Doc/whatsnew/3.0.rst

****************************
  What's New In Python 3.0  
****************************

.. XXX Add trademark info for Apple, Microsoft.

.. XXX Remove duplicates; just put info in the most relevant section.

:Author: Guido van Rossum
:Release: |release|
:Date: |today|

.. $Id$
   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.  (Note: I didn't get to this for 3.0.
   GvR.)
   
   * 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.  (Due to time
   constraints I haven't managed to do this for 3.0.  GvR.)
   
   * 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.  (Again, I didn't get to this for 3.0.
   GvR.)

This article explains the new features in Python 3.0, compared to 2.6.
Python 3.0, also known as "Python 3000" or "Py3K", is the first ever
*intentionally incompatible* release.  There are more changes than in
a typical release, and more that are important for all Python users.
Nevertheless, after digesting the changes, you'll find that Python
really hasn't changed all that much -- by and large, we're merely
fixing well-known annoyances and warts.

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 lists those few changes that are most likely to trip you
up if you're used to Python 2.5.

Print Is A Function
-------------------

The :keyword:`print` statement has been replaced with a :func:`print`
function, with keyword arguments to replace most of the special syntax
of the old :keyword:`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!

Note:

* The :func:`print` function doesn't support the "softspace" feature of
  the old :keyword:`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
  :keyword:`print` statements are automatically converted to
  :func:`print` function calls, so this is mostly a non-issue for
  larger projects.

Text Vs. Data Instead Of Unicode Vs. 8-bit
------------------------------------------

Everything you thought you knew about binary data and Unicode has
changed:

XXX HIRO

* Python 3.0 uses the concepts of *text* and (binary) *data* instead
  of Unicode strings and 8-bit strings.  All text is Unicode; however
  *encoded* Unicode is represented as binary data.  The type used to
  hold text is :class:`str`, the type used to hold data is
  :class:`bytes`.  The difference is that any attempt to mix text and
  data in Python 3.0 raises a TypeError exception, whereas if you were
  to mix Unicode and 8-bit strings in Python 2.x, you would only get
  an exception if the 8-bit string contained non-ASCII values.  As a
  consequence, pretty much all code that uses Unicode, encodings or
  binary data most likely 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.

* You no longer use ``u"..."`` literals for Unicode text.  However,
  you must use ``b"..."`` literals for binary data.

* Files opened as text files (still the default mode for :func:`open`)
  always use an encoding to map between strings (in memory) and bytes
  (on disk).  Binary files (opened with a ``b`` in the mode argument)
  always use bytes in memory.  This means that if a file is opened
  using an incorrect mode or encoding, I/O will likely fail.  It also
  means that even Unix users will have to specify the correct mode
  (text or binary) when opening a file.  There is a platform-dependent
  default encoding, which on Unixy platforms can be set with the
  ``LANG`` environment variable (and sometimes also with some other
  platform-specific locale-related environment variables).  In many
  cases, but not all, the system default is UTF-8; you should never
  count on this default.  Any application reading or writing more than
  pure ASCII text should probably have a way to override the encoding.

* The builtin :class:`basestring` abstract type was removed.  Use
  :class:`str` instead.  The :class:`str` and :class:`bytes` types
  don't have functionality enough in common to warrant a shared base
  class.

* Filenames are passed to and returned from APIs as (Unicode) strings.
  This can present platform-specific problems because on some
  platforms filenames are arbitrary byte strings.  (On the other hand
  on Windows, filenames are natively stored as Unicode.)  As a
  work-around, most APIs (e.g. :func:`open` and many functions in the
  :mod:`os` module) that take filenames accept :class:`bytes` objects
  as well as strings, and a few APIs have a way to ask for a
  :class:`bytes` return value: :func:`os.listdir` returns a
  :class:`bytes` instance if the argument is a :class:`bytes`
  instance, and :func:`os.getcwdu` returns the current working
  directory as a :class:`bytes` instance.

* Some system APIs like :data:`os.environ` and :data:`sys.argv` can
  also present problems when the bytes made available by the system is
  not interpretable using the default encoding.  Setting the ``LANG``
  variable and rerunning the program is probably the best approach.

