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# What’s New in Python 2.7

Author: | A.M. Kuchling (amk at amk.ca)
Release: | 3.1c2
Date: | June 13, 2009

This article explains the new features in Python 2.7. No release schedule has been decided yet for 2.7.

## 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 -3 switch now automatically enables the -Qwarn switch that causes warnings about using classic division with integers and long integers.

## PEP 372: Adding an ordered dictionary to collections

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.

## Other Language Changes

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.)

### Optimizations

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.)

## 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 `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`.

  - `assertIsNone()` and `assertIsNotNone()` take one expression and verify that the result is or is not `None`.
  - `assertIs()` and `assertIsNot()` take two values and check whether the two values evaluate to the same object or not. (Added by Michael Foord; issue 2578.)
  - `assertGreater()`, `assertGreaterEqual()`, `assertLess()`, and `assertLessEqual()` compare two quantities.
  - `assertMultiLineEqual()` compares two strings, and if they’re not equal, displays a helpful comparison that highlights the differences in the two strings.
  - `assertRegexpMatches()` checks whether its first argument is a string matching a regular expression provided as its second argument.
  - `assertRaisesRegexp()` checks whether a particular exception is raised, and then also checks that the string representation of the exception matches the provided regular expression.
  - `assertIn()` and `assertNotIn()` tests whether first is or is not in second.
  - `assertSameElements()` tests whether two provided sequences contain the same elements.
  - `assertSetEqual()` compares whether two sets are equal, and only reports the differences between the sets in case of error.
  - Similarly, `assertListEqual()` and `assertTupleEqual()` compare the specified types and explain the differences. More generally, `assertSequenceEqual()` compares two sequences and can optionally check whether both sequences are of a particular type.
  - `assertDictEqual()` compares two dictionaries and reports the differences. `assertDictContainsSubset()` checks whether all of the key/value pairs in first are found in second.
  - A new hook, `addTypeEqualityFunc()` takes a type object and a function. The `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.

  `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.)

### importlib: Importing Modules

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.

### 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 `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 `.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.)
- `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.)
- Global symbols defined by the `ctypes` module are now prefixed with `Py`, or with ``_ctypes`. (Implemented by Thomas Heller; issue 3102.)
- The configure script now checks for floating-point rounding bugs on certain 32-bit Intel chips and defines a `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 `msvcrt` module now contains some constants from the `crtassem.h` header file: `CRT_ASSEMBLY_VERSION`, `VC_ASSEMBLY_PUBLICKEYTOKEN`, and `LIBRARIES_ASSEMBLY_NAME_PREFIX`. (Contributed by David Cournapeau; issue 4365.)
- The new `_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 `.pyc` or `.pyo` file with an existing `.py` counterpart, the `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 `regrtest.py` script now takes a --randseed= switch that takes an integer that will be used as the random seed for the -r option that executes tests in random order. The -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:

- Because of an optimization for the `with` statement, the special methods `__enter__()` and `__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 `object`) and C extension types. (issue 6101.)

## 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.

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