blob: b8cd7c48b359b2ef43b2dc03fc587b7e9d2098f7 [file] [log] [blame]
What's New In Python 3.0
.. XXX Add trademark info for Apple, Microsoft.
:Author: Guido van Rossum
.. $Id$
Rules for maintenance:
* Anyone can add text to this document. Do not spend very much time
on the wording of your changes, because your text will probably
get rewritten to some degree.
* The maintainer will go through Misc/NEWS periodically and add
changes; it's therefore more important to add your changes to
Misc/NEWS than to this file. (Note: I didn't get to this for 3.0.
* 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
* It's OK to just add a fragmentary note about a change. For
example: "XXX Describe the transmogrify() function added to the
socket module." The maintainer will research the change and
write the necessary text.
* You can comment out your additions if you like, but it's not
necessary (especially when a final release is some months away).
* Credit the author of a patch or bugfix. Just the name is
sufficient; the e-mail address isn't necessary. (Due to time
constraints I haven't managed to do this for 3.0. GvR.)
* It's helpful to add the bug/patch number as a comment:
% Patch 12345
XXX Describe the transmogrify() function added to the socket
(Contributed by P.Y. Developer.)
This saves the maintainer the effort of going through the SVN log
when researching a change. (Again, I didn't get to this for 3.0.
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 backwards incompatible* Python release. Python 3.0 was released on December 3, 2008.
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
mostly fixing well-known annoyances and warts, and removing a lot of
old cruft.
This article doesn't attempt to provide a complete specification of
all new features, but instead tries to give a convenient overview.
For full details, you should refer to the documentation for Python
3.0, and/or the many PEPs referenced in the text. If you want to
understand the complete implementation and design rationale for a
particular feature, PEPs usually have more details than the regular
documentation; but note that PEPs usually are not kept up-to-date once
a feature has been fully implemented.
Due to time constraints this document is not as complete as it should
have been. As always for a new release, the ``Misc/NEWS`` file in the
source distribution contains a wealth of detailed information about
every small thing that was changed.
.. 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 ``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:
.. code-block:: none
There are <4294967296> possibilities!
* 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 automatically converted to
:func:`print` function calls, so this is mostly a non-issue for
larger projects.
Views And Iterators Instead Of Lists
Some well-known APIs no longer return lists:
* :class:`dict` methods :meth:`dict.keys`, :meth:`dict.items` and
:meth:`dict.values` return "views" instead of lists. For example,
this no longer works: ``k = d.keys(); k.sort()``. Use ``k =
sorted(d)`` instead (this works in Python 2.5 too and is just
as efficient).
* Also, the :meth:`dict.iterkeys`, :meth:`dict.iteritems` and
:meth:`dict.itervalues` methods are no longer supported.
* :func:`map` and :func:`filter` return iterators. If you really need
a list and the input sequences are all of equal length, a quick
fix is to wrap :func:`map` in :func:`list`, e.g. ``list(map(...))``,
but a better fix is
often to use a list comprehension (especially when the original code
uses :keyword:`lambda`), or rewriting the code so it doesn't need a
list at all. Particularly tricky is :func:`map` invoked for the
side effects of the function; the correct transformation is to use a
regular :keyword:`for` loop (since creating a list would just be
If the input sequences are not of equal length, :func:`map` will
stop at the termination of the shortest of the sequences. For full
compatibility with :func:`map` from Python 2.x, also wrap the sequences in
:func:`itertools.zip_longest`, e.g. ``map(func, *sequences)`` becomes
``list(map(func, itertools.zip_longest(*sequences)))``.
* :func:`range` now behaves like :func:`xrange` used to behave, except
it works with values of arbitrary size. The latter no longer
* :func:`zip` now returns an iterator.
Ordering Comparisons
Python 3.0 has simplified the rules for ordering comparisons:
* The ordering comparison operators (``<``, ``<=``, ``>=``, ``>``)
raise a TypeError exception when the operands don't have a
meaningful natural ordering. Thus, expressions like ``1 < ''``, ``0
> None`` or ``len <= len`` are no longer valid, and e.g. ``None <
None`` raises :exc:`TypeError` instead of returning
``False``. 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 incomparable types always compare
unequal to each other.