* All backslashes in raw strings are interpreted literally.  This
  means that ``'\U'`` and ``'\u'`` escapes in raw strings are not
  treated specially.

XXX Deal with dupes below

* There is only one text 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 (see below) replaces every occurrence 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
  it is decoded).  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.

* Like :class:`str`, the :class:`bytes` type is immutable.  There is a
  separate *mutable* type to hold buffered binary data,
  :class:`bytearray`.  Nearly all APIs that accept :class:`bytes` also
  accept :class:`bytearray`.  The mutable API is based on
  :class:`collections.MutableSequence`.

* :pep:`3138`: The :func:`repr` of a string no longer escapes
  non-ASCII characters.  It still escapes control characters and code
  points with non-printable status in the Unicode standard, however.

* :pep:`3120`: The default source encoding is now UTF-8.

* :pep:`3131`: Non-ASCII letters are now allowed in 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 largely
  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`, for text and
  data respectively.

* See also the :ref:`unicode-howto`, which was updated for Python 3.0.

Views And Iterators Instead Of Lists
-------------------------------------

Some well-known APIs no longer return lists:

* :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 (this works in Python 2.5 too, and is just
  as efficient).

* Also, the :meth:`dict.iterkeys`, :meth:`dict.iteritems` and
  :meth:`dict.itervalues` methods are no longer supported.

* :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.

* :func:`range` now behaves like :func:`xrange` used to behave, except
  it works with values of arbitrary size.  The latter no longer
  exists.

* :func:`zip` now returns an iterator.

Ordering Comparisons
--------------------

Python 3.0 has simplified the rules for ordering comparisons:

* The ordering comparison operators (``<``, ``<=``, ``>=``, ``>``)
  raise a TypeError exception when the operands don't have a
  meaningful natural ordering.  Thus, expressions like ``1 < ''``, ``0
  > None`` or ``len <= len`` are no longer valid.  A corollary is that
  sorting a heterogeneous list no longer makes sense -- all the
  elements must be comparable to each other.  Note that this does not
  apply to the ``==`` and ``!=`` operators: objects of different
  uncomparable types always compare unequal to each other, and an
  object always compares equal to itself (i.e., ``x is y`` implies ``x
  = y``; this is true even for ``NaN``).

* :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".

* The :func:`cmp` function is gone, and the :meth:`__cmp__` special
  method is no longer supported.  Use :meth:`__lt__` for sorting,
  :meth:`__eq__` with :meth:`__hash__`, and other rich comparisons as
  needed.  (If you really need the :func:`cmp` functionality, you
  could use the expression ``(a > b) - (a < b)`` as the equivalent for
  ``cmp(a, b)``.)

Integers
--------

* :pep:`0237`: Essentially, :class:`long` renamed to :class:`int`.
  That is, there is only one built-in integral type, named
  :class:`int`; but it behaves mostly like the old :class:`long` type.

* :pep:`0238`: An expression like ``1/2`` returns a float.  Use
  ``1//2`` to get the truncating behavior.  (The latter syntax has
  existed for years, at least since Python 2.2.)

* The :data:`sys.maxint` constant was removed, since there is no
  longer a limit to the value of ints.  However, :data:`sys.maxsize`
  can be used as an integer larger than any practical list or string
  index.  It conforms to the implementation's "natural" integer size
  and is typically the same as :data:`sys.maxint` in previous releases
  on the same platform (assuming the same build options).

* 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.  (Use :func:`str` instead.)

* Octal literals are no longer of the form ``0720``; use ``0o720``
  instead.


Overview Of Syntax Changes
==========================

This section gives a brief overview of every *syntactic* change in
Python 3.0.

New Syntax
----------

* :pep:`3107`: Function argument and return value annotations.  This
  provides a standardized way of annotating a function's parameters
  and return value.  There are no semantics attached to such
  annotations except that they can be introspected at runtime using
  the :attr:`__annotations__` attribute.  The intent is to encourage
  experimentation through metaclasses, decorators or frameworks.

* :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.

* Keyword arguments are allowed after the list of base classes in a
  class definition.  This is used by the new convention for specifying
  a metaclass (see :pep:`3115`), but can be used for other purposes as
  well, as long as the metaclass supports it.

