cpython/Doc/whatsnew/3.0.rst

****************************
  What's New in Python 3.0  
****************************

:Author: Guido van Rossum
:Release: 0.1

.. 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 as a comment:
   
   % Patch 12345
   XXX Describe the transmogrify() function added to the socket
   module.
   (Contributed by P.Y. Developer.)
   
   This saves the maintainer the effort of going through the SVN log
   when researching a change.

This article explains the new features in Python 3.0, comparing to 2.6
(or in some cases 2.5, since 2.6 isn't released yet).

The best estimate for a release date is August 2008.

This article doesn't attempt to provide a complete specification of
the new features, but instead provides a convenient overview.  For
full details, you should refer to the documentation for Python 3.0. If
you want to understand the complete implementation and design
rationale, refer to the PEP for a particular new feature.

.. 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?
.. sets module deprecated
.. ======================================================================


Common Stumbling Blocks
=======================

This section briefly lists the changes that are more likely to trip
people up, without necessarily raising obvious errors.  These are all
explained in more detail below.  (I'm not listing syntactic changes
and removed or renamed features here, since those tend to produce hard
and fast errors; it's the subtle behavioral changes in code that
remains syntactically valid that trips people up.  I'm also omitting
changes to rarely used features.)

* The ``print`` statement has been replaced with a :func:`print` function,
  with keyword arguments to replace most of the special syntax of the
  old ``print`` statement (PEP 3105).  Examples::

    Old: print "The answer is", 2*2
    New: print("The answer is", 2*2)

    Old: print x,           # Trailing comma suppresses newline
    New: print(x, end=" ")  # Appends a space instead of a newline

    Old: print              # Prints a newline
    New: print()            # You must call the function!

    Old: print >>sys.stderr, "fatal error"
    New: print("fatal error", file=sys.stderr)

    Old: print (x, y)       # prints repr((x, y))
    New: print((x, y))      # Not the same as print(x, y)!

  You can also customize the separator between items, e.g.::

    print("There are <", 2**32, "> possibilities!", sep="")

  which produces::

   There are <4294967296> possibilities!

  Notes about the :func:`print` function:

  * The :func:`print` function doesn't support the "softspace" feature of
    the old ``print`` statement.  For example, in Python 2.x,
    ``print "A\n", "B"`` would write ``"A\nB\n"``; but in Python 3.0,
    ``print("A\n", "B")`` writes ``"A\n B\n"``.

  * Initially, you'll be finding yourself typing the old ``print x``
    a lot in interactive mode.  Time to retrain your fingers to type
    ``print(x)`` instead!

  * When using the ``2to3`` source-to-source conversion tool, all
    ``print`` statements are autmatically converted to :func:`print`
    function calls, so this is mostly a non-issue for larger projects.

* Python 3.0 uses strings and bytes instead of the Unicode strings and
  8-bit strings.  This means that pretty much all code that uses
  Unicode, encodings or binary data in any way has to change.  The
  change is for the better, as in the 2.x world there were numerous
  bugs having to do with mixing encoded and unencoded text.

* Text files enforce an encoding; binary files use bytes.  This means
  that if a file is opened using an incorrect mode or encoding, I/O
  will likely fail.

* :func:`map` and :func:`filter` return iterators.  A quick fix is e.g.
  ``list(map(...))``, but a better fix is often to use a list
  comprehension (especially when the original code uses :keyword:`lambda`).
  Particularly tricky is :func:`map` invoked for the side effects of the
  function; the correct transformation is to use a for-loop.

* :class:`dict` methods :meth:`dict.keys`, :meth:`dict.items` and
  :meth:`dict.values` return views instead of lists.  For example, this no
  longer works: ``k = d.keys(); k.sort()``.  Use ``k = sorted(d)`` instead.

* :meth:`builtin.sorted` and :meth:`list.sort` no longer accept the *cmp*
  argument providing a comparison function.  Use the *key* argument
  instead. N.B. the *key* and *reverse* arguments are now "keyword-only".

* ``1/2`` returns a float.  Use ``1//2`` to get the truncating behavior.

* The :func:`repr` of a long integer doesn't include the trailing ``L``
  anymore, so code that unconditionally strips that character will
  chop off the last digit instead.


Strings and Bytes
=================

* There is only one string type; its name is :class:`str` but its behavior and
  implementation are like :class:`unicode` in 2.x.

* The :class:`basestring` superclass has been removed. The ``2to3`` tool
  replaces every occurence of :class:`basestring` with :class:`str`.

* PEP 3137: There is a new type, :class:`bytes`, to represent binary data (and
  encoded text, which is treated as binary data until you decide to decode it).
  The :class:`str` and :class:`bytes` types cannot be mixed; you must always
  explicitly convert between them, using the :meth:`str.encode` (str -> bytes)
  or :meth:`bytes.decode` (bytes -> str) methods.

