, both the standard error message and any additional message you
provide will be printed for failures. (Added by Michael Foord; :issue:`5663`.)
The :meth:`assertRaises` and :meth:`failUnlessRaises` methods now
return a context handler when called without providing a callable
object to run. For example, you can write this::
with self.assertRaises(KeyError):
raise ValueError
(Implemented by Antoine Pitrou; :issue:`4444`.)
The methods :meth:`addCleanup` and :meth:`doCleanups` were added.
:meth:`addCleanup` allows you to add cleanup functions that
will be called unconditionally (after :meth:`setUp` if
:meth:`setUp` fails, otherwise after :meth:`tearDown`). This allows
for much simpler resource allocation and deallocation during tests.
:issue:`5679`
A number of new methods were added that provide more specialized
tests. Many of these methods were written by Google engineers
for use in their test suites; Gregory P. Smith, Michael Foord, and
GvR worked on merging them into Python's version of :mod:`unittest`.
* :meth:`assertIsNone` and :meth:`assertIsNotNone` take one
expression and verify that the result is or is not ``None``.
* :meth:`assertIs` and :meth:`assertIsNot` take two values and check
whether the two values evaluate to the same object or not.
(Added by Michael Foord; :issue:`2578`.)
* :meth:`assertIsInstance` and :meth:`assertNotIsInstance` check whether
the resulting object is an instance of a particular class, or of
one of a tuple of classes. (Added by Georg Brandl; :issue:`7031`.)
* :meth:`assertGreater`, :meth:`assertGreaterEqual`,
:meth:`assertLess`, and :meth:`assertLessEqual` compare
two quantities.
* :meth:`assertMultiLineEqual` compares two strings, and if they're
not equal, displays a helpful comparison that highlights the
differences in the two strings.
* :meth:`assertRegexpMatches` checks whether its first argument is a
string matching a regular expression provided as its second argument.
* :meth:`assertRaisesRegexp` checks whether a particular exception
is raised, and then also checks that the string representation of
the exception matches the provided regular expression.
* :meth:`assertIn` and :meth:`assertNotIn` tests whether
*first* is or is not in *second*.
* :meth:`assertSameElements` tests whether two provided sequences
contain the same elements.
* :meth:`assertSetEqual` compares whether two sets are equal, and
only reports the differences between the sets in case of error.
* Similarly, :meth:`assertListEqual` and :meth:`assertTupleEqual`
compare the specified types and explain the differences.
More generally, :meth:`assertSequenceEqual` compares two sequences
and can optionally check whether both sequences are of a
particular type.
* :meth:`assertDictEqual` compares two dictionaries and reports the
differences. :meth:`assertDictContainsSubset` checks whether
all of the key/value pairs in *first* are found in *second*.
* :meth:`assertAlmostEqual` and :meth:`assertNotAlmostEqual` short-circuit
(automatically pass or fail without checking decimal places) if the objects
are equal.
* :meth:`loadTestsFromName` properly honors the ``suiteClass`` attribute of
the :class:`TestLoader`. (Fixed by Mark Roddy; :issue:`6866`.)
* A new hook, :meth:`addTypeEqualityFunc` takes a type object and a
function. The :meth:`assertEqual` method will use the function
when both of the objects being compared are of the specified type.
This function should compare the two objects and raise an
exception if they don't match; it's a good idea for the function
to provide additional information about why the two objects are
matching, much as the new sequence comparison methods do.
:func:`unittest.main` now takes an optional ``exit`` argument.
If False ``main`` doesn't call :func:`sys.exit` allowing it to
be used from the interactive interpreter. :issue:`3379`.
:class:`TestResult` has new :meth:`startTestRun` and
:meth:`stopTestRun` methods; called immediately before
and after a test run. :issue:`5728` by Robert Collins.
With all these changes, the :file:`unittest.py` was becoming awkwardly
large, so the module was turned into a package and the code split into
several files (by Benjamin Peterson). This doesn't affect how the
module is imported.