* :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
* The :func:`cmp` function should be treated as gone, and the :meth:`__cmp__`
special method is no longer supported. Use :meth:`__lt__` for sorting,
:meth:`__eq__` with :meth:`__hash__`, and other rich comparisons as needed.
(If you really need the :func:`cmp` functionality, you could use the
expression ``(a > b) - (a < b)`` as the equivalent for ``cmp(a, b)``.)
* :pep:`237`: Essentially, :class:`long` renamed to :class:`int`.
That is, there is only one built-in integral type, named
:class:`int`; but it behaves mostly like the old :class:`long` type.
* :pep:`238`: An expression like ``1/2`` returns a float. Use
``1//2`` to get the truncating behavior. (The latter syntax has
existed for years, at least since Python 2.2.)
* The :data:`sys.maxint` constant was removed, since there is no
longer a limit to the value of integers. However, :data:`sys.maxsize`
can be used as an integer larger than any practical list or string
index. It conforms to the implementation's "natural" integer size
and is typically the same as :data:`sys.maxint` in previous releases
on the same platform (assuming the same build options).
* The :func:`repr` of a long integer doesn't include the trailing ``L``
anymore, so code that unconditionally strips that character will
chop off the last digit instead. (Use :func:`str` instead.)
* Octal literals are no longer of the form ``0720``; use ``0o720``
Text Vs. Data Instead Of Unicode Vs. 8-bit
Everything you thought you knew about binary data and Unicode has
* Python 3.0 uses the concepts of *text* and (binary) *data* instead
of Unicode strings and 8-bit strings. All text is Unicode; however
*encoded* Unicode is represented as binary data. The type used to
hold text is :class:`str`, the type used to hold data is
:class:`bytes`. The biggest difference with the 2.x situation is
that any attempt to mix text and data in Python 3.0 raises
:exc:`TypeError`, whereas if you were to mix Unicode and 8-bit
strings in Python 2.x, it would work if the 8-bit string happened to
contain only 7-bit (ASCII) bytes, but you would get
:exc:`UnicodeDecodeError` if it contained non-ASCII values. This
value-specific behavior has caused numerous sad faces over the
* As a consequence of this change in philosophy, 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. To be prepared in Python 2.x, start using :class:`unicode`
for all unencoded text, and :class:`str` for binary or encoded data
only. Then the ``2to3`` tool will do most of the work for you.
* You can no longer use ``u"..."`` literals for Unicode text.
However, you must use ``b"..."`` literals for binary data.
* As the :class:`str` and :class:`bytes` types cannot be mixed, you
must always explicitly convert between them. Use :meth:`str.encode`
to go from :class:`str` to :class:`bytes`, and :meth:`bytes.decode`
to go from :class:`bytes` to :class:`str`. You can also use
``bytes(s, encoding=...)`` and ``str(b, encoding=...)``,
* Like :class:`str`, the :class:`bytes` type is immutable. There is a
separate *mutable* type to hold buffered binary data,
:class:`bytearray`. Nearly all APIs that accept :class:`bytes` also
accept :class:`bytearray`. The mutable API is based on
* All backslashes in raw string literals are interpreted literally.
This means that ``'\U'`` and ``'\u'`` escapes in raw strings are not
treated specially. For example, ``r'\u20ac'`` is a string of 6
characters in Python 3.0, whereas in 2.6, ``ur'\u20ac'`` was the
single "euro" character. (Of course, this change only affects raw
string literals; the euro character is ``'\u20ac'`` in Python 3.0.)
* The built-in :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. The ``2to3`` tool (see below) replaces every occurrence of
:class:`basestring` with :class:`str`.
* 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 loudly,
instead of silently producing incorrect data. It also means that
even Unix users will have to specify the correct mode (text or
binary) when opening a file. There is a platform-dependent default
encoding, which on Unixy platforms can be set with the ``LANG``
environment variable (and sometimes also with some other
platform-specific locale-related environment variables). In many
cases, but not all, the system default is UTF-8; you should never
count on this default. Any application reading or writing more than
pure ASCII text should probably have a way to override the encoding.
There is no longer any need for using the encoding-aware streams
in the :mod:`codecs` module.