.. XXX add bytearray

* PEP 3112: Bytes literals, e.g. ``b"abc"``, create :class:`bytes` instances.

* PEP 3120: UTF-8 default source encoding.

* PEP 3131: Non-ASCII identifiers.  (However, the standard library remains
  ASCII-only with the exception of contributor names in comments.)

* PEP 3116: New I/O Implementation.  The API is nearly 100% backwards
  compatible, but completely reimplemented (currently mostly in Python).  Also,
  binary files use bytes instead of strings.

* The :mod:`StringIO` and :mod:`cStringIO` modules are gone.  Instead, import
  :class:`io.StringIO` or :class:`io.BytesIO`.


PEP 3101: A New Approach to String Formatting
=============================================

.. XXX expand this

* A new system for built-in string formatting operations replaces the ``%``
  string formatting operator.


PEP 3106: Revamping dict :meth:`dict.keys`, :meth:`dict.items` and :meth:`dict.values`
======================================================================================

.. XXX expand this

* The :meth:`dict.iterkeys`, :meth:`dict.itervalues` and :meth:`dict.iteritems`
  methods have been removed.

* :meth:`dict.keys`, :meth:`dict.values` and :meth:`dict.items` return objects
  with set behavior that reference the underlying dict.


PEP 3107: Function Annotations
==============================

.. XXX expand this

* A standardized way of annotating a function's parameters and return values.


Exception Stuff
===============

* PEP 352: Exceptions must derive from :exc:`BaseException`.  This is the root
  of the exception hierarchy.

* :exc:`StandardError` was removed (already in 2.6).

* Dropping sequence behavior (slicing!) and :attr:`message` attribute of
  exception instances.

* PEP 3109: Raising exceptions.  You must now use ``raise Exception(args)``
  instead of ``raise Exception, args``.

* PEP 3110: Catching exceptions.

* PEP 3134: Exception chaining.  (The :attr:`__context__` feature from the PEP
  hasn't been implemented yet in 3.0a2.)

* A few exception messages are improved when Windows fails to load an extension
  module.  For example, ``error code 193`` is now ``%1 is not a valid Win32
  application``.  Strings now deal with non-English locales.


New Class and Metaclass Stuff
=============================

* Classic classes are gone.

* PEP 3115: New Metaclass Syntax.

* PEP 3119: Abstract Base Classes (ABCs); ``@abstractmethod`` and
  ``@abstractproperty`` decorators; collection ABCs.

* PEP 3129: Class decorators.

* PEP 3141: Numeric ABCs.


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

Here are most of the changes that Python 3.0 makes to the core Python
language and built-in functions.

* Removed backticks (use :func:`repr` instead).

* Removed ``<>`` (use ``!=`` instead).

* ``!=`` now returns the opposite of ``==``, unless ``==`` returns
  ``NotImplemented``.

* :keyword:`as` and :keyword:`with` are keywords.

* ``True``, ``False``, and ``None`` are keywords.

* PEP 237: :class:`long` renamed to :class:`int`.  That is, there is only one
  built-in integral type, named :class:`int`; but it behaves like the old
  :class:`long` type, with the exception that the literal suffix ``L`` is
  neither supported by the parser nor produced by :func:`repr` anymore.
  :data:`sys.maxint` was also removed since the int type has no maximum value
  anymore.

* PEP 238: int division returns a float.

* The ordering operators behave differently: for example, ``x < y`` where ``x``
  and ``y`` have incompatible types raises :exc:`TypeError` instead of returning
  a pseudo-random boolean.

* :meth:`__getslice__` and friends killed.  The syntax ``a[i:j]`` now translates
  to ``a.__getitem__(slice(i, j))`` (or :meth:`__setitem__` or
  :meth:`__delitem__`, depending on context).

* PEP 3102: Keyword-only arguments.  Named parameters occurring after ``*args``
  in the parameter list *must* be specified using keyword syntax in the call.
  You can also use a bare ``*`` in the parameter list to indicate that you don't
  accept a variable-length argument list, but you do have keyword-only
  arguments.

* PEP 3104: :keyword:`nonlocal` statement.  Using ``nonlocal x`` you can now
  assign directly to a variable in an outer (but non-global) scope.

* PEP 3111: :func:`raw_input` renamed to :func:`input`.  That is, the new
  :func:`input` function reads a line from :data:`sys.stdin` and returns it with
  the trailing newline stripped.  It raises :exc:`EOFError` if the input is
  terminated prematurely.  To get the old behavior of :func:`input`, use
  ``eval(input())``.

* :func:`xrange` renamed to :func:`range`.

* PEP 3113: Tuple parameter unpacking removed.  You can no longer write ``def
  foo(a, (b, c)): ...``.  Use ``def foo(a, b_c): b, c = b_c`` instead.