.. _importlib-section:
importlib: Importing Modules
------------------------------
Python 3.1 includes the :mod:`importlib` package, a re-implementation
of the logic underlying Python's :keyword:`import` statement.
:mod:`importlib` is useful for implementors of Python interpreters and
to users who wish to write new importers that can participate in the
import process. Python 2.7 doesn't contain the complete
:mod:`importlib` package, but instead has a tiny subset that contains
a single function, :func:`import_module`.
``import_module(name, package=None)`` imports a module. *name* is
a string containing the module or package's name. It's possible to do
relative imports by providing a string that begins with a ``.``
character, such as ``..utils.errors``. For relative imports, the
*package* argument must be provided and is the name of the package that
will be used as the anchor for
the relative import. :func:`import_module` both inserts the imported
module into ``sys.modules`` and returns the module object.
Here are some examples::
>>> from importlib import import_module
>>> anydbm = import_module('anydbm') # Standard absolute import
>>> anydbm
<module 'anydbm' from '/p/python/Lib/anydbm.py'>
>>> # Relative import
>>> sysconfig = import_module('..sysconfig', 'distutils.command')
>>> sysconfig
<module 'distutils.sysconfig' from '/p/python/Lib/distutils/sysconfig.pyc'>
:mod:`importlib` was implemented by Brett Cannon and introduced in
Python 3.1.
ttk: Themed Widgets for Tk
--------------------------
Tcl/Tk 8.5 includes a set of themed widgets that re-implement basic Tk
widgets but have a more customizable appearance and can therefore more
closely resemble the native platform's widgets. This widget
set was originally called Tile, but was renamed to Ttk (for "themed Tk")
on being added to Tcl/Tck release 8.5.
XXX write a brief discussion and an example here.
The :mod:`ttk` module was written by Guilherme Polo and added in
:issue:`2983`. An alternate version called ``Tile.py``, written by
Martin Franklin and maintained by Kevin Walzer, was proposed for
inclusion in :issue:`2618`, but the authors argued that Guilherme
Polo's work was more comprehensive.
Deprecations and Removals
=========================
* :func:`contextlib.nested`, which allows handling more than one context manager
with one :keyword:`with` statement, has been deprecated; :keyword:`with`
supports multiple context managers syntactically now.
.. ======================================================================
Build and C API Changes
=======================
Changes to Python's build process and to the C API include:
* If you use the :file:`.gdbinit` file provided with Python,
the "pyo" macro in the 2.7 version now works correctly when the thread being
debugged doesn't hold the GIL; the macro now acquires it before printing.
(Contributed by Victor Stinner; :issue:`3632`.)
* :cfunc:`Py_AddPendingCall` is now thread-safe, letting any
worker thread submit notifications to the main Python thread. This
is particularly useful for asynchronous IO operations.
(Contributed by Kristjan Valur Jonsson; :issue:`4293`.)
* New function: :cfunc:`PyCode_NewEmpty` creates an empty code object;
only the filename, function name, and first line number are required.
This is useful to extension modules that are attempting to
construct a more useful traceback stack. Previously such
extensions needed to call :cfunc:`PyCode_New`, which had many
more arguments. (Added by Jeffrey Yasskin.)
* New function: :cfunc:`PyErr_NewExceptionWithDoc` creates a new
exception class, just as the existing :cfunc:`PyErr_NewException` does,
but takes an extra ``char *`` argument containing the docstring for the
new exception class. (Added by the 'lekma' user on the Python bug tracker;
:issue:`7033`.)
* New function: :cfunc:`PyFrame_GetLineNumber` takes a frame object
and returns the line number that the frame is currently executing.
Previously code would need to get the index of the bytecode
instruction currently executing, and then look up the line number
corresponding to that address. (Added by Jeffrey Yasskin.)
* New function: :cfunc:`PyLong_AsLongAndOverflow` approximates a Python long
integer as a C :ctype:`long`. If the number is too large to fit into
a :ctype:`long`, an *overflow* flag is set and returned to the caller.
