What’s New In Python 3.1
************************

Author:
   Raymond Hettinger

This article explains the new features in Python 3.1, compared to 3.0.
Python 3.1 was released on June 27, 2009.


PEP 372: Ordered Dictionaries
=============================

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
"collections.OrderedDict" class has been introduced.

The 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.  If a new entry overwrites an
existing entry, the original insertion position is left unchanged.
Deleting an entry and reinserting it will move it to the end.

The standard library now supports use of ordered dictionaries in
several modules.  The "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
"collections.namedtuple()" now returns an ordered dictionary with the
values appearing in the same order as the underlying tuple indices.
The "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.

See also:

  **PEP 372** - Ordered Dictionaries
     PEP written by Armin Ronacher and Raymond Hettinger.
     Implementation written by Raymond Hettinger.


PEP 378: Format Specifier for Thousands Separator
=================================================

The built-in "format()" function and the "str.format()" method use a
mini-language that now includes a simple, non-locale aware way to
format a number with a thousands separator.  That provides a way to
humanize a program’s output, improving its professional appearance and
readability:

   >>> format(1234567, ',d')
   '1,234,567'
   >>> format(1234567.89, ',.2f')
   '1,234,567.89'
   >>> format(12345.6 + 8901234.12j, ',f')
   '12,345.600000+8,901,234.120000j'
   >>> format(Decimal('1234567.89'), ',f')
   '1,234,567.89'

The supported types are "int", "float", "complex" and
"decimal.Decimal".

Discussions are underway about how to specify alternative separators
like dots, spaces, apostrophes, or underscores.  Locale-aware
applications should use the existing *n* format specifier which
already has some support for thousands separators.

See also:

  **PEP 378** - Format Specifier for Thousands Separator
     PEP written by Raymond Hettinger and implemented by Eric Smith
     and Mark Dickinson.


Other Language Changes
======================

Some smaller changes made to the core Python language are:

* Directories and zip archives containing a "__main__.py" file can now
  be executed directly by passing their name to the interpreter. The
  directory/zipfile is automatically inserted as the first entry in
  sys.path.  (Suggestion and initial patch by Andy Chu; revised patch
  by Phillip J. Eby and Nick Coghlan; bpo-1739468.)

* The "int()" type 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, Victor Stinner, Raymond
  Hettinger, and Mark Dickinson; bpo-3439.)

* The fields in "format()" strings can now be automatically numbered:

     >>> 'Sir {} of {}'.format('Gallahad', 'Camelot')
     'Sir Gallahad of Camelot'

  Formerly, the string would have required numbered fields such as:
  "'Sir {0} of {1}'".

  (Contributed by Eric Smith; bpo-5237.)

* The "string.maketrans()" function is deprecated and is replaced by
  new static methods, "bytes.maketrans()" and "bytearray.maketrans()".
  This change solves the confusion around which types were supported
  by the "string" module. Now, "str", "bytes", and "bytearray" each
  have their own **maketrans** and **translate** methods with
  intermediate translation tables of the appropriate type.

  (Contributed by Georg Brandl; bpo-5675.)

* The syntax of the "with" statement now allows multiple context
  managers in a single statement:

     >>> with open('mylog.txt') as infile, open('a.out', 'w') as outfile:
     ...     for line in infile:
     ...         if '<critical>' in line:
     ...             outfile.write(line)

  With the new syntax, the "contextlib.nested()" function is no longer
  needed and is now deprecated.

  (Contributed by Georg Brandl and Mattias Brändström; appspot issue
  53094.)

* "round(x, n)" now returns an integer if *x* is an integer.
  Previously it returned a float:

     >>> round(1123, -2)
     1100

  (Contributed by Mark Dickinson; bpo-4707.)

* Python now uses David Gay’s algorithm for finding the shortest
  floating point representation that doesn’t change its value.  This
  should help mitigate some of the confusion surrounding binary
  floating point numbers.

  The significance is easily seen with a number like "1.1" which does
  not have an exact equivalent in binary floating point.  Since there
  is no exact equivalent, an expression like "float('1.1')" evaluates
  to the nearest representable value which is "0x1.199999999999ap+0"
  in hex or "1.100000000000000088817841970012523233890533447265625" in
  decimal. That nearest value was and still is used in subsequent
  floating point calculations.

