What’s New In Python 3.0 & Python v3.0.1 documentation
Navigation
What’s New In Python 3.0
Author:Guido van Rossum
Release:3.0.1
Date:February 14, 2009
This article explains the new features in Python 3.0, compared to 2.6.
Python 3.0, also known as “Python ; or “Py3K”, is the first ever
intentionally backwards incompatible Python release.
There are more
changes than in a typical release, and more that are important for all
Python users.
Nevertheless, after digesting the changes, you’ll find
that Python really hasn’t changed all that much – by and large, we’re
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
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.
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
function, with keyword arguments to replace most of the special syntax
of the old print statement ().
Old: print &The answer is&, 2*2
New: print(&The answer is&, 2*2)
Old: print x,
# Trailing comma suppresses newline
New: print(x, end=& &)
# Appends a space instead of a newline
Old: print
# Prints a newline
New: print()
# You must call the function!
Old: print &&sys.stderr, &fatal error&
New: print(&fatal error&, file=sys.stderr)
Old: print (x, y)
# prints repr((x, y))
New: print((x, y))
# Not the same as print(x, y)!
You can also customize the separator between items, e.g.:
print(&There are &&, 2**32, && possibilities!&, sep=&&)
which produces:
There are && possibilities!
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
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:
return “views” instead of lists.
For example,
this no longer works: k = d.keys(); k.sort().
sorted(d) instead (this works in Python 2.5 too and is just
as efficient).
Also, the dict.iterkeys(), dict.iteritems() and
dict.itervalues() methods are no longer supported.
return iterators.
If you really need
a list, a quick fix is e.g.
list(map(...)), but a better fix is
often to use a list comprehension (especially when the original code
uses ), or rewriting the code so it doesn’t need a
list at all.
Particularly tricky is
invoked for the
side eff the correct transformation is to use a
loop (since creating a list would just be
wasteful).
now behaves like xrange() used to behave, except
it works with values of arbitrary size.
The latter no longer
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
instead of returning
A corollary is that sorting a heterogeneous list
no longer makes sense – all the elements must be comparable to each
Note that this does not apply to the == and !=
operators: objects of different incomparable types always compare
unequal to each other.
builtin.sorted() and 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 cmp() function should be treated as gone, and the __cmp__()
special method is no longer supported.
for sorting,
with , and other rich comparisons as needed.
(If you really need the cmp() functionality, you could use the
expression (a & b) - (a & b) as the equivalent for cmp(a, b).)
: Essentially, long renamed to .
That is, there is only one built-in integral type, named
; but it behaves mostly like the old long type.
: An expression like 1/2 returns a float.
1//2 to get the truncating behavior.
(The latter syntax has
existed for years, at least since Python 2.2.)
The sys.maxint constant was removed, since there is no
longer a limit to the value of integers.
can be used as an integer larger than any practical list or string
It conforms to the implementation’s “natural” integer size
and is typically the same as sys.maxint in previous releases
on the same platform (assuming the same build options).
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.
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 U however
encoded Unicode is represented as binary data.
The type used to
hold text is , the type used to hold data is
The biggest difference with the 2.x situation is
that any attempt to mix text and data in Python 3.0 raises
, 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
if it contained non-ASCII values.
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
The change is for the better, as in the 2.x world there
were numerous bugs having to do with mixing encoded and unencoded
To be prepared in Python 2.x, start using unicode
for all unencoded text, and
for binary or encoded data
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.
types cannot be mixed, you
must always explicitly convert between them.
to go from
to , and bytes.decode()
to go from
You can also use
bytes(s, encoding=...) and str(b, encoding=...),
respectively.
Like , the
type is immutable.
There is a
separate mutable type to hold buffered binary data,
Nearly all APIs that accept
The mutable API is based on
collections.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
the euro character is '\u20ac' in Python 3.0.)
The builtin basestring abstract type was removed.
don’t have functionality enough in common to warrant a shared base
The 2to3 tool (see below) replaces every occurrence of
basestring with .
Files opened as text files (still the default mode for )
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).
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
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.)
work-around, most APIs (e.g.
and many functions in the
module) that take filenames accept
as well as strings, and a few APIs have a way to ask for a
return value.
instances if the argument is a
instance, and
returns the current working
directory as a
Note that when
returns a list of strings, filenames that
cannot be decoded properly are omitted rather than raising
Some system APIs like
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.
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.
: The default source encoding is now UTF-8.
: Non-ASCII letters are now allowed in identifiers.
(However, the standard library remains ASCII-only with the exception
of contributor names in comments.)
The StringIO and cStringIO modules are gone.
import the
module and use
for text and data respectively.
See also the , 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
: Function argument and return value annotations.
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 __annotations__ attribute.