(Contributed by Case Van Horsen; :issue:`7528`.)
* New function: stemming from the rewrite of string-to-float conversion,
a new :cfunc:`PyOS_string_to_double` function was added. The old
:cfunc:`PyOS_ascii_strtod` and :cfunc:`PyOS_ascii_atof` functions
are now deprecated.
* New macros: the Python header files now define the following macros:
:cmacro:`Py_ISALNUM`,
:cmacro:`Py_ISALPHA`,
:cmacro:`Py_ISDIGIT`,
:cmacro:`Py_ISLOWER`,
:cmacro:`Py_ISSPACE`,
:cmacro:`Py_ISUPPER`,
:cmacro:`Py_ISXDIGIT`,
and :cmacro:`Py_TOLOWER`, :cmacro:`Py_TOUPPER`.
All of these functions are analogous to the C
standard macros for classifying characters, but ignore the current
locale setting, because in
several places Python needs to analyze characters in a
locale-independent way. (Added by Eric Smith;
:issue:`5793`.)
.. XXX these macros don't seem to be described in the c-api docs.
* New format codes: the :cfunc:`PyFormat_FromString`,
:cfunc:`PyFormat_FromStringV`, and :cfunc:`PyErr_Format` now
accepts ``%lld`` and ``%llu`` format codes for displaying values of
C's :ctype:`long long` types.
(Contributed by Mark Dickinson; :issue:`7228`.)
* The complicated interaction between threads and process forking has
been changed. Previously, the child process created by
:func:`os.fork` might fail because the child is created with only a
single thread running, the thread performing the :func:`os.fork`.
If other threads were holding a lock, such as Python's import lock,
when the fork was performed, the lock would still be marked as
"held" in the new process. But in the child process nothing would
ever release the lock, since the other threads weren't replicated,
and the child process would no longer be able to perform imports.
Python 2.7 now acquires the import lock before performing an
:func:`os.fork`, and will also clean up any locks created using the
:mod:`threading` module. C extension modules that have internal
locks, or that call :cfunc:`fork()` themselves, will not benefit
from this clean-up.
(Fixed by Thomas Wouters; :issue:`1590864`.)
* The :cfunc:`Py_Finalize` function now calls the internal
:func:`threading._shutdown` function; this prevents some exceptions from
being raised when an interpreter shuts down.
(Patch by Adam Olsen; :issue:`1722344`.)
* Global symbols defined by the :mod:`ctypes` module are now prefixed
with ``Py``, or with ``_ctypes``. (Implemented by Thomas
Heller; :issue:`3102`.)
* New configure option: the :option:`--with-system-expat` switch allows
building the :mod:`pyexpat` module to use the system Expat library.
(Contributed by Arfrever Frehtes Taifersar Arahesis; :issue:`7609`.)
* New configure option: Compiling Python with the
:option:`--with-valgrind` option will now disable the pymalloc
allocator, which is difficult for the Valgrind to analyze correctly.
Valgrind will therefore be better at detecting memory leaks and
overruns. (Contributed by James Henstridge; :issue:`2422`.)
* New configure option: you can now supply no arguments to
:option:`--with-dbmliborder=` in order to build none of the various
DBM modules. (Added by Arfrever Frehtes Taifersar Arahesis;
:issue:`6491`.)
* The :program:`configure` script now checks for floating-point rounding bugs
on certain 32-bit Intel chips and defines a :cmacro:`X87_DOUBLE_ROUNDING`
preprocessor definition. No code currently uses this definition,
but it's available if anyone wishes to use it.
(Added by Mark Dickinson; :issue:`2937`.)
* The build process now creates the necessary files for pkg-config
support. (Contributed by Clinton Roy; :issue:`3585`.)
* The build process now supports Subversion 1.7. (Contributed by
Arfrever Frehtes Taifersar Arahesis; :issue:`6094`.)