  What is new is how the number gets displayed.  Formerly, Python used
  a simple approach.  The value of "repr(1.1)" was computed as
  "format(1.1, '.17g')" which evaluated to "'1.1000000000000001'". The
  advantage of using 17 digits was that it relied on IEEE-754
  guarantees to assure that "eval(repr(1.1))" would round-trip exactly
  to its original value.  The disadvantage is that many people found
  the output to be confusing (mistaking intrinsic limitations of
  binary floating point representation as being a problem with Python
  itself).

  The new algorithm for "repr(1.1)" is smarter and returns "'1.1'".
  Effectively, it searches all equivalent string representations (ones
  that get stored with the same underlying float value) and returns
  the shortest representation.

  The new algorithm tends to emit cleaner representations when
  possible, but it does not change the underlying values.  So, it is
  still the case that "1.1 + 2.2 != 3.3" even though the
  representations may suggest otherwise.

  The new algorithm depends on certain features in the underlying
  floating point implementation.  If the required features are not
  found, the old algorithm will continue to be used.  Also, the text
  pickle protocols assure cross-platform portability by using the old
  algorithm.

  (Contributed by Eric Smith and Mark Dickinson; bpo-1580)


New, Improved, and Deprecated Modules
=====================================

* Added a "collections.Counter" class to support convenient counting
  of unique items in a sequence or iterable:

     >>> Counter(['red', 'blue', 'red', 'green', 'blue', 'blue'])
     Counter({'blue': 3, 'red': 2, 'green': 1})

  (Contributed by Raymond Hettinger; bpo-1696199.)

* Added a new module, "tkinter.ttk" for access to the Tk themed widget
  set. The basic idea of ttk is to separate, to the extent possible,
  the code implementing a widget’s behavior from the code implementing
  its appearance.

  (Contributed by Guilherme Polo; bpo-2983.)

* The "gzip.GzipFile" and "bz2.BZ2File" classes now support the
  context management protocol:

     >>> # Automatically close file after writing
     >>> with gzip.GzipFile(filename, "wb") as f:
     ...     f.write(b"xxx")

  (Contributed by Antoine Pitrou.)

* The "decimal" module now supports methods for creating a decimal
  object from a binary "float".  The conversion is exact but can
  sometimes be surprising:

     >>> Decimal.from_float(1.1)
     Decimal('1.100000000000000088817841970012523233890533447265625')

  The long decimal result shows the actual binary fraction being
  stored for *1.1*.  The fraction has many digits because *1.1* cannot
  be exactly represented in binary.

  (Contributed by Raymond Hettinger and Mark Dickinson.)

* The "itertools" module grew two new functions.  The
  "itertools.combinations_with_replacement()" function is one of four
  for generating combinatorics including permutations and Cartesian
  products.  The "itertools.compress()" function mimics its namesake
  from APL.  Also, the existing "itertools.count()" function now has
  an optional *step* argument and can accept any type of counting
  sequence including "fractions.Fraction" and "decimal.Decimal":

     >>> [p+q for p,q in combinations_with_replacement('LOVE', 2)]
     ['LL', 'LO', 'LV', 'LE', 'OO', 'OV', 'OE', 'VV', 'VE', 'EE']

     >>> list(compress(data=range(10), selectors=[0,0,1,1,0,1,0,1,0,0]))
     [2, 3, 5, 7]

     >>> c = count(start=Fraction(1,2), step=Fraction(1,6))
     >>> [next(c), next(c), next(c), next(c)]
     [Fraction(1, 2), Fraction(2, 3), Fraction(5, 6), Fraction(1, 1)]

  (Contributed by Raymond Hettinger.)

* "collections.namedtuple()" now supports a keyword argument *rename*
  which lets invalid fieldnames be automatically converted to
  positional names in the form _0, _1, etc.  This is useful when the
  field names are being created by an external source such as a CSV
  header, SQL field list, or user input:

     >>> query = input()
     SELECT region, dept, count(*) FROM main GROUPBY region, dept

     >>> cursor.execute(query)
     >>> query_fields = [desc[0] for desc in cursor.description]
     >>> UserQuery = namedtuple('UserQuery', query_fields, rename=True)
     >>> pprint.pprint([UserQuery(*row) for row in cursor])
     [UserQuery(region='South', dept='Shipping', _2=185),
      UserQuery(region='North', dept='Accounting', _2=37),
      UserQuery(region='West', dept='Sales', _2=419)]

  (Contributed by Raymond Hettinger; bpo-1818.)

