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ReStructuredText
483 lines
17 KiB
ReStructuredText
****************************
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What's New in Python 3.0
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****************************
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:Author: Guido van Rossum
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:Release: 0.1
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.. Rules for maintenance:
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* Anyone can add text to this document. Do not spend very much time
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on the wording of your changes, because your text will probably
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get rewritten to some degree.
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* The maintainer will go through Misc/NEWS periodically and add
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changes; it's therefore more important to add your changes to
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Misc/NEWS than to this file.
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* This is not a complete list of every single change; completeness
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is the purpose of Misc/NEWS. Some changes I consider too small
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or esoteric to include. If such a change is added to the text,
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I'll just remove it. (This is another reason you shouldn't spend
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too much time on writing your addition.)
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* If you want to draw your new text to the attention of the
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maintainer, add 'XXX' to the beginning of the paragraph or
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section.
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* It's OK to just add a fragmentary note about a change. For
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example: "XXX Describe the transmogrify() function added to the
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socket module." The maintainer will research the change and
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write the necessary text.
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* You can comment out your additions if you like, but it's not
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necessary (especially when a final release is some months away).
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* Credit the author of a patch or bugfix. Just the name is
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sufficient; the e-mail address isn't necessary.
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* It's helpful to add the bug/patch number as a comment:
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% Patch 12345
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XXX Describe the transmogrify() function added to the socket
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module.
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(Contributed by P.Y. Developer.)
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This saves the maintainer the effort of going through the SVN log
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when researching a change.
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This article explains the new features in Python 3.0, comparing to 2.6
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(or in some cases 2.5, since 2.6 isn't released yet).
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The best estimate for a release date is August 2008.
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This article doesn't attempt to provide a complete specification of
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the new features, but instead provides a convenient overview. For
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full details, you should refer to the documentation for Python 3.0. If
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you want to understand the complete implementation and design
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rationale, refer to the PEP for a particular new feature.
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.. Compare with previous release in 2 - 3 sentences here.
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.. add hyperlink when the documentation becomes available online.
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.. ======================================================================
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.. Large, PEP-level features and changes should be described here.
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.. Should there be a new section here for 3k migration?
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.. Or perhaps a more general section describing module changes/deprecation?
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.. sets module deprecated
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.. ======================================================================
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Common Stumbling Blocks
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=======================
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This section briefly lists the changes that are more likely to trip
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people up, without necessarily raising obvious errors. These are all
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explained in more detail below. (I'm not listing syntactic changes
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and removed or renamed features here, since those tend to produce hard
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and fast errors; it's the subtle behavioral changes in code that
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remains syntactically valid that trips people up. I'm also omitting
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changes to rarely used features.)
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* The ``print`` statement has been replaced with a :func:`print` function,
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with keyword arguments to replace most of the special syntax of the
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old ``print`` statement (PEP 3105). Examples::
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Old: print "The answer is", 2*2
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New: print("The answer is", 2*2)
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Old: print x, # Trailing comma suppresses newline
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New: print(x, end=" ") # Appends a space instead of a newline
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Old: print # Prints a newline
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New: print() # You must call the function!
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Old: print >>sys.stderr, "fatal error"
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New: print("fatal error", file=sys.stderr)
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Old: print (x, y) # prints repr((x, y))
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New: print((x, y)) # Not the same as print(x, y)!
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You can also customize the separator between items, e.g.::
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print("There are <", 2**32, "> possibilities!", sep="")
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which produces::
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There are <4294967296> possibilities!
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Notes about the :func:`print` function:
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* The :func:`print` function doesn't support the "softspace" feature of
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the old ``print`` statement. For example, in Python 2.x,
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``print "A\n", "B"`` would write ``"A\nB\n"``; but in Python 3.0,
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``print("A\n", "B")`` writes ``"A\n B\n"``.
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* Initially, you'll be finding yourself typing the old ``print x``
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a lot in interactive mode. Time to retrain your fingers to type
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``print(x)`` instead!
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* When using the ``2to3`` source-to-source conversion tool, all
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``print`` statements are automatically converted to :func:`print`
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function calls, so this is mostly a non-issue for larger projects.