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

* :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 (possibly empty) list; the
  right-hand side may be any iterable.  Example::

    (a, *rest, b) = range(5)

  This sets *a* to ``0``, *b* to ``4``, and \*rest to ``[1, 2, 3]``.

* Dictionary comprehensions: ``{k: v for k, v in stuff}`` means the
  same thing as ``dict(stuff)`` but is more flexible.  (This is
  :pep:`0274` vindicated. :-)

* Set literals, e.g. ``{1, 2}``.  Note that ``{}`` is an empty
  dictionary; use ``set()`` for an empty set.  Set comprehensions are
  also supported; e.g., ``{x for x in stuff}`` means the same thing as
  ``set(stuff)`` but is more flexible.

* New octal literals, e.g. ``0o720`` (already in 2.6).  The old octal
  literals (``0720``) are gone.

* New binary literals, e.g. ``0b1010`` (already in 2.6).

* Bytes literals are introduced with a leading ``b`` or ``B``.

Changed Syntax
--------------

* New :keyword:`raise` statement syntax: ``raise [expr [from expr]]``.
  Also note that string exceptions are no longer legal (:pep:`0352`).

* :keyword:`as` and :keyword:`with` are now reserved words.  (Since
  2.6, actually.)

* :keyword:`True`, :keyword:`False`, and :keyword:`None` are reserved
  words.  (2.6 partially enforced the restrictions on :keyword:`None`
  already.)

* Change from :keyword:`except` *exc*, *var* to
  :keyword:`except` *exc* :keyword:`as` *var*.  See :pep:`3110`.

* List comprehensions no longer support the syntactic form
  ``[... for var in item1, item2, ...]``.  Use
  ``[... for var in (item1, item2, ...)]`` instead.
  Also note that list comprehensions have different semantics: they
  are closer to syntactic sugar for a generator expression inside a
  :func:`list` constructor, and in particular the loop control
  variables are no longer leaked into the surrounding scope.

* The *ellipsis* (``...``) can be used as an atomic expression
  anywhere.  (Previously it was only allowed in slices.)  Also, it
  *must* now be spelled as ``...``.  (Previously it could also be
  spelled as ``. . .``, by a mere accident of the grammar.)

Removed Syntax
--------------

* :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.

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

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

* Removed keyword: :func:`exec` is no longer a keyword; it remains as
  a function.  (Fortunately the function syntax was also accepted in
  2.x.)  Also note that :func:`exec` no longer takes a stream argument;
  instead of ``exec(f)`` you can use ``exec(f.read())``.

* Integer literals no longer support a trailing ``l`` or ``L``.

* String literals no longer support a leading ``u`` or ``U``.

* The :keyword:`from` *module* :keyword:`import` ``*`` syntax is only
  allowed at the module level, no longer inside functions.

* The only acceptable syntax for relative imports is :keyword:`from`
  ``.``[*module*] :keyword:`import` *name*; :keyword:`import` forms
  not starting with ``.`` are always interpreted as absolute imports.
  (:pep:`0328`)


Changes Already Present In Python 2.6
=====================================

Since many users presumably make the jump straight from Python 2.5 to
Python 3.0, this section reminds the reader of new features that were
originally designed for Python 3.0 but that were back-ported to Python
2.6.  The corresponding sections in :ref:`whats-new-in-2.6` should be
consulted for longer descriptions.

* :ref:`pep-0343`.  The :keyword:`with` statement is now a standard
  feature and no longer needs to be imported from the ``__future__``.
  Also check out :ref:`new-26-context-managers` and
  :ref:`new-module-contextlib`.

* :ref:`pep-0366`.  This enhances the usefulness of the :option:`-m`
  option when the referenced module lives in a package.

* :ref:`pep-0370`.

* :ref:`pep-0371`.

* :ref:`pep-3101`.  Note: the 2.6 description mentions the
  :meth:`format` method for both 8-bit and Unicode strings.  In 3.0,
  only the :class:`str` type (text strings with Unicode support)
  supports this method; the :class:`bytes` type does not.  The plan is
  to eventually make this the only API for string formatting, and to
  start deprecating the ``%`` operator in Python 3.1.

