**************************** 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. * 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, 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 Strings Vs. Bytes ---------------------- Everything you thought you knew about binary data and Unicode has changed. There's a longer section below; here's a summary of the changes: * Python 3.0 uses *strings* and *bytes* instead of *Unicode strings* and *8-bit strings*. The difference is that any attempt to mix strings and bytes 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. * 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. 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. * 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. * 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. The latter no longer exists. * :func:`zip` now returns an iterator. * XXX More below? 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, the expression ``(a > b) - (a < b)`` is equivalent to ``cmp(a, b)``. * XXX More below? Integers -------- * :pep:`0237`: :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. * 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.) * 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). * ``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.) See :pep:`0238`. Overview Of Syntactic Changes ============================= This section gives a brief overview of every *syntactic* change. Additions --------- * Function argument and return value annotations (see below). XXX * :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, 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. * Set literals, e.g. ``{1, 2}``. Note that ``{}`` is an empty dictionary; use ``set()`` for an empty set. Set comprehensions are also supported; ``{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``. Changes ------- * 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.) Removals -------- * :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__`. * :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`. More about :class:`bytes` below. * :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`. * :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 slightly 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`. This speaks for itself. * :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. 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. Strings And Bytes ================= This section discusses the many changes in string XXX * 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 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 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 * All backslashes in raw strings are interpreted literally. This means that ``'\U'`` and ``'\u'`` escapes in raw strings are not treated specially. * :pep:`3138`: :func:`repr` of a string no longer escapes all non-ASCII characters. XXX * :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 ================================================ * 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.) .. XXX expand this :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*. :pep:`3107`: Function Annotations ================================= .. XXX expand this * A standardized way of annotating a function's parameters and return values. 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 ============================= * 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 ====================== * 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())``. * :func:`xrange` renamed to :func:`range`, so :func:`range` will no longer produce a list but an iterable yielding integers when iterated over. XXX dupe * :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`. .. ======================================================================