* The "re.sub()", "re.subn()" and "re.split()" functions now accept a
  flags parameter.

  (Contributed by Gregory Smith.)

* The "logging" module now implements a simple "logging.NullHandler"
  class for applications that are not using logging but are calling
  library code that does.  Setting-up a null handler will suppress
  spurious warnings such as “No handlers could be found for logger
  foo”:

     >>> h = logging.NullHandler()
     >>> logging.getLogger("foo").addHandler(h)

  (Contributed by Vinay Sajip; bpo-4384).

* The "runpy" module which supports the "-m" command line switch now
  supports the execution of packages by looking for and executing a
  "__main__" submodule when a package name is supplied.

  (Contributed by Andi Vajda; bpo-4195.)

* The "pdb" module can now access and display source code loaded via
  "zipimport" (or any other conformant **PEP 302** loader).

  (Contributed by Alexander Belopolsky; bpo-4201.)

* "functools.partial" objects can now be pickled.

   (Suggested by Antoine Pitrou and Jesse Noller.  Implemented by Jack
   Diederich; bpo-5228.)

* Add "pydoc" help topics for symbols so that "help('@')" works as
  expected in the interactive environment.

  (Contributed by David Laban; bpo-4739.)

* The "unittest" module now supports skipping individual tests or
  classes of tests. And it supports marking a test as an expected
  failure, a test that is known to be broken, but shouldn’t be counted
  as a failure on a TestResult:

     class TestGizmo(unittest.TestCase):

         @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
         def test_gizmo_on_windows(self):
             ...

         @unittest.expectedFailure
         def test_gimzo_without_required_library(self):
             ...

  Also, tests for exceptions have been builtout to work with context
  managers using the "with" statement:

     def test_division_by_zero(self):
         with self.assertRaises(ZeroDivisionError):
             x / 0

  In addition, several new assertion methods were added including
  "assertSetEqual()", "assertDictEqual()",
  "assertDictContainsSubset()", "assertListEqual()",
  "assertTupleEqual()", "assertSequenceEqual()",
  "assertRaisesRegexp()", "assertIsNone()", and "assertIsNotNone()".

  (Contributed by Benjamin Peterson and Antoine Pitrou.)

* The "io" module has three new constants for the "seek()" method
  "SEEK_SET", "SEEK_CUR", and "SEEK_END".

* The "sys.version_info" tuple is now a named tuple:

     >>> sys.version_info
     sys.version_info(major=3, minor=1, micro=0, releaselevel='alpha', serial=2)

  (Contributed by Ross Light; bpo-4285.)

* The "nntplib" and "imaplib" modules now support IPv6.

  (Contributed by Derek Morr; bpo-1655 and bpo-1664.)

* The "pickle" module has been adapted for better interoperability
  with Python 2.x when used with protocol 2 or lower.  The
  reorganization of the standard library changed the formal reference
  for many objects.  For example, "__builtin__.set" in Python 2 is
  called "builtins.set" in Python 3. This change confounded efforts to
  share data between different versions of Python.  But now when
  protocol 2 or lower is selected, the pickler will automatically use
  the old Python 2 names for both loading and dumping. This remapping
  is turned-on by default but can be disabled with the *fix_imports*
  option:

     >>> s = {1, 2, 3}
     >>> pickle.dumps(s, protocol=0)
     b'c__builtin__\nset\np0\n((lp1\nL1L\naL2L\naL3L\natp2\nRp3\n.'
     >>> pickle.dumps(s, protocol=0, fix_imports=False)
     b'cbuiltins\nset\np0\n((lp1\nL1L\naL2L\naL3L\natp2\nRp3\n.'

  An unfortunate but unavoidable side-effect of this change is that
  protocol 2 pickles produced by Python 3.1 won’t be readable with
  Python 3.0. The latest pickle protocol, protocol 3, should be used
  when migrating data between Python 3.x implementations, as it
  doesn’t attempt to remain compatible with Python 2.x.

  (Contributed by Alexandre Vassalotti and Antoine Pitrou, bpo-6137.)