* The initial values of :data:`sys.stdin`, :data:`sys.stdout` and
:data:`sys.stderr` are now unicode-only text files (i.e., they are
instances of :class:`io.TextIOBase`). To read and write bytes data
with these streams, you need to use their :data:`io.TextIOBase.buffer`
* Filenames are passed to and returned from APIs as (Unicode) strings.
This can present platform-specific problems because on some
platforms filenames are arbitrary byte strings. (On the other hand,
on Windows filenames are natively stored as Unicode.) As a
work-around, most APIs (e.g. :func:`open` and many functions in the
:mod:`os` module) that take filenames accept :class:`bytes` objects
as well as strings, and a few APIs have a way to ask for a
:class:`bytes` return value. Thus, :func:`os.listdir` returns a
list of :class:`bytes` instances if the argument is a :class:`bytes`
instance, and :func:`os.getcwdb` returns the current working
directory as a :class:`bytes` instance. Note that when
:func:`os.listdir` returns a list of strings, filenames that
cannot be decoded properly are omitted rather than raising
* Some system APIs like :data:`os.environ` and :data:`sys.argv` can
also present problems when the bytes made available by the system is
not interpretable using the default encoding. Setting the ``LANG``
variable and rerunning the program is probably the best approach.
* :pep:`3138`: The :func:`repr` of a string no longer escapes
non-ASCII characters. It still escapes control characters and code
points with non-printable status in the Unicode standard, however.
* :pep:`3120`: The default source encoding is now UTF-8.
* :pep:`3131`: Non-ASCII letters are now allowed in identifiers.
(However, the standard library remains ASCII-only with the exception
of contributor names in comments.)
* The :mod:`StringIO` and :mod:`cStringIO` modules are gone. Instead,
import the :mod:`io` module and use :class:`io.StringIO` or
:class:`io.BytesIO` for text and data respectively.
* See also the :ref:`unicode-howto`, which was updated for Python 3.0.
Overview Of Syntax Changes
This section gives a brief overview of every *syntactic* change in
Python 3.0.
New Syntax
* :pep:`3107`: Function argument and return value annotations. This
provides a standardized way of annotating a function's parameters
and return value. There are no semantics attached to such
annotations except that they can be introspected at runtime using
the :attr:`__annotations__` attribute. The intent is to encourage
experimentation through metaclasses, decorators or frameworks.
* :pep:`3102`: Keyword-only arguments. Named parameters occurring
after ``*args`` in the parameter list *must* be specified using
keyword syntax in the call. You can also use a bare ``*`` in the
parameter list to indicate that you don't accept a variable-length
argument list, but you do have keyword-only arguments.
* Keyword arguments are allowed after the list of base classes in a
class definition. This is used by the new convention for specifying
a metaclass (see next section), but can be used for other purposes
as well, as long as the metaclass supports it.
* :pep:`3104`: :keyword:`nonlocal` statement. Using ``nonlocal x``
you can now assign directly to a variable in an outer (but
non-global) scope. :keyword:`!nonlocal` is a new reserved word.
* :pep:`3132`: Extended Iterable Unpacking. You can now write things
like ``a, b, *rest = some_sequence``. And even ``*rest, a =
stuff``. The ``rest`` object is always a (possibly empty) list; the
right-hand side may be any iterable. Example::
(a, *rest, b) = range(5)
This sets *a* to ``0``, *b* to ``4``, and *rest* to ``[1, 2, 3]``.
* Dictionary comprehensions: ``{k: v for k, v in stuff}`` means the
same thing as ``dict(stuff)`` but is more flexible. (This is
:pep:`274` vindicated. :-)
* Set literals, e.g. ``{1, 2}``. Note that ``{}`` is an empty
dictionary; use ``set()`` for an empty set. Set comprehensions are
also supported; e.g., ``{x for x in stuff}`` means the same thing as
``set(stuff)`` but is more flexible.
* New octal literals, e.g. ``0o720`` (already in 2.6). The old octal
literals (``0720``) are gone.
* New binary literals, e.g. ``0b1010`` (already in 2.6), and
there is a new corresponding built-in function, :func:`bin`.
