cpython/Doc/whatsnew/2.7.rst

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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.
.. $Id$
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.
No release schedule has been decided yet for 2.7.
.. 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?
.. ========================================================================
Other Language Changes
======================
Some smaller changes made to the core Python language are:
* 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`.)
* 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
--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`.)
.. ======================================================================
Optimizations
-------------
A few performance enhancements have been added:
* The garbage collector now performs better when many objects are
being allocated without deallocating any. A full garbage collection
pass is only performed 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. The second condition was added to reduce the number
of full garbage collections as the number of objects on the heap grows,
avoiding quadratic performance when allocating very many objects.
(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. As of now, this is 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 bring
down the individual cost of each garbage collection, since it decreases the
number of objects to be considered and traversed by the collector.
To help diagnosing this optimization, a new function in the :mod:`gc`
module, :func:`is_tracked`, returns True if a given instance is tracked
by the garbage collector, False otherwise.
(Contributed by Antoine Pitrou; :issue:`4688`.)
.. ======================================================================
New, Improved, and Deprecated 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.
* In Distutils, distutils.sdist.add_defaults now uses package_dir and data_files
to feed MANIFEST.
* It is not mandatory anymore 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, with the initial
contribution of Nathan Van Gheem; :issue:`4394`.)
* 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`.)
* A new :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`::
>>> 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 :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`.)
* The :class:`io.FileIO` class now raises an :exc:`OSError` when passed
an invalid file descriptor. (Implemented by Benjamin Peterson;
:issue:`4991`.)
* 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`.)
* A new function in the :mod:`subprocess` module,
:func:`check_output`, runs a command with a specified set of arguments
and returns the command's output as a string if 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.)
* The :func:`is_zipfile` function in the :mod:`zipfile` module will now
accept a file object, in addition to the path names accepted in earlier
versions. (Contributed by Gabriel Genellina; :issue:`4756`.)
.. ======================================================================
.. whole new modules get described in subsections here
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.
.. ======================================================================
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 will now work when the thread being
debugged doesn't hold the GIL; the macro will now acquire 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`.)
.. ======================================================================
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`.)
.. ======================================================================
Port-Specific Changes: Mac OS X
-----------------------------------
.. ======================================================================
Porting to Python 2.7
=====================
This section lists previously described changes and other bugfixes
that may require changes to your code:
To be written.
.. ======================================================================
.. _acks27:
Acknowledgements
================
The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: no one yet.