.. ======================================================================
Port-Specific Changes: Windows
-----------------------------------
* The :mod:`msvcrt` module now contains some constants from
the :file:`crtassem.h` header file:
:data:`CRT_ASSEMBLY_VERSION`,
:data:`VC_ASSEMBLY_PUBLICKEYTOKEN`,
and :data:`LIBRARIES_ASSEMBLY_NAME_PREFIX`.
(Contributed by David Cournapeau; :issue:`4365`.)
* The new :cfunc:`_beginthreadex` API is used to start threads, and
the native thread-local storage functions are now used.
(Contributed by Kristjan Valur Jonsson; :issue:`3582`.)
* The :func:`os.listdir` function now correctly fails
for an empty path. (Fixed by Hirokazu Yamamoto; :issue:`5913`.)
* The :mod:`mimelib` module will now read the MIME database from
the Windows registry when initializing.
(Patch by Gabriel Genellina; :issue:`4969`.)
.. ======================================================================
Port-Specific Changes: Mac OS X
-----------------------------------
* The path ``/Library/Python/2.7/site-packages`` is now appended to
``sys.path``, in order to share added packages between the system
installation and a user-installed copy of the same version.
(Changed by Ronald Oussoren; :issue:`4865`.)
Other Changes and Fixes
=======================
* When importing a module from a :file:`.pyc` or :file:`.pyo` file
with an existing :file:`.py` counterpart, the :attr:`co_filename`
attributes of the resulting code objects are overwritten when the
original filename is obsolete. This can happen if the file has been
renamed, moved, or is accessed through different paths. (Patch by
Ziga Seilnacht and Jean-Paul Calderone; :issue:`1180193`.)
* The :file:`regrtest.py` script now takes a :option:`--randseed=`
switch that takes an integer that will be used as the random seed
for the :option:`-r` option that executes tests in random order.
The :option:`-r` option also reports the seed that was used
(Added by Collin Winter.)
* Another :file:`regrtest.py` switch is :option:`-j`, which
takes an integer specifying how many tests run in parallel. This
allows reducing the total runtime on multi-core machines.
This option is compatible with several other options, including the
:option:`-R` switch which is known to produce long runtimes.
(Added by Antoine Pitrou, :issue:`6152`.) This can also be used
with a new :option:`-F` switch that runs selected tests in a loop
until they fail. (Added by Antoine Pitrou; :issue:`7312`.)
.. ======================================================================
Porting to Python 2.7
=====================
This section lists previously described changes and other bugfixes
that may require changes to your code:
* When using :class:`Decimal` instances with a string's
:meth:`format` method, the default alignment was previously
left-alignment. This has been changed to right-alignment, which might
change the output of your programs.
(Changed by Mark Dickinson; :issue:`6857`.)
Another :meth:`format`-related change: the default precision used
for floating-point and complex numbers was changed from 6 decimal
places to 12, which matches the precision used by :func:`str`.
(Changed by Eric Smith; :issue:`5920`.)
* Because of an optimization for the :keyword:`with` statement, the special
methods :meth:`__enter__` and :meth:`__exit__` must belong to the object's
type, and cannot be directly attached to the object's instance. This
affects new-style classes (derived from :class:`object`) and C extension
types. (:issue:`6101`.)
* The :meth:`readline` method of :class:`StringIO` objects now does
nothing when a negative length is requested, as other file-like
objects do. (:issue:`7348`).
For C extensions:
* C extensions that use integer format codes with the ``PyArg_Parse*``
family of functions will now raise a :exc:`TypeError` exception
instead of triggering a :exc:`DeprecationWarning` (:issue:`5080`).
* Use the new :cfunc:`PyOS_string_to_double` function instead of the old
:cfunc:`PyOS_ascii_strtod` and :cfunc:`PyOS_ascii_atof` functions,
which are now deprecated.
.. ======================================================================
.. _acks27:
Acknowledgements
================
The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: Ryan Lovett, Hugh Secker-Walker.
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