**************************** 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. .. OrderedDict .. $Id$ 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. Python 3.1 ================ Much as Python 2.6 incorporated features from Python 3.0, version 2.7 is influenced by features from 3.1. XXX mention importlib; anything else? 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. .. ======================================================================== .. Large, PEP-level features and changes should be described here. .. ======================================================================== PEP 372: Adding an ordered dictionary to collections ==================================================== XXX write this Several modules will now use :class:`OrderedDict` by default. The :mod:`ConfigParser` module uses :class:`OrderedDict` for the list of sections and the options within a section. The :meth:`namedtuple._asdict` method returns an :class:`OrderedDict` as well. 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`.) .. ====================================================================== 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. * 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. 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`.) * 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`.) * 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`.) .. ====================================================================== 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. * 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`. * In Distutils, :func:`distutils.sdist.add_defaults` now uses *package_dir* and *data_files* to create the MANIFEST file. 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, with the initial contribution by Nathan Van Gheem; :issue:`4394`.) * 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`.) * 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`.) * 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 :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`.) * 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 :mod:`unittest` module was enhanced in several ways. Test cases can raise the :exc:`SkipTest` exception to skip a test. (:issue:`1034053`.) It will now use 'x' for expected failures and 'u' for unexpected successes when run in its verbose mode. (Contributed by Benjamin Peterson.) * 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`.) :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 importlib: Importing Modules ------------------------------ XXX write this 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`.) * 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`.) .. ====================================================================== 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 ----------------------------------- * The ``/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 all code objects if the original filename is obsolete, which 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 now reports the seed that was used (Added by Collin Winter.) .. ====================================================================== 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.