First draft of a what's new document.
(There's something wrong with my network right now so I can't build it.)
This commit is contained in:
Guido van Rossum
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What's New in Python 3.0
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****************************
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:Author: A.M. Kuchling
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:Author: A.M. Kuchling, Guido van Rossum
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.. |release| replace:: 0.0
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@ -48,14 +48,16 @@
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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. No release date for
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Python 3.0 has been set; it will probably be released in mid 2008.
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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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This article doesn't attempt to provide a complete specification of the new
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features, but instead provides a convenient overview. For full details, you
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should refer to the documentation for Python 3.0. If you want to understand the
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complete implementation and design rationale, refer to the PEP for a particular
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new feature.
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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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@ -68,12 +70,224 @@ new feature.
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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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* 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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* Bytes aren't hashable, and don't support certain operations like
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``b.lower()``, ``b.strip()`` or ``b.split()``.
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For the latter two, use ``b.strip(b" \t\r\n\f")`` or
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``b.split(b" \t\r\n\f")``.
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* ``map()`` and ``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 ``lambda``).
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Particularly tricky is ``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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* ``dict`` methods ``.keys()``, ``.items()`` and ``.values()`` return
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views instead of lists. For example, this no longer works:
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``k = d.keys(); k.sort()``. Use ``k = sorted(d)`` instead.
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* ``1/2`` returns a float. Use ``1//2`` to get the truncating behavior.
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* Code that unconditionally strips the trailing ``L`` from the ``repr()``
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of a long integer will chop off the last digit instead.
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* The ``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\n"`` would write ``"A\nB\n"``; but in Python 3.0,
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``print("A\n", "B\n")`` writes ``"A\n B\n"``.
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* Also, ``print`` and ``print (x, y)`` behave differently without
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warning: the former used to add a newline in 2.x, but does nothing
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in 3.0; the latter used to print the ``repr()`` of a tuple in 2.x,
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but prints the individual values in 3.0.
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* You'll be finding yourself typing ``print x`` a lot in interactive
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mode. Time to retrain your fingers. :-)
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Strings and Bytes
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=================
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* There is only on string type; its name is ``str`` but its behavior
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and implementation are more like ``unicode`` in 2.x.
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* PEP 358: There is a new type, ``bytes``, to represent binary data
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(and encoded text, which is treated as binary data until you decide
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to decode it). The ``str`` and ``bytes`` types cannot be mixed; you
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must always explicitly convert between them, using the ``.encode()``
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(str -> bytes) or ``.decode()`` (bytes -> str) methods. Comparing a
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bytes and a str instance for equality raises a TypeError; this
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catches common mistakes.
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* PEP 3112: Bytes literals. E.g. b"abc".
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* PEP 3120: UTF-8 default source encoding.
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* PEP 3131: Non-ASCII identifiers. (However, the standard library
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remains ASCII-only with the exception of contributor names in
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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
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Python). Also, binary files use bytes instead of strings.
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* The ``StringIO`` and ``cStringIO`` modules are gone. Instead,
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import ``StringIO`` or ``BytesIO`` from the ``io`` module.
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PEP 3101: A New Approach to String Formatting
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=============================================
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XXX
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PEP 3106: Revamping ``.keys()``, ``.items()`` and ``.values()``
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===============================================================
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XXX
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PEP 3107: Function Annotations
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==============================
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XXX
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Exception Stuff
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===============
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* PEP 352: Exceptions must derive from BaseException. This is the
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root of the exception hierarchy; only Exception,
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* StandardException was removed (already in 2.6).
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* Dropping sequence behavior and ``.message`` attribute of exception
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instances.
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* PEP 3109: Raising exceptions. You must now use ``raise
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Exception(args)`` instead of ``raise Exception, args``.
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* PEP 3110: Catching exceptions.
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* PEP 3134: Exception chaining. (The ``__context__`` feature from the
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PEP hasn't been implemented yet in 3.0a1.)
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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; ``@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 all of the changes that Python 2.6 makes to the core Python language.
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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 ``repr()`` instead).
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* Removed ``<>`` (use ``!=`` instead).
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* ``as`` and ``with`` are keywords.
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* PEP 237: ``long`` renamed to ``int``. That is, there is only one
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built-in integral type, named ``int``; but it behaves like the old
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``long`` type.
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* PEP 238: int division returns a float.