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* Python 3.0 uses strings and bytes instead of the Unicode strings and
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8-bit strings. This means that pretty much all code that uses
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Unicode, encodings or binary data in any way has to change. The
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change is for the better, as in the 2.x world there were numerous
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bugs having to do with mixing encoded and unencoded text.
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* Text files enforce an encoding; binary files use bytes. This means
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that if a file is opened using an incorrect mode or encoding, I/O
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will likely fail.
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* :func:`map` and :func:`filter` return iterators. A quick fix is e.g.
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``list(map(...))``, but a better fix is often to use a list
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comprehension (especially when the original code uses :keyword:`lambda`).
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Particularly tricky is :func:`map` invoked for the side effects of the
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function; the correct transformation is to use a for-loop.
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* :class:`dict` methods :meth:`dict.keys`, :meth:`dict.items` and
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:meth:`dict.values` return views instead of lists. For example, this no
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longer works: ``k = d.keys(); k.sort()``. Use ``k = sorted(d)`` instead.
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* :meth:`builtin.sorted` and :meth:`list.sort` no longer accept the *cmp*
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argument providing a comparison function. Use the *key* argument
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instead. N.B. the *key* and *reverse* arguments are now "keyword-only".
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* ``1/2`` returns a float. Use ``1//2`` to get the truncating behavior.
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* The :func:`repr` of a long integer doesn't include the trailing ``L``
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anymore, so code that unconditionally strips that character will
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chop off the last digit instead.
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Strings and Bytes
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=================
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* There is only one string type; its name is :class:`str` but its behavior and
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implementation are like :class:`unicode` in 2.x.
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* The :class:`basestring` superclass has been removed. The ``2to3`` tool
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replaces every occurrence of :class:`basestring` with :class:`str`.
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* PEP 3137: There is a new type, :class:`bytes`, to represent binary data (and
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encoded text, which is treated as binary data until you decide to decode it).
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The :class:`str` and :class:`bytes` types cannot be mixed; you must always
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explicitly convert between them, using the :meth:`str.encode` (str -> bytes)
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or :meth:`bytes.decode` (bytes -> str) methods.
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* All backslashes in raw strings are interpreted literally. This means that
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Unicode escapes are not treated specially.
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.. XXX add bytearray
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* PEP 3112: Bytes literals, e.g. ``b"abc"``, create :class:`bytes` instances.
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* PEP 3120: UTF-8 default source encoding.
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* PEP 3131: Non-ASCII identifiers. (However, the standard library remains
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ASCII-only with the exception of contributor names in comments.)
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* PEP 3116: New I/O Implementation. The API is nearly 100% backwards
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compatible, but completely reimplemented (currently mostly in Python). Also,
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binary files use bytes instead of strings.
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* The :mod:`StringIO` and :mod:`cStringIO` modules are gone. Instead, import
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:class:`io.StringIO` or :class:`io.BytesIO`.
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* ``'\U'`` and ``'\u'`` escapes in raw strings are not treated specially.
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PEP 3101: A New Approach to String Formatting
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=============================================
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.. XXX expand this
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* A new system for built-in string formatting operations replaces the ``%``
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string formatting operator.
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PEP 3106: Revamping dict :meth:`dict.keys`, :meth:`dict.items` and :meth:`dict.values`
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======================================================================================
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.. XXX expand this
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* The :meth:`dict.iterkeys`, :meth:`dict.itervalues` and :meth:`dict.iteritems`
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methods have been removed.
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* :meth:`dict.keys`, :meth:`dict.values` and :meth:`dict.items` return objects
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with set behavior that reference the underlying dict.
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PEP 3107: Function Annotations
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==============================
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.. XXX expand this
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* A standardized way of annotating a function's parameters and return values.
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Exception Stuff
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===============
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* PEP 352: Exceptions must derive from :exc:`BaseException`. This is the root
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of the exception hierarchy.
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* :exc:`StandardError` was removed (already in 2.6).
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* Dropping sequence behavior (slicing!) and :attr:`message` attribute of
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exception instances.
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* PEP 3109: Raising exceptions. You must now use ``raise Exception(args)``
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instead of ``raise Exception, args``.
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* PEP 3110: Catching exceptions. You must now use ``except SomeException as
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identifier:`` instead of ``except Exception, identifier:``
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* PEP 3134: Exception chaining. (The :attr:`__context__` feature from the PEP
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hasn't been implemented yet in 3.0a2.)