* :ref:`pep-3105`.  This is now a standard feature and no longer needs
  to be imported from :mod:`__future__`.  More details were given above.

* :ref:`pep-3110`.  The :keyword:`except` *exc* :keyword:`as` *var*
  syntax is now standard and :keyword:`except` *exc*, *var* is no
  longer supported.  (Of course, the :keyword:`as` *var* part is still
  optional.)

* :ref:`pep-3112`.  The ``b"..."`` string literal notation (and its
  variants like ``b'...'``, ``b"""..."""``, and ``br"..."``) now
  produces a literal of type :class:`bytes`.

* :ref:`pep-3116`.  The :mod:`io` module is now the standard way of
  doing file I/O, and the initial values of :data:`sys.stdin`,
  :data:`sys.stdout` and :data:`sys.stderr` are now instances of
  :class:`io.TextIOBase`.  The builtin :func:`open` function is now an
  alias for :func:`io.open` and has additional keyword arguments
  *encoding*, *errors*, *newline* and *closefd*.  Also note that an
  invalid *mode* argument now raises :exc:`ValueError`, not
  :exc:`IOError`.  The binary file object underlying a text file
  object can be accessed as :attr:`f.buffer` (but beware that the
  text object maintains a buffer of itself in order to speed up
  the encoding and decoding operations).

* :ref:`pep-3118`.  The old builtin :func:`buffer` is now really gone;
  the new builtin :func:`memoryview` provides (mostly) similar
  functionality.

* :ref:`pep-3119`.  The :mod:`abc` module and the ABCs defined in the
  :mod:`collections` module plays a somewhat more prominent role in
  the language now, and builtin collection types like :class:`dict`
  and :class:`list` conform to the :class:`collections.MutableMapping`
  and :class:`collections.MutableSequence` ABC, respectively.

* :ref:`pep-3127`.  As mentioned above, the new octal literal
  notation is the only one supported, and binary literals have been
  added.

* :ref:`pep-3129`.

* :ref:`pep-3141`.  The :mod:`numbers` module is another new use of
  ABCs, defining Python's "numeric tower".  Also note the new
  :mod:`fractions` module which implements :class:`numbers.Rational`.


Library Changes
===============

Due to time constraints, this document does not exhaustively cover the
very extensive changes to the standard library.  :pep:`3108` is the
reference for the major changes to the library.  Here's a capsule
review:

* Many old modules were removed.  Some, like :mod:`gopherlib` (no
  longer used) and :mod:`md5` (replaced by :mod:`hashlib`), were
  already deprecated by :pep:`0004`.  Others were removed as a result
  of the removal of support for various platforms such as Irix, BeOS
  and Mac OS 9 (see :pep:`0011`).  Some modules were also selected for
  removal in Python 3.0 due to lack of use or because a better
  replacement exists.  See :pep:`3108` for an exhaustive list.

* The :mod:`bsddb3` package was removed because its presence in the
  core standard library has proved over time to be a particular burden
  for the core developers due to testing instability and Berlekey DB's
  release schedule.  However, the package is alive and well,
  externally maintained at http://www.jcea.es/programacion/pybsddb.htm.

* Some modules were renamed because their old name flaunted
  :pep:`0008`, or for various other reasons:

  =======================  =======================
  Old Name                 New Name
  =======================  =======================
  _winreg                  winreg
  ConfigParser             configparser
  copy_reg                 copyreg
  Queue                    queue
  SocketServer             socketserver
  markupbase               _markupbase
  repr                     reprlib
  test.test_support        test.support
  =======================  =======================

* A common pattern in Python 2.x is to have one version of a module
  implemented in pure Python, with an optional accelerated version
  implemented as a C extension; for example, :mod:`pickle` and
  :mod:`cPickle`.  This places the burden of importing the accelerated
  version and falling back on the pure Python version on each user of
  these modules.  In Python 3.0, the accelerated versions are
  considered implementation details of the pure Python versions.
  Users should always import the standard version, which attempts to
  import the accelerated version and falls back to the pure Python
  version.  The :mod:`pickle` module received this treatment.  The
  :mod:`profile` module is on the list for 3.1.  The :mod:`StringIO`
  module has been turned into a class in the :mod:`io` module.

