Doc/whatsnew/3.0.rst
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. GvR.)
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. (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 module. (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. GvR.)
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 .. ======================================================================
This section lists those few changes that are most likely to trip you up if you're used to Python 2.5.
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", 22 New: print("The answer is", 22)
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!
Note:
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.
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
wasteful).
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
exists.
:func:zip now returns an iterator.
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
orlen <= lenare no longer valid, and e.g.None < Noneraises :exc:`TypeError` instead of returningFalse. 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: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 should be treated as gone, and the :meth:!__cmp__
special method is no longer supported. Use :meth:~object.__lt__ for sorting,
:meth:~object.__eq__ with :meth:~object.__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
instead.
Everything you thought you knew about binary data and Unicode has changed.
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
years.
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=...),
respectively.
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
:class:collections.MutableSequence <collections.abc.MutableSequence>.
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
attribute.
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
:exc:UnicodeError.
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.
This section gives a brief overview of every syntactic change in Python 3.0.
: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:~object.__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.
: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
:class:object.)
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.)
: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 :samp:from .[{module}] import {name}. All :keyword:import forms not starting with . are
interpreted as absolute imports. (:pep:328)
Classic classes are gone.
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: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__. 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
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.
: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: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. 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
functionality.
: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 <collections.abc.MutableMapping>
and :class:collections.MutableSequence <collections.abc.MutableSequence> ABCs, 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.
: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.
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: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 https://www.jcea.es/programacion/pybsddb.htm.
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 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 / :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
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 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:!robotparse).
:mod:xmlrpc (:mod:!xmlrpclib, :mod:!DocXMLRPCServer,
:mod:!SimpleXMLRPCServer).
Some other changes to standard library modules, not covered by
:pep:3108:
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:~array.array.fromfile and
:meth:~array.array.tofile instead. Also, the 'c' typecode for array is
gone -- use either 'b' for bytes or 'u' for Unicode
characters.
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 :meth:~threading.Lock.acquire and
:meth:~threading.Lock.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
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.
: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__!
3101: A New Approach To String Formatting% 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
scoop.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:~BaseException.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:~BaseException.__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:~BaseException.__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:~BaseException.__cause__ attribute of the secondary exception. The
traceback printed when an unhandled exception occurs walks the chain
of :attr:!__cause__ and :attr:~BaseException.__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:~BaseException.__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
locales.
!= now returns the opposite of ==, unless == returns
:data:NotImplemented.
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:~object.__setitem__ or
:meth:~object.__delitem__, when used as an assignment or deletion target,
respectively).
:pep:3114: the standard :meth:next method has been renamed to
:meth:~iterator.__next__.
The :meth:!__oct__ and :meth:!__hex__ special methods are removed
-- :func:oct and :func:hex use :meth:~object.__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 :attr:!__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:~function.__closure__,
:attr:~function.__code__, :attr:~function.__defaults__,
:attr:~function.__dict__, :attr:~function.__doc__,
:attr:~function.__globals__, :attr:~function.__name__,
respectively.
:meth:!__nonzero__ is now :meth:~object.__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
arguments.
Moved :func:!intern to :func:sys.intern.
Removed: :func:!apply. Instead of apply(f, args) use
f(*args).
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
exec(open(fn).read()).
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
instead.
.. ======================================================================
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 released!
.. ======================================================================
For porting existing Python 2.5 or 2.6 source code to Python 3.0, the best strategy is the following:
(Prerequisite:) Start with excellent test coverage.
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.
(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.
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.
.. ======================================================================