The intent is to encourage
experimentation through metaclasses, decorators or frameworks.
: 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.
statement.
Using nonlocal x
you can now assign directly to a variable in an outer (but
non-global) scope.
is a new reserved word.
: Extended Iterable Unpacking.
You can now write things
like a, b, *rest = some_sequence.
And even *rest, a =
The rest object is always a (possibly empty) the
right-hand side may be any iterable.
(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.
vindicated. :-)
Set literals, e.g. {1, 2}.
Note that {} is an empty
use set() for an empty set.
Set comprehensions are
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 builtin function, .
Bytes literals are introduced with a leading b or B, and
there is a new corresponding builtin function, .
Changed Syntax
statement syntax:
raise [expr [from expr]].
See below.
are now reserved words.
2.6, actually.)
True, False, and None are reserved
(2.6 partially enforced the restrictions on None
Change from
exc, var to
: New Metaclass Syntax.
Instead of:
__metaclass__ = M
you must now use:
class C(metaclass=M):
The module-global __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
[... for var in item1, item2, ...].
[... 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
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
(Previously it was only allowed in slices.)
must now be spelled as ....
(Previously it could also be
spelled as . . ., by a mere accident of the grammar.)
Removed Syntax
: 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
Removed && (use != instead).
Removed keyword:
is it remains as
a function.
(Fortunately the function syntax was also accepted in
Also note that
no longer tak
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.
* syntax is only
allowed at the module level, no longer inside functions.
The only acceptable syntax for relative imports is from .[module]
import name.
forms not starting with . are
interpreted as absolute imports.
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
The corresponding sections in
consulted for longer descriptions.
statement is now a standard
feature and no longer needs to be imported from the .
Also check out
This enhances the usefulness of the
option when the referenced module lives in a package.
Note: the 2.6 description mentions the
method for both 8-bit and Unicode strings.
type (text strings with Unicode support)
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.
This is now a standard feature and no longer needs
to be imported from .
More details were given above.
syntax is now standard and
exc, var is no
longer supported.
(Of course, the
var part is still
optional.)
The b&...& string literal notation (and its
variants like b'...', b&&&...&&&, and br&...&) now
produces a literal of type .
module is now the standard way of
doing file I/O, and the initial values of ,
are now instances of
The builtin
function is now an
and has additional keyword arguments
encoding, errors, newline and closefd.
Also note that an
invalid mode argument now raises , not
The binary file object underlying a text file
object can be accessed as f.buffer (but beware that the
text object maintains a buffer of itself in order to speed up
the encoding and decoding operations).
The old builtin buffer()
the new builtin
provides (mostly) similar
functionality.
module and the ABCs defined in the
module plays a somewhat more prominent role in
the language now, and builtin collection types like
conform to the collections.MutableMapping
and collections.MutableSequence ABCs, respectively.
As mentioned above, the new octal literal
notation is the only one supported, and binary literals have been
module is another new use of
ABCs, defining Python’s “numeric tower”.
Also note the new
module which implements .
Library Changes
Due to time constraints, this document does not exhaustively cover the
very extensive changes to the standard library.
reference for the major changes to the library.
Here’s a capsule
Many old modules were removed.
Some, like gopherlib (no
longer used) and md5 (replaced by ), were
already deprecated by .
Others were removed as a result
of the removal of support for various platforms such as Irix, BeOS
and Mac OS 9 (see ).
Some modules were also selected for
removal in Python 3.0 due to lack of use or because a better
replacement exists.
for an exhaustive list.
The 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
, or for various other reasons.
Here’s the list:
ConfigParser
configparser
SocketServer
socketserver
markupbase
_markupbase
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 for example,
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
/ cPickle pair received this
treatment.
The profile module is on the list for 3.1.
StringIO module has been turned into a class in the
Some related modules have been grouped into packages, and usually
the submodule names have been simplified.
The resulting new
packages are:
(anydbm, dbhash, ,
dumbdbm, gdbm, whichdb).
html (HTMLParser, htmlentitydefs).
http (httplib, BaseHTTPServer,
CGIHTTPServer, SimpleHTTPServer, Cookie,
cookielib).
(all Tkinter-related modules except
The target audience of
doesn’t
really care about .
Also note that as of Python
2.6, the functionality of
has been greatly enhanced.
urllib (urllib, urllib2, urlparse,
robotparse).
xmlrpc (xmlrpclib, DocXMLRPCServer,
SimpleXMLRPCServer).
Some other changes to standard library modules, not covered by
Killed sets.
Use the builtin
Cleanup of the
module: removed sys.exitfunc(),
sys.exc_clear(), sys.exc_type, sys.exc_value,
sys.exc_traceback.
(Note that
etc. remain.)