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* A few exception messages are improved when Windows fails to load an extension
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module. For example, ``error code 193`` is now ``%1 is not a valid Win32
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application``. Strings now deal with non-English locales.
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New Class and Metaclass Stuff
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=============================
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* Classic classes are gone.
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* PEP 3115: New Metaclass Syntax.
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* PEP 3119: Abstract Base Classes (ABCs); ``@abstractmethod`` and
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``@abstractproperty`` decorators; collection ABCs.
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* PEP 3129: Class decorators.
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* PEP 3141: Numeric ABCs.
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Other Language Changes
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======================
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Here are most of the changes that Python 3.0 makes to the core Python
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language and built-in functions.
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* Removed backticks (use :func:`repr` instead).
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* Removed ``<>`` (use ``!=`` instead).
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* ``!=`` now returns the opposite of ``==``, unless ``==`` returns
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``NotImplemented``.
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* :keyword:`as` and :keyword:`with` are keywords.
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* ``True``, ``False``, and ``None`` are keywords.
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* PEP 237: :class:`long` renamed to :class:`int`. That is, there is only one
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built-in integral type, named :class:`int`; but it behaves like the old
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:class:`long` type, with the exception that the literal suffix ``L`` is
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neither supported by the parser nor produced by :func:`repr` anymore.
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:data:`sys.maxint` was also removed since the int type has no maximum value
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anymore.
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* PEP 238: int division returns a float.
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* The ordering operators behave differently: for example, ``x < y`` where ``x``
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and ``y`` have incompatible types raises :exc:`TypeError` instead of returning
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a pseudo-random boolean.
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* :meth:`__getslice__` and friends killed. The syntax ``a[i:j]`` now translates
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to ``a.__getitem__(slice(i, j))`` (or :meth:`__setitem__` or
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:meth:`__delitem__`, depending on context).
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* PEP 3102: Keyword-only arguments. Named parameters occurring after ``*args``
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in the parameter list *must* be specified using keyword syntax in the call.
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You can also use a bare ``*`` in the parameter list to indicate that you don't
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accept a variable-length argument list, but you do have keyword-only
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arguments.
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* PEP 3104: :keyword:`nonlocal` statement. Using ``nonlocal x`` you can now
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assign directly to a variable in an outer (but non-global) scope.
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* PEP 3111: :func:`raw_input` renamed to :func:`input`. That is, the new
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:func:`input` function reads a line from :data:`sys.stdin` and returns it with
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the trailing newline stripped. It raises :exc:`EOFError` if the input is
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terminated prematurely. To get the old behavior of :func:`input`, use
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``eval(input())``.
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* :func:`xrange` renamed to :func:`range`, so :func:`range` will no longer
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produce a list but an iterable yielding integers when iterated over.
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* PEP 3113: Tuple parameter unpacking removed. You can no longer write ``def
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foo(a, (b, c)): ...``. Use ``def foo(a, b_c): b, c = b_c`` instead.
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* PEP 3114: ``.next()`` renamed to :meth:`__next__`, new builtin :func:`next` to
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call the :meth:`__next__` method on an object.
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* PEP 3127: New octal literals; binary literals and :func:`bin`. Instead of
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``0666``, you write ``0o666``. The :func:`oct` function is modified
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accordingly. Also, ``0b1010`` equals 10, and ``bin(10)`` returns
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``"0b1010"``. ``0666`` is now a :exc:`SyntaxError`.
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* PEP 3132: Extended Iterable Unpacking. You can now write things like ``a, b,
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*rest = some_sequence``. And even ``*rest, a = stuff``. The ``rest`` object
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is always a list; the right-hand side may be any iterable.
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* PEP 3135: New :func:`super`. You can now invoke :func:`super` without
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arguments and the right class and instance will automatically be chosen. With
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arguments, its behavior is unchanged.
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* :func:`zip`, :func:`map` and :func:`filter` return iterators.
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* :data:`string.letters` and its friends (:data:`string.lowercase` and
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:data:`string.uppercase`) are gone. Use :data:`string.ascii_letters`
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etc. instead.
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* Removed: :func:`apply`, :func:`callable`, :func:`coerce`, :func:`execfile`,
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:func:`file`, :func:`reduce`, :func:`reload`.
