cpython/Doc/whatsnew/3.0.rst

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

:Author: A.M. Kuchling, Guido van Rossum

.. |release| replace:: 0.0

.. % $Id: whatsnew26.tex 55506 2007-05-22 07:43:29Z neal.norwitz $
.. % 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.)

* 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.

* Bytes aren't hashable, and don't support certain operations like
  ``b.lower()``, ``b.strip()`` or ``b.split()``.
  For the latter two, use ``b.strip(b" \t\r\n\f")`` or
  ``b.split(b" \t\r\n\f")``.

* ``map()`` and ``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 ``lambda``).
  Particularly tricky is ``map()`` invoked for the side effects of the
  function; the correct transformation is to use a for-loop.

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

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

* Code that unconditionally strips the trailing ``L`` from the ``repr()``
  of a long integer will chop off the last digit instead.

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

* Also, ``print`` and ``print (x, y)`` behave differently without
  warning: the former used to add a newline in 2.x, but does nothing
  in 3.0; the latter used to print the ``repr()`` of a tuple in 2.x,
  but prints the individual values in 3.0.

* You'll be finding yourself typing ``print x`` a lot in interactive
  mode.  Time to retrain your fingers. :-)


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

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

* PEP 358: There is a new type, ``bytes``, to represent binary data
  (and encoded text, which is treated as binary data until you decide
  to decode it).  The ``str`` and ``bytes`` types cannot be mixed; you
  must always explicitly convert between them, using the ``.encode()``
  (str -> bytes) or ``.decode()`` (bytes -> str) methods.  Comparing a
  bytes and a str instance for equality raises a TypeError; this
  catches common mistakes.

* PEP 3112: Bytes literals.  E.g. b"abc".

* 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 ``StringIO`` and ``cStringIO`` modules are gone.  Instead,
  import ``StringIO`` or ``BytesIO`` from the ``io`` module.


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

XXX


PEP 3106: Revamping ``.keys()``, ``.items()`` and ``.values()``
===============================================================

XXX


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

XXX


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

* PEP 352: Exceptions must derive from BaseException.  This is the
  root of the exception hierarchy; only Exception, 

* StandardException was removed (already in 2.6).

* Dropping sequence behavior and ``.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 ``__context__`` feature from the
  PEP hasn't been implemented yet in 3.0a1.)


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

* Classic classes are gone.

* PEP 3115: New Metaclass Syntax.

* PEP 3119: Abstract Base Classes; ``@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 ``repr()`` instead).

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

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

* PEP 237: ``long`` renamed to ``int``.  That is, there is only one
  built-in integral type, named ``int``; but it behaves like the old
  ``long`` type.

* PEP 238: int division returns a float.

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

* ``__getslice__()`` and friends killed.  The syntax ``a[i:j]`` now
  translates to ``a.__getitem__(slice(i, j))`` (or ``__setitem__``
  or ``__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 ``*`` 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: ``nonlocal`` statement.  Using ``nonlocal x`` you can now
  assign directly to a variable in an outer (but non-global) scope.

* PEP 3105: ``print`` is now a function.  Keyword argumemts
  ``file=sys.stdout``, ``sep=" "`` and ``end="\n"`` let you customize
  it.

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

* ``xrange()`` renamed to ``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 ``.__next__()``.

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

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

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

* ``zip()``, ``map()`` and ``filter()`` return iterators.

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

* Removed: apply(), callable(), coerce(), execfile(), file(),
  reduce(), reload().

* Removed: ``dict.has_key()``.

* ``exec`` is now a function.


.. % ======================================================================


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

* Detailed changes are listed here.

The net result of the 3.0 generalizations is that Python 3.0 runs the
pystone benchmark around 25% slower than Python 2.5.  There's room for
improvement!

.. % ======================================================================


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 CVS logs for all the details.

* The ``cPickle`` module is gone.  Use ``pickle`` instead.  Eventually
  we'll have a transparent accelerator 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 PyObject_HEAD conform to standard C.

.. % ======================================================================


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!

.. % ======================================================================


.. _acks:

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

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