* Bytes literals are introduced with a leading ``b`` or ``B``, and
there is a new corresponding built-in function, :func:`bytes`.
Changed Syntax
* :pep:`3109` and :pep:`3134`: new :keyword:`raise` statement syntax:
:samp:`raise [{expr} [from {expr}]]`. See below.
* :keyword:`!as` and :keyword:`with` are now reserved words. (Since
2.6, actually.)
* ``True``, ``False``, and ``None`` are reserved words. (2.6 partially enforced
the restrictions on ``None`` already.)
* Change from :keyword:`except` *exc*, *var* to
:keyword:`!except` *exc* :keyword:`!as` *var*. See :pep:`3110`.
* :pep:`3115`: New Metaclass Syntax. Instead of::
class C:
__metaclass__ = M
you must now use::
class C(metaclass=M):
The module-global :data:`__metaclass__` variable is no longer
supported. (It was a crutch to make it easier to default to
new-style classes without deriving every class from
* List comprehensions no longer support the syntactic form
:samp:`[... for {var} in {item1}, {item2}, ...]`. Use
:samp:`[... for {var} in ({item1}, {item2}, ...)]` instead.
Also note that list comprehensions have different semantics: they
are closer to syntactic sugar for a generator expression inside a
:func:`list` constructor, and in particular the loop control
variables are no longer leaked into the surrounding scope.
* The *ellipsis* (``...``) can be used as an atomic expression
anywhere. (Previously it was only allowed in slices.) Also, it
*must* now be spelled as ``...``. (Previously it could also be
spelled as ``. . .``, by a mere accident of the grammar.)
Removed Syntax
* :pep:`3113`: Tuple parameter unpacking removed. You can no longer
write ``def foo(a, (b, c)): ...``.
Use ``def foo(a, b_c): b, c = b_c`` instead.
* Removed backticks (use :func:`repr` instead).
* Removed ``<>`` (use ``!=`` instead).
* Removed keyword: :func:`exec` is no longer a keyword; it remains as
a function. (Fortunately the function syntax was also accepted in
2.x.) Also note that :func:`exec` no longer takes a stream argument;
instead of ``exec(f)`` you can use ``exec(``.
* 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 :samp:`from .[{module}]
import {name}`. All :keyword:`import` forms not starting with ``.`` are
interpreted as absolute imports. (:pep:`328`)
* Classic classes are gone.
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 :mod:`__future__`.
Also check out :ref:`new-26-context-managers` and
* :ref:`pep-0366`. This enhances the usefulness of the :option:`-m`
option when the referenced module lives in a package.
* :ref:`pep-0370`.
* :ref:`pep-0371`.
* :ref:`pep-3101`. Note: the 2.6 description mentions the
:meth:`format` method for both 8-bit and Unicode strings. In 3.0,
only the :class:`str` type (text strings with Unicode support)
supports this method; the :class:`bytes` type does not. The plan is
to eventually make this the only API for string formatting, and to
start deprecating the ``%`` operator in Python 3.1.
* :ref:`pep-3105`. This is now a standard feature and no longer needs
to be imported from :mod:`__future__`. More details were given above.
* :ref:`pep-3110`. The :keyword:`except` *exc* :keyword:`!as` *var*
syntax is now standard and :keyword:`!except` *exc*, *var* is no
longer supported. (Of course, the :keyword:`!as` *var* part is still
* :ref:`pep-3112`. The ``b"..."`` string literal notation (and its
variants like ``b'...'``, ``b"""..."""``, and ``br"..."``) now
produces a literal of type :class:`bytes`.
* :ref:`pep-3116`. The :mod:`io` module is now the standard way of
doing file I/O. The built-in :func:`open` function is now an
alias for :func:`` and has additional keyword arguments
*encoding*, *errors*, *newline* and *closefd*. Also note that an
invalid *mode* argument now raises :exc:`ValueError`, not
:exc:`IOError`. The binary file object underlying a text file
object can be accessed as :attr:`f.buffer` (but beware that the
text object maintains a buffer of itself in order to speed up
the encoding and decoding operations).