Cleanup of the
type: the read() and
write() use fromfile() and
tofile() instead.
Also, the 'c' typecode for array is
gone – use either 'b' for bytes or 'u' for Unicode
characters.
Cleanup of the
module: removed
sequenceIncludes() and isCallable().
Cleanup of the thread module: acquire_lock() and
release_lock() use acquire() and
release() instead.
Cleanup of the
module: removed the jumpahead() API.
The new module is gone.
The functions os.tmpnam(), os.tempnam() and
os.tmpfile() have been removed in favor of the
module has been changed to work with bytes.
main entry point is now , instead of
generate_tokens.
string.letters and its friends (string.lowercase and
string.uppercase) are gone.
etc. instead.
(The reason for the
removal is that string.letters and friends had
locale-specific behavior, which is a bad idea for such
attractively-named global “constants”.)
Renamed module __builtin__ to
(removing the
underscores, adding an ‘s’).
The __builtins__ variable
found in most global namespaces is unchanged.
To modify a builtin,
you should use , not __builtins__!
: A New Approach To String Formatting
A new system for
built-in string formatting operations replaces the
formatting operator.
(However, the %
operator is
be deprecated in
Python 3.1
and removed
from the language at some later time.)
for the full
Changes To Exceptions
The APIs for raising and catching exception have been cleaned up and
new powerful features added:
: All exceptions must be derived (directly or indirectly)
This is the root of the exception
hierarchy.
This is not new as a recommendation, but the
requirement to inherit from
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 a
should only be used as a base class for
exceptions that should only be handled at the top level, such as
The recommended
idiom for handling all exceptions except for this latter category is
StandardError was removed (in 2.6 already).
Exceptions no longer behave as sequences.
Use the args
attribute instead.
: Raising exceptions.
You must now use raise
Exception(args) instead of raise Exception, args.
Additionally, you can no longer explicitly
instead, if you have to do this, you can assign directly to the
__traceback__ attribute (see below).
: Catching exceptions.
You must now use
except SomeException as variable instead
of except SomeException, variable.
Moreover, the
variable is explicitly deleted when the
: Exception chaining.
There are two cases: implicit
chaining and explicit chaining.
Implicit chaining happens when an
exception is raised in an
handler block.
This usually happens due to a bug in the handler
we call this a secondary exception.
In this case, the
original exception (that was being handled) is saved as the
__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
__cause__ attribute of the secondary exception.
traceback printed when an unhandled exception occurs walks the chain
of __cause__ and __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.)
: Exception objects now store their traceback as the
__traceback__ attribute.
This means that an exception
object now contains all the information pertaining to an exception,
and there are fewer reasons to use
(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.
__getslice__(), __setslice__() and __delslice__()
were killed.
The syntax a[i:j] now translates to
a.__getitem__(slice(i, j)) (or
, when used as an assignment or deletion target,
respectively).
: the standard
method has been renamed to
__next__().
The __oct__() and __hex__() special methods are removed
now to convert
the argument to an integer.
Removed support for __members__ and __methods__.
The function attributes named func_X have been renamed to
use the __X__ form, freeing up these names in the function
attribute namespace for user-defined attributes.
func_closure, func_code, func_defaults,
func_dict, func_doc, func_globals,
func_name were renamed to __closure__,
__code__, __defaults__, __dict__,
__doc__, __globals__, __name__,
respectively.
__nonzero__() is now .
You can now invoke
without arguments and (assuming this is in a regular instance method
defined inside a
statement) the right class and
instance will automatically be chosen.
With arguments, the behavior
is unchanged.
: raw_input() was renamed to .
is, the new
function reads a line from
and returns it with the trailing newline stripped.
if the input is terminated prematurely.
To get the old behavior of , use eval(input()).
A new builtin
was added to call the __next__()
method on an object.
Moved intern() to .
Removed: apply().
Instead of apply(f, args) use
Removed callable().
Instead of callable(f) you can use
hasattr(f, '__call__').
The operator.isCallable() function
is also gone.
Removed coerce().
This function no longer serves a purpose
now that classic classes are gone.
Removed execfile().
Instead of execfile(fn) use
exec(open(fn).read()).
Removed file.
Removed reduce().
if you really
however, 99 percent of the time an explicit
loop is more readable.
Removed reload().
Removed. dict.has_key() – use the
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.
: New Buffer API.
: Extension Module Initialization & Finalization.
conform to standard C.
No more C API support for restricted execution.
PyNumber_Coerce, PyNumber_CoerceEx,
PyMember_Get, and PyMember_Set C APIs are removed.
New C API , works like
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 METH_OLDARGS and WITH_CYCLE_GC from the C API.
Performance
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:
(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
(Still using 2.6:) Turn on the -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
for more on this tool.)
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 .
Navigation