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