* Some related modules have been grouped into packages, and usually
  the submodule names have been simplified.  The resulting new
  packages are:

  * :mod:`dbm` (:mod:`anydbm`, :mod:`dbhash`, :mod:`dbm`,
    :mod:`dumbdbm`, :mod:`gdbm`, :mod:`whichdb`).

  * :mod:`html` (:mod:`HTMLParser`, :mod:`htmlentitydefs`).

  * :mod:`http` (:mod:`httplib`, :mod:`BaseHTTPServer`,
    :mod:`CGIHTTPServer`, :mod:`SimpleHTTPServer`, :mod:`Cookie`,
    :mod:`cookielib`).

  * :mod:`tkinter` (all :mod:`Tkinter`-related modules except
    :mod:`turtle`).  The target audience of :mod:`turtle` doesn't
    really care about :mod:`tkinter`.  Also note that as of Python
    2.6, the functionality of :mod:`turtle` has been greatly enhanced.

  * :mod:`urllib` (:mod:`urllib`, :mod:`urllib`2, :mod:`urlparse`,
    :mod:`robotparse`).

  * :mod:`xmlrpc` (:mod:`xmlrpclib`, :mod:`DocXMLRPCServer`,
    :mod:`SimpleXMLRPCServer`).

Some other library changes (not covered by :pep:`3108`):

* Killed :mod:`sets`.  Use the builtin :func:`set` function.

* Cleanup of the :mod:`sys` module: removed :func:`sys.exitfunc`,
  :func:`sys.exc_clear`, :data:`sys.exc_type`, :data:`sys.exc_value`,
  :data:`sys.exc_traceback`.  (Note that :data:`sys.last_type`
  etc. remain.)

* Cleanup of the :class:`array.array` type: the :meth:`read` and
  :meth:`write` methods are gone; use :meth:`fromfile` and
  :meth:`tofile` instead.

* Cleanup of the :mod:`operator` module: removed
  :func:`sequenceIncludes` and :func:`isCallable`.

* Cleanup of the :mod:`thread` module: :func:`acquire_lock` and
  :func:`release_lock` are gone; use :func:`acquire` and
  :func:`release` instead.

* Cleanup of the :mod:`random` module: removed the :func:`jumpahead` API.


:pep:`3101`: A New Approach To String Formatting
================================================

* A new system for  built-in string formatting operations replaces the
  ``%`` string  formatting operator.   (However, the ``%``  operator is
  still supported;  it will  be deprecated in  Python 3.1  and removed
  from the language at some later time.)  Read :pep:`3101` for the full
  scoop.


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

.. XXX expand this (but note that the "pitfalls" section currently has
.. XXX more detail :-)

* 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; these are often
  referred to as *dictionary views*.


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

* :pep:`0352`: All exceptions must be derived (directly or indirectly)
  from :exc:`BaseException`.  This is the root of the exception
  hierarchy.  Most exceptions should actually be derived from
  :exc:`Exception`.  This is not a new recommendation, but the
  *requirement* to inherit from :exc:`BaseException` is new.  (Python
  2.6 still allowed classic classes to be raised, and placed no
  restriction on what you can catch.)

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

* 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.  You must now use ``except SomeException as
  identifier:`` instead of ``except Exception, identifier:``

* :pep:`3134`: Exception chaining.  XXX

* 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
=============================

XXX Move to new syntax section???

* Classic classes are gone.

* :pep:`3115`: New Metaclass Syntax.  Instead of::

    class C:
        __metaclass__ = M
        ...

  you now use::

    class C(metaclass=M):
        ...

  The module-global :data:`__metaclass__` variable is no longer supported.
  (It was a crutch to make it easier to default to new-style classes
  without deriving every class from :class:`object`.)


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

* Moved :func:`intern` to :func:`sys.intern`.

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

* The concept of "unbound methods" was removed from the language.
  When referencing a method as a class attribute, you now get a plain
  function object.