* PEP 3114: ``.next()`` renamed to :meth:`__next__`, new builtin :func:`next` to
  call the :meth:`__next__` method on an object.

* PEP 3127: New octal literals; binary literals and :func:`bin`.  Instead of
  ``0666``, you write ``0o666``.  The :func:`oct` function is modified
  accordingly.  Also, ``0b1010`` equals 10, and ``bin(10)`` returns
  ``"0b1010"``.  ``0666`` is now a :exc:`SyntaxError`.

* PEP 3132: Extended Iterable Unpacking.  You can now write things like ``a, b,
  *rest = some_sequence``.  And even ``*rest, a = stuff``.  The ``rest`` object
  is always a list; the right-hand side may be any iterable.

* PEP 3135: New :func:`super`.  You can now invoke :func:`super` without
  arguments and the right class and instance will automatically be chosen.  With
  arguments, its behavior is unchanged.

* :func:`zip`, :func:`map` and :func:`filter` return iterators.

* :data:`string.letters` and its friends (:data:`string.lowercase` and
  :data:`string.uppercase`) are gone.  Use :data:`string.ascii_letters`
  etc. instead.

* Removed: :func:`apply`, :func:`callable`, :func:`coerce`, :func:`execfile`,
  :func:`file`, :func:`reduce`, :func:`reload`.

* Removed: :meth:`dict.has_key`.

* :func:`exec` is now a function.

* There is a new free format floating point representation, which is based on
  "Floating-Point Printer Sample Code", by Robert G. Burger.  ``repr(11./5)``
  now returns ``2.2`` instead of ``2.2000000000000002``.

* The :meth:`__oct__` and :meth:`__hex__` special methods are removed --
  :func:`oct` and :func:`hex` use :meth:`__index__` now to convert the argument
  to an integer.

* Support is removed for :attr:`__members__` and :attr:`__methods__`.

* Renamed the boolean conversion C-level slot and method: ``nb_nonzero`` is now
  ``nb_bool`` and :meth:`__nonzero__` is now :meth:`__bool__`.

* Removed :data:`sys.maxint`. Use :data:`sys.maxsize`.


.. ======================================================================


Optimizations
-------------

* Detailed changes are listed here.

The net result of the 3.0 generalizations is that Python 3.0 runs the pystone
benchmark around 33% slower than Python 2.5.  There's room for improvement; we
expect to be optimizing string and integer operations significantly before the
final 3.0 release!

.. ======================================================================


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

As usual, 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:`cPickle` module is gone.  Use :mod:`pickle` instead.  Eventually
  we'll have a transparent accelerator module.

* The :mod:`imageop` module is gone.

* The :mod:`audiodev`, :mod:`Bastion`, :mod:`bsddb185`, :mod:`exceptions`,
  :mod:`linuxaudiodev`, :mod:`md5`, :mod:`MimeWriter`, :mod:`mimify`,
  :mod:`popen2`, :mod:`rexec`, :mod:`sets`, :mod:`sha`, :mod:`stringold`,
  :mod:`strop`, :mod:`sunaudiodev`, :mod:`timing`, and :mod:`xmllib` modules are
  gone.

* The :mod:`new` module is gone.

* The functions :func:`os.tmpnam`, :func:`os.tempnam` and :func:`os.tmpfile`
  have been removed in favor of the :mod:`tempfile` module.

.. ======================================================================
.. whole new modules get described in subsections here

.. ======================================================================


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

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

* PEP 3118: New Buffer API.

* PEP 3121: Extension Module Initialization & Finalization.

* PEP 3123: Making :cmacro:`PyObject_HEAD` conform to standard C.

* No more C API support for restricted execution.

* :cfunc:`PyNumber_Coerce`, :cfunc:`PyNumber_CoerceEx`, :cfunc:`PyMember_Get`,
  and :cfunc:`PyMember_Set` C APIs are removed.

* New C API :cfunc:`PyImport_ImportModuleNoBlock`, works like
  :cfunc:`PyImport_ImportModule` but won't block on the import lock (returning
  an error instead).

.. ======================================================================


Port-Specific Changes
---------------------

Platform-specific changes go here.


.. ======================================================================


.. _section-other:

Other Changes and Fixes
=======================

As usual, there were a bunch of other improvements and bugfixes
scattered throughout the source tree.  A search through the change
logs finds there were XXX patches applied and YYY bugs fixed between
Python 2.6 and 3.0.  Both figures are likely to be underestimates.

Some of the more notable changes are:

* Details go here.

.. ======================================================================


Porting to Python 3.0
=====================

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

* Everything is all in the details!

* Developers can include :file:`intobject.h` after :file:`Python.h` for
  some ``PyInt_`` aliases.

.. ======================================================================


.. _acks:

Acknowledgements
================

The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: Georg Brandl.