| Author: | A.M. Kuchling (amk at amk.ca) |
|---|---|
| Release: | 3.1.2 |
| Date: | March 20, 2010 |
This article explains the new features in Python 2.7. No release schedule has been decided yet for 2.7.
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 -3 switch now automatically enables the -Qwarn switch that causes warnings about using classic division with integers and long integers.
XXX write this
Several modules will now use OrderedDict by default. The ConfigParser module uses OrderedDict for the list of sections and the options within a section. The namedtuple._asdict() method returns an OrderedDict as well.
Some smaller changes made to the core Python language are:
str.format() method now supports automatic numbering of the replacement fields. This makes using 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 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`.)
The int() and 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 will now approximate 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.)
The bytearray type’s translate() method will now accept None as its first argument. (Fixed by Georg Brandl; issue 4759.)
Several 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. 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 --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.)
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.)
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 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 bz2 module’s 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 Counter class in the collections module is useful for tallying data. Counter instances behave mostly like dictionaries but return zero for missing keys instead of raising a 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 Counter methods: most_common() returns the N most common elements and their counts, and 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 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 deque data type now exposes its maximum length as the read-only maxlen attribute. (Added by Raymond Hettinger.)
In Distutils, distutils.sdist.add_defaults() now uses package_dir and data_files to create the MANIFEST file. distutils.sysconfig will now read the AR environment variable.
It is no longer mandatory to store clear-text passwords in the .pypirc file when registering and uploading packages to PyPI. As long as the username is present in that file, the 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.)
New method: the Decimal class gained a from_float() class method that performs an exact conversion of a floating-point number to a 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 Fraction class will now accept two rational numbers as arguments to its constructor. (Implemented by Mark Dickinson; issue 5812.)
New function: the gc module’s is_tracked() returns true if a given instance is tracked by the garbage collector, false otherwise. (Contributed by Antoine Pitrou; issue 4688.)
The gzip module’s 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 io.FileIO class now raises an 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 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 itertools.count function now has a step argument that allows incrementing by values other than 1. count() also now allows keyword arguments, and using non-integer values such as floats or Decimal instances. (Implemented by Raymond Hettinger; issue 5032.)
itertools.combinations() and itertools.product() were previously raising 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 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 OrderedDict type, 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 multiprocessing module’s 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 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 re module’s split(), sub(), and subn() now accept an optional flags argument, for consistency with the other functions in the module. (Added by Gregory P. Smith.)
New function: the subprocess module’s 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 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: is_declared_global() in the 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 threading module’s Event.wait() method now returns the internal flag on exit. This means the method will usually return true because 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 XXX; issue 1674032.)
The unittest module was enhanced in several ways. The progress messages will now show ‘x’ for expected failures and ‘u’ for unexpected successes when run in verbose mode. (Contributed by Benjamin Peterson.) Test cases can raise the SkipTest exception to skip a test. (issue 1034053.)
The error messages for assertEqual(), assertTrue(), and assertFalse() failures now provide more information. If you set the longMessage attribute of your 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 assertRaises() and 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 addCleanup() and doCleanups() were added. addCleanup() allows you to add cleanup functions that will be called unconditionally (after setUp() if setUp() fails, otherwise after 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 unittest.
unittest.main() now takes an optional exit argument. If False main doesn’t call sys.exit() allowing it to be used from the interactive interpreter. issue 3379.
TestResult has new startTestRun() and stopTestRun() methods; called immediately before and after a test run. issue 5728 by Robert Collins.
The is_zipfile() function in the zipfile module will now accept a file object, in addition to the path names accepted in earlier versions. (Contributed by Gabriel Genellina; issue 4756.)
zipfile now supports archiving empty directories and extracts them correctly. (Fixed by Kuba Wieczorek; issue 4710.)
Python 3.1 includes the importlib package, a re-implementation of the logic underlying Python’s import statement. importlib is useful for implementors of Python interpreters and to user who wish to write new importers that can participate in the import process. Python 2.7 doesn’t contain the complete importlib package, but instead has a tiny subset that contains a single function, 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. 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'>
importlib was implemented by Brett Cannon and introduced in Python 3.1.
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 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.
Changes to Python’s build process and to the C API include:
This section lists previously described changes and other bugfixes that may require changes to your code:
The author would like to thank the following people for offering suggestions, corrections and assistance with various drafts of this article: no one yet.