* :meth:`__getslice__`, :meth:`__setslice__` and :meth:`__delslice__`
  were killed.  The syntax ``a[i:j]`` now translates to
  ``a.__getitem__(slice(i, j))`` (or :meth:`__setitem__` or
  :meth:`__delitem__`, when used as an assignment or deletion target,
  respectively).

* :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())``.

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

* :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.  (The reason for the
  removal is that :data:string.letters` and friends had
  locale-specific behavior, which is a bad idea for such
  attractively-named global "constants".)

* Removed: :func:`apply`.  Instead of ``apply(f, args)`` use
  ``f(*args)``.

* Removed :func:`callable`.  Instead of ``callable(f)`` you can use
  ``hasattr(f, '__call__')``.  The :func:`operator.isCallable` function
  is also gone.

* Removed :func:`coerce`.  This function no longer serves a purpose
  now that classic classes are gone.

* Removed :func:`execfile`.  Instead of ``execfile(fn)`` use
  ``exec(open(fn).read())``.

* Removed :class:`file`.  Use :func:`open`.

* Removed :func:`reduce`.  Use :func:`functools.reduce` if you really
  need it; however, 99 percent of the time an explicit :keyword:`for`
  loop is more readable.

* Removed :func:`reload`.  Use :func:`imp.reload`.  

* Removed. :meth:`dict.has_key` -- use the :keyword:`in` operator
  instead.

* 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.

* Removed support 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__`.

* Renamed module :mod:`__builtin__` to :mod:`builtins` (removing the
  underscores, adding an 's').  The :data:`__builtins__` variable
  found in most global namespaces is unchanged.  To modify a builtin,
  you should use :mod:`builtins`, not :data:`__builtins__`!

* Renamed function attributes :attr:`func_whatever` to
  :attr:`__whatever__`.  XXX list every single one.

* Removed :exc:`StandardError`.

* Removed METH_OLDARGS and WITH_CYCLE_GC.  XXX more.

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


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

The net result of the 3.0 generalizations is that Python 3.0 runs the
pystone benchmark around 10% slower than Python 2.5.  Most likely the
biggest cause is the removal of special-casing for small integers.
There's room for improvement, but it will happen after 3.0 is
released!

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


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:`bsddb` module is gone.  It is being maintained externally
  with its own release schedule better mirroring that of BerkeleyDB.
  See http://www.jcea.es/programacion/pybsddb.htm.

* 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.

* The :mod:`tokenize` module has been changed to work with bytes.  The main
  entry point is now :func:`tokenize.tokenize`, instead of generate_tokens.

.. ======================================================================
.. 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. XXX

* :pep:`3121`: Extension Module Initialization & Finalization. XXX

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

* 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
---------------------

XXX Platform-specific changes go here.

* XXX BeOS, RISCOS, Irix, Tru64 support

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


Porting To Python 3.0
=====================

For porting existing Python 2.5 or 2.6 source code to Python 3.0, the
best strategy is the following:

0. (Prerequisite:) Start with excellent test coverage.

1. Port to Python 2.6.  This should be no more work than the average
   port from Python 2.x to Python 2.(x+1).  Make sure all your tests
   pass.

2. (Still using 2.6:) Turn on the :option:`-3` command line switch.
   This enables warnings about features that will be removed (or
   change) in 3.0.  Run your test suite again, and fix code that you
   get warnings about until there are no warnings left, and all your
   tests still pass.

3. Run the ``2to3`` source-to-source translator over your source code
   tree.  (See :ref:`2to3-reference` for more on this tool.)  Run the
   result of the translation under Python 3.0.  Manually fix up any
   remaining issues, fixing problems until all tests pass again.

It is not recommended to try to write source code that runs unchanged
under both Python 2.6 and 3.0; you'd have to use a very contorted
coding style, e.g. avoiding :keyword:`print` statements, metaclasses,
and much more.  If you are maintaining a library that needs to support
both Python 2.6 and Python 3.0, the best approach is to modify step 3
above by editing the 2.6 version of the source code and running the
``2to3`` translator again, rather than editing the 3.0 version of the
source code.

For porting C extensions to Python 3.0, please see :ref:`cporting-howto`.

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