* A new module, "importlib" was added.  It provides a complete,
  portable, pure Python reference implementation of the "import"
  statement and its counterpart, the "__import__()" function.  It
  represents a substantial step forward in documenting and defining
  the actions that take place during imports.

  (Contributed by Brett Cannon.)


Optimizations
=============

Major performance enhancements have been added:

* The new I/O library (as defined in **PEP 3116**) was mostly written
  in Python and quickly proved to be a problematic bottleneck in
  Python 3.0. In Python 3.1, the I/O library has been entirely
  rewritten in C and is 2 to 20 times faster depending on the task at
  hand. The pure Python version is still available for experimentation
  purposes through the "_pyio" module.

  (Contributed by Amaury Forgeot d’Arc and Antoine Pitrou.)

* Added a heuristic so that tuples and dicts containing only
  untrackable objects are not tracked by the garbage collector. This
  can reduce the size of collections and therefore the garbage
  collection overhead on long-running programs, depending on their
  particular use of datatypes.

  (Contributed by Antoine Pitrou, bpo-4688.)

* Enabling a configure option named "--with-computed-gotos" on
  compilers that support it (notably: gcc, SunPro, icc), the bytecode
  evaluation loop is compiled with a new dispatch mechanism which
  gives speedups of up to 20%, depending on the system, the compiler,
  and the benchmark.

  (Contributed by Antoine Pitrou along with a number of other
  participants, bpo-4753).

* The decoding of UTF-8, UTF-16 and LATIN-1 is now two to four times
  faster.

  (Contributed by Antoine Pitrou and Amaury Forgeot d’Arc, bpo-4868.)

* The "json" module now has a C extension to substantially improve its
  performance.  In addition, the API was modified so that json works
  only with "str", not with "bytes".  That change makes the module
  closely match the JSON specification which is defined in terms of
  Unicode.

  (Contributed by Bob Ippolito and converted to Py3.1 by Antoine
  Pitrou and Benjamin Peterson; bpo-4136.)

* Unpickling now interns the attribute names of pickled objects.  This
  saves memory and allows pickles to be smaller.

  (Contributed by Jake McGuire and Antoine Pitrou; bpo-5084.)


IDLE
====

* IDLE’s format menu now provides an option to strip trailing
  whitespace from a source file.

  (Contributed by Roger D. Serwy; bpo-5150.)


Build and C API Changes
=======================

Changes to Python’s build process and to the C API include:

* 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 "sys.int_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.int_info
     sys.int_info(bits_per_digit=30, sizeof_digit=4)

  (Contributed by Mark Dickinson; bpo-4258.)

* The "PyLong_AsUnsignedLongLong()" function now handles a negative
  *pylong* by raising "OverflowError" instead of "TypeError".

  (Contributed by Mark Dickinson and Lisandro Dalcrin; bpo-5175.)

* Deprecated "PyNumber_Int()".  Use "PyNumber_Long()" instead.

  (Contributed by Mark Dickinson; bpo-4910.)

* Added a new "PyOS_string_to_double()" function to replace the
  deprecated functions "PyOS_ascii_strtod()" and "PyOS_ascii_atof()".

  (Contributed by Mark Dickinson; bpo-5914.)

* Added "PyCapsule" as a replacement for the "PyCObject" API. The
  principal difference is that the new type has a well defined
  interface for passing typing safety information and a less
  complicated signature for calling a destructor.  The old type had a
  problematic API and is now deprecated.

  (Contributed by Larry Hastings; bpo-5630.)


Porting to Python 3.1
=====================

This section lists previously described changes and other bugfixes
that may require changes to your code:

* The new floating point string representations can break existing
  doctests. For example:

     def e():
         '''Compute the base of natural logarithms.

         >>> e()
         2.7182818284590451

         '''
         return sum(1/math.factorial(x) for x in reversed(range(30)))

     doctest.testmod()

     **********************************************************************
     Failed example:
         e()
     Expected:
         2.7182818284590451
     Got:
         2.718281828459045
     **********************************************************************

* The automatic name remapping in the pickle module for protocol 2 or
  lower can make Python 3.1 pickles unreadable in Python 3.0.  One
  solution is to use protocol 3.  Another solution is to set the
  *fix_imports* option to "False". See the discussion above for more
  details.