* :ref:`pep-3118`. The old builtin :func:`buffer` is now really gone;
the new builtin :func:`memoryview` provides (mostly) similar
* :ref:`pep-3119`. The :mod:`abc` module and the ABCs defined in the
:mod:`collections` module plays a somewhat more prominent role in
the language now, and built-in collection types like :class:`dict`
and :class:`list` conform to the :class:`collections.MutableMapping`
and :class:`collections.MutableSequence` ABCs, respectively.
* :ref:`pep-3127`. As mentioned above, the new octal literal
notation is the only one supported, and binary literals have been
* :ref:`pep-3129`.
* :ref:`pep-3141`. The :mod:`numbers` module is another new use of
ABCs, defining Python's "numeric tower". Also note the new
:mod:`fractions` module which implements :class:`numbers.Rational`.
Library Changes
Due to time constraints, this document does not exhaustively cover the
very extensive changes to the standard library. :pep:`3108` is the
reference for the major changes to the library. Here's a capsule
* Many old modules were removed. Some, like :mod:`gopherlib` (no
longer used) and :mod:`md5` (replaced by :mod:`hashlib`), were
already deprecated by :pep:`4`. Others were removed as a result
of the removal of support for various platforms such as Irix, BeOS
and Mac OS 9 (see :pep:`11`). 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 Berkeley DB's
release schedule. However, the package is alive and well,
externally maintained at
* Some modules were renamed because their old name disobeyed
:pep:`8`, or for various other reasons. Here's the list:
======================= =======================
Old Name New Name
======================= =======================
_winreg winreg
ConfigParser configparser
copy_reg copyreg
Queue queue
SocketServer socketserver
markupbase _markupbase
repr reprlib
======================= =======================
* 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` / :mod:`cPickle` pair 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`
* 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:`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:`urllib2`, :mod:`urlparse`,
* :mod:`xmlrpc` (:mod:`xmlrpclib`, :mod:`DocXMLRPCServer`,
Some other changes to standard library modules, not covered by
* Killed :mod:`sets`. Use the built-in :func:`set` class.
* 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. Also, the ``'c'`` typecode for array is
gone -- use either ``'b'`` for bytes or ``'u'`` for Unicode
* 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.
* 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`
* The :mod:`tokenize` module has been changed to work with bytes. The
main entry point is now :func:`tokenize.tokenize`, instead of
* :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".)
* 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__`!
:pep:`3101`: A New Approach To String Formatting
* A new system for built-in string formatting operations replaces the
``%`` string formatting operator. (However, the ``%`` operator is
still supported; it will be deprecated in Python 3.1 and removed
from the language at some later time.) Read :pep:`3101` for the full
Changes To Exceptions
The APIs for raising and catching exception have been cleaned up and
new powerful features added:
* :pep:`352`: All exceptions must be derived (directly or indirectly)
from :exc:`BaseException`. This is the root of the exception
hierarchy. This is not new as a 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.) As a consequence, string
exceptions are finally truly and utterly dead.
* Almost all exceptions should actually derive from :exc:`Exception`;
:exc:`BaseException` should only be used as a base class for
exceptions that should only be handled at the top level, such as
:exc:`SystemExit` or :exc:`KeyboardInterrupt`. The recommended
idiom for handling all exceptions except for this latter category is
to use :keyword:`except` :exc:`Exception`.
* :exc:`StandardError` was removed.
* Exceptions no longer behave as sequences. Use the :attr:`args`
attribute instead.
* :pep:`3109`: Raising exceptions. You must now use :samp:`raise
{Exception}({args})` instead of :samp:`raise {Exception}, {args}`.
Additionally, you can no longer explicitly specify a traceback;
instead, if you *have* to do this, you can assign directly to the
:attr:`__traceback__` attribute (see below).
* :pep:`3110`: Catching exceptions. You must now use
:samp:`except {SomeException} as {variable}` instead
of :samp:`except {SomeException}, {variable}`. Moreover, the
*variable* is explicitly deleted when the :keyword:`except` block
is left.
* :pep:`3134`: Exception chaining. There are two cases: implicit
chaining and explicit chaining. Implicit chaining happens when an
exception is raised in an :keyword:`except` or :keyword:`finally`
handler block. This usually happens due to a bug in the handler
block; we call this a *secondary* exception. In this case, the
original exception (that was being handled) is saved as the
:attr:`__context__` attribute of the secondary exception.