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* Removed: :meth:`dict.has_key` -- use the ``in`` operator instead.
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* :func:`exec` is now a function.
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* The :meth:`__oct__` and :meth:`__hex__` special methods are removed --
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:func:`oct` and :func:`hex` use :meth:`__index__` now to convert the argument
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to an integer.
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* Support is removed for :attr:`__members__` and :attr:`__methods__`.
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* Renamed the boolean conversion C-level slot and method: ``nb_nonzero`` is now
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``nb_bool`` and :meth:`__nonzero__` is now :meth:`__bool__`.
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* Removed :data:`sys.maxint`. Use :data:`sys.maxsize`.
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.. ======================================================================
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Optimizations
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-------------
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* Detailed changes are listed here.
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The net result of the 3.0 generalizations is that Python 3.0 runs the pystone
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benchmark around 33% slower than Python 2.5. There's room for improvement; we
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expect to be optimizing string and integer operations significantly before the
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final 3.0 release!
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.. ======================================================================
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New, Improved, and Deprecated Modules
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=====================================
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As usual, Python's standard library received a number of enhancements and bug
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fixes. Here's a partial list of the most notable changes, sorted alphabetically
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by module name. Consult the :file:`Misc/NEWS` file in the source tree for a more
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complete list of changes, or look through the Subversion logs for all the
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details.
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* The :mod:`cPickle` module is gone. Use :mod:`pickle` instead. Eventually
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we'll have a transparent accelerator module.
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* The :mod:`imageop` module is gone.
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* The :mod:`audiodev`, :mod:`Bastion`, :mod:`bsddb185`, :mod:`exceptions`,
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:mod:`linuxaudiodev`, :mod:`md5`, :mod:`MimeWriter`, :mod:`mimify`,
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:mod:`popen2`, :mod:`rexec`, :mod:`sets`, :mod:`sha`, :mod:`stringold`,
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:mod:`strop`, :mod:`sunaudiodev`, :mod:`timing`, and :mod:`xmllib` modules are
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gone.
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* The :mod:`bsddb` module is gone. It is being maintained externally
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with its own release schedule better mirroring that of BerkeleyDB.
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See http://www.jcea.es/programacion/pybsddb.htm.
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* The :mod:`new` module is gone.
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* The functions :func:`os.tmpnam`, :func:`os.tempnam` and :func:`os.tmpfile`
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have been removed in favor of the :mod:`tempfile` module.
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* The :mod:`tokenize` module has been changed to work with bytes. The main
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entry point is now :func:`tokenize.tokenize`, instead of generate_tokens.
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.. ======================================================================
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.. whole new modules get described in subsections here
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.. ======================================================================
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Build and C API Changes
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=======================
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Changes to Python's build process and to the C API include:
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* PEP 3118: New Buffer API.
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* PEP 3121: Extension Module Initialization & Finalization.
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* PEP 3123: Making :cmacro:`PyObject_HEAD` conform to standard C.
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* No more C API support for restricted execution.
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* :cfunc:`PyNumber_Coerce`, :cfunc:`PyNumber_CoerceEx`, :cfunc:`PyMember_Get`,
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and :cfunc:`PyMember_Set` C APIs are removed.
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* New C API :cfunc:`PyImport_ImportModuleNoBlock`, works like
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:cfunc:`PyImport_ImportModule` but won't block on the import lock (returning
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an error instead).
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.. ======================================================================
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Port-Specific Changes
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---------------------
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Platform-specific changes go here.
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.. ======================================================================
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.. _30section-other:
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Other Changes and Fixes
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=======================
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As usual, there were a bunch of other improvements and bugfixes
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scattered throughout the source tree. A search through the change
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logs finds there were XXX patches applied and YYY bugs fixed between
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Python 2.6 and 3.0. Both figures are likely to be underestimates.
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Some of the more notable changes are:
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* Details go here.
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.. ======================================================================
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Porting to Python 3.0
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=====================
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This section lists previously described changes that may require
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changes to your code:
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* Everything is all in the details!
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* Developers can include :file:`intobject.h` after :file:`Python.h` for
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some ``PyInt_`` aliases.
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.. ======================================================================
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.. _acks:
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Acknowledgements
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================
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The author would like to thank the following people for offering
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suggestions, corrections and assistance with various drafts of this
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article: Georg Brandl.
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