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* The ordering operators behave differently: for example, ``x < y``
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where ``x`` and ``y`` have incompatible types raises ``TypeError``
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instead of returning a pseudo-random boolean.
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* ``__getslice__()`` and friends killed. The syntax ``a[i:j]`` now
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translates to ``a.__getitem__(slice(i, j))`` (or ``__setitem__``
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or ``__delitem``, depending on context).
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* PEP 3102: Keyword-only arguments. Named parameters occurring after
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``*args`` in the parameter list *must* be specified using keyword
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syntax in the call. You can also use ``*`` in the parameter list to
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indicate that you don't accept a variable-length argument list, but
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you do have keyword-only arguments.
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* PEP 3104: ``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 3105: ``print`` is now a function. Keyword argumemts
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``file=sys.stdout``, ``sep=" "`` and ``end="\n"`` let you customize
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it.
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* PEP 3111: ``raw_input()`` renamed to ``input()``. That is, the new
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``input()`` function reads a line from ``sys.stdin`` and returns it
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with the trailing newline stripped. It raises ``EOFError`` if the
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input is terminated prematurely. To get the old behavior of
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``input()``, use ``eval(input())``.
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* ``xrange()`` renamed to ``range()``.
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* PEP 3113: Tuple parameter unpacking removed. You can no longer write
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``def foo(a, (b, c)): ...``. Use ``def foo(a, b_c): b, c = b_c``
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instead.
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* PEP 3114: ``.next()`` renamed to ``.__next__()``.
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* PEP 3127: New octal literals; binary literals and ``bin()``.
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Instead of ``0666``, you write ``0o666``. The oct() function is
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modified accordingly. Also, ``0b1010`` equals 10, and ``bin(10)``
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returns ``"0b1010"``.
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* PEP 3132: Extended Iterable Unpacking. You can now write things
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like ``a, b, *rest = some_sequence``. And even ``*rest, a =
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stuff``. The ``rest`` variable is always a list; the right-hand
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side may be any iterable.
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* PEP 3135: New ``super()``. You can now invoke ``super()`` without
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arguments and the right class and instance will automatically be
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chosen. With arguments, its behavior is unchanged.
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* ``zip()``, ``map()`` and ``filter()`` return iterators.
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* ``string.letters`` and its friends (``.lowercase`` and
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``.uppercase``) are gone. Use ``string.ascii_letters``
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etc. instead.
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* Removed: apply(), callable(), coerce(), execfile(), file(),
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reduce(), reload().
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* Removed: ``dict.has_key()``.
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* ``exec`` is now a function.
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* Detailed changes are listed here.
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.. % ======================================================================
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@ -83,8 +297,9 @@ Optimizations
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* Detailed changes are listed here.
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The net result of the 3.0 optimizations is that Python 3.0 runs the pystone
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benchmark around XX% slower than Python 2.6.
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The net result of the 3.0 generalizations is that Python 3.0 runs the
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pystone benchmark around 25% slower than Python 2.5. There's room for
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improvement!
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.. % ======================================================================
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@ -92,12 +307,14 @@ benchmark around XX% slower than Python 2.6.
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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 CVS logs for all the details.
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As usual, Python's standard library received a number of enhancements
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and bug fixes. Here's a partial list of the most notable changes,
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sorted alphabetically by module name. Consult the :file:`Misc/NEWS`
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file in the source tree for a more complete list of changes, or look
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through the CVS logs for all the details.
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* Detailed changes are listed here.
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* The ``cPickle`` module is gone. Use ``pickle`` instead. Eventually
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we'll have a transparent accelerator module.
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.. % ======================================================================
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.. % whole new modules get described in \subsections here
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@ -110,7 +327,11 @@ Build and C API Changes
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Changes to Python's build process and to the C API include:
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* Detailed changes are listed here.
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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 PyObject_HEAD conform to standard C.
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.. % ======================================================================
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@ -128,10 +349,10 @@ Platform-specific changes go here.
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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 scattered
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throughout the source tree. A search through the change logs finds there were
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XXX patches applied and YYY bugs fixed between Python 2.6 and 3.0. Both figures
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are likely to be underestimates.
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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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@ -143,8 +364,8 @@ Some of the more notable changes are:
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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 changes to your
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code:
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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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@ -156,6 +377,7 @@ code:
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Acknowledgements
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================
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The author would like to thank the following people for offering suggestions,
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corrections and assistance with various drafts of this article: .
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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: .
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