Explicit chaining is invoked with this syntax::
raise SecondaryException() from primary_exception
(where *primary_exception* is any expression that produces an
exception object, probably an exception that was previously caught).
In this case, the primary exception is stored on the
:attr:`__cause__` attribute of the secondary exception. The
traceback printed when an unhandled exception occurs walks the chain
of :attr:`__cause__` and :attr:`__context__` attributes and prints a
separate traceback for each component of the chain, with the primary
exception at the top. (Java users may recognize this behavior.)
* :pep:`3134`: Exception objects now store their traceback as the
:attr:`__traceback__` attribute. This means that an exception
object now contains all the information pertaining to an exception,
and there are fewer reasons to use :func:`sys.exc_info` (though the
latter is not removed).
* 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
Miscellaneous Other Changes
Operators And Special Methods
* ``!=`` now returns the opposite of ``==``, unless ``==`` returns
* The concept of "unbound methods" has been 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,
* :pep:`3114`: the standard :meth:`next` method has been renamed to
* 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__`.
* The function attributes named :attr:`func_X` have been renamed to
use the :data:`__X__` form, freeing up these names in the function
attribute namespace for user-defined attributes. To wit,
:attr:`func_closure`, :attr:`func_code`, :attr:`func_defaults`,
:attr:`func_dict`, :attr:`func_doc`, :attr:`func_globals`,
:attr:`func_name` were renamed to :attr:`__closure__`,
:attr:`__code__`, :attr:`__defaults__`, :attr:`~object.__dict__`,
:attr:`__doc__`, :attr:`__globals__`, :attr:`~definition.__name__`,
* :meth:`__nonzero__` is now :meth:`__bool__`.
* :pep:`3135`: New :func:`super`. You can now invoke :func:`super`
without arguments and (assuming this is in a regular instance method
defined inside a :keyword:`class` statement) the right class and
instance will automatically be chosen. With arguments, the behavior
of :func:`super` is unchanged.
* :pep:`3111`: :func:`raw_input` was 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())``.
* A new built-in function :func:`next` was added to call the
:meth:`~iterator.__next__` method on an object.
* The :func:`round` function rounding strategy and return type have
changed. Exact halfway cases are now rounded to the nearest even
result instead of away from zero. (For example, ``round(2.5)`` now
returns ``2`` rather than ``3``.) ``round(x[, n])`` now
delegates to ``x.__round__([n])`` instead of always returning a
float. It generally returns an integer when called with a single
argument and a value of the same type as ``x`` when called with two
* Moved :func:`intern` to :func:`sys.intern`.
* Removed: :func:`apply`. Instead of ``apply(f, args)`` use
* Removed :func:`callable`. Instead of ``callable(f)`` you can use
``isinstance(f, collections.Callable)``. 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
* Removed the :class:`file` type. Use :func:`open`. There are now several
different kinds of streams that open can return in the :mod:`io` module.
* 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
.. ======================================================================
Build and C API Changes
Due to time constraints, here is a *very* incomplete list of changes
to the C API.
* Support for several platforms was dropped, including but not limited
to Mac OS 9, BeOS, RISCOS, Irix, and Tru64.
* :pep:`3118`: New Buffer API.
* :pep:`3121`: Extension Module Initialization & Finalization.
* :pep:`3123`: Making :c:macro:`PyObject_HEAD` conform to standard C.
* No more C API support for restricted execution.
* :c:func:`PyNumber_Coerce`, :c:func:`PyNumber_CoerceEx`,
:c:func:`PyMember_Get`, and :c:func:`PyMember_Set` C APIs are removed.
* New C API :c:func:`PyImport_ImportModuleNoBlock`, works like
:c:func:`PyImport_ImportModule` but won't block on the import lock
(returning an error instead).
* Renamed the boolean conversion C-level slot and method:
``nb_nonzero`` is now ``nb_bool``.
* Removed :c:macro:`METH_OLDARGS` and :c:macro:`WITH_CYCLE_GC` from the C API.
.. ======================================================================
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
.. ======================================================================
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
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. 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 ``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`.
.. ======================================================================