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 3000” 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
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.

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 print()
function, with keyword arguments to replace most of the special syntax
of the old print statement (PEP 3105). Examples:

Old
:
 print
 "The answer is"
,
 2
*
2
New
:
 print
(
"The answer is"
,
 2
*
2
)

Old
:
 print
 x
,
           # Trailing comma suppresses newline
New
:
 print
(
x
,
 end
=
" "
)
  # Appends a space instead of a newline

Old
:
 print
              # Prints a newline
New
:
 print
()
            # You must call the function!

Old
:
 print
 >>
sys
.
stderr
,
 "fatal error"
New
:
 print
(
"fatal error"
,
 file
=
sys
.
stderr
)

Old
:
 print
 (
x
,
 y
)
       # prints repr((x, y))
New
:
 print
((
x
,
 y
))
      # Not the same as print(x, y)!

You can also customize the separator between items, e.g.:

print
(
"There are <"
,
 2
**
32
,
 "> possibilities!"
,
 sep
=
""
)

which produces:

There are <4294967296> possibilities!

Note:

The
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

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:

dict

methods

dict.keys()

,

dict.items()

and

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
dict.iterkeys()

,

dict.iteritems()

and

dict.itervalues()

methods are no longer supported.
map()

and

filter()

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

lambda

), or rewriting the code so it doesn’t need a
list at all. Particularly tricky is

map()

invoked for the
side effects of the function; the correct transformation is to use a
regular

for

loop (since creating a list would just be
wasteful).
range()

now behaves like

xrange()

used to behave, except
it works with values of arbitrary size. The latter no longer
exists.
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

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.
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. Use

__lt__()

for sorting,

__eq__()

with

__hash__()

, 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)

.)

Integers¶

PEP 0237: Essentially,

long

renamed to

int

.
That is, there is only one built-in integral type, named

int

; but it behaves mostly like the old

long

type.
PEP 0238: 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
sys.maxint

constant was removed, since there is no
longer a limit to the value of integers. However,

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

sys.maxint

in previous releases
on the same platform (assuming the same build options).
The
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

str()

instead.)
Octal literals are no longer of the form
0720

; use

0o720

instead.

Text Vs. Data Instead Of Unicode Vs. 8-bit¶

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

str

, the type used to hold data is

bytes

. The biggest difference with the 2.x situation is
that any attempt to mix text and data in Python 3.0 raises

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

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

unicode

for all unencoded text, and

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
str

and

bytes

types cannot be mixed, you
must always explicitly convert between them. Use

str.encode()

to go from

str

to

bytes

, and

bytes.decode()

to go from

bytes

to

str

. You can also use

bytes(s,

encoding=…)

and

str(b,

encoding=…)

,
respectively.
Like
str

, the

bytes

type is immutable. There is a
separate mutable type to hold buffered binary data,

bytearray

. Nearly all APIs that accept

bytes

also
accept

bytearray

. 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
string literals; the euro character is

‘\u20ac’

in Python 3.0.)
The builtin
basestring

abstract type was removed. Use

str

instead. The

str

and

bytes

types
don’t have functionality enough in common to warrant a shared base
class. The

2to3

tool (see below) replaces every occurrence of

basestring

with

str

.
Files opened as text files (still the default mode for
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

codecs

module.
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.

open()

and many functions in the

os

module) that take filenames accept

bytes

objects
as well as strings, and a few APIs have a way to ask for a

bytes

return value. Thus,

os.listdir()

returns a
list of

bytes

instances if the argument is a

bytes

instance, and

os.getcwdb()

returns the current working
directory as a

bytes

instance. Note that when

os.listdir()

returns a list of strings, filenames that
cannot be decoded properly are omitted rather than raising

UnicodeError

.
Some system APIs like
os.environ

and

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

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
StringIO

and

cStringIO

modules are gone. Instead,
import the

io

module and use

io.StringIO

or

io.BytesIO

for text and data respectively.
See also the 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 __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: nonlocal statement. Using nonlocal x
you can now assign directly to a variable in an outer (but
non-global) scope. 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 0274 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 builtin function, bin().
Bytes literals are introduced with a leading b or B, and
there is a new corresponding builtin function, bytes().

Changed Syntax¶

PEP 3109 and PEP 3134: new raise statement syntax:
raise [expr [from expr]]. See below.
as and 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 except exc, var to
except exc 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 __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
object.)
List comprehensions no longer support the syntactic form
[... for var in item1, item2, ...]. Use
[... 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
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
repr()

instead).
Removed
<>

(use

!=

instead).
Removed keyword:
exec()

is no longer a keyword; it remains as
a function. (Fortunately the function syntax was also accepted in
2.x.) Also note that

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
from

module

import

*

syntax is only
allowed at the module level, no longer inside functions.
The only acceptable syntax for relative imports is
from

.[

module

]

import

name

. All

import

forms not starting with

.

are
interpreted as absolute imports. (

PEP 0328)
Classic classes are gone.

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
review:

Many old modules were removed. Some, like gopherlib (no
longer used) and md5 (replaced by hashlib), were
already deprecated by PEP 0004. Others were removed as a result
of the removal of support for various platforms such as Irix, BeOS
and Mac OS 9 (see PEP 0011). 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 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 http://www.jcea.es/programacion/pybsddb.htm.
Some modules were renamed because their old name disobeyed
PEP 0008, 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, pickle and
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 pickle / cPickle pair received this
treatment. The profile module is on the list for 3.1. The
StringIO module has been turned into a class in the io
module.
Some related modules have been grouped into packages, and usually
the submodule names have been simplified. The resulting new
packages are:
- dbm
  
  (
  
  anydbm
  
  ,
  
  dbhash
  
  ,
  
  dbm
  
  ,
  
  dumbdbm
  
  ,
  
  gdbm
  
  ,
  
  whichdb
  
  ).
- html
  
  (
  
  HTMLParser
  
  ,
  
  htmlentitydefs
  
  ).
- http
  
  (
  
  httplib
  
  ,
  
  BaseHTTPServer
  
  ,
  
  CGIHTTPServer
  
  ,
  
  SimpleHTTPServer
  
  ,
  
  Cookie
  
  ,
  
  cookielib
  
  ).
- tkinter
  
  (all
  
  Tkinter
  
  -related modules except
  
  turtle
  
  ). The target audience of
  
  turtle
  
  doesn’t
  really care about
  
  tkinter
  
  . Also note that as of Python
  2.6, the functionality of
  
  turtle
  
  has been greatly enhanced.
- urllib
  
  (
  
  urllib
  
  ,
  
  urllib2
  
  ,
  
  urlparse
  
  ,
  
  robotparse
  
  ).
- xmlrpc
  
  (
  
  xmlrpclib
  
  ,
  
  DocXMLRPCServer
  
  ,
  
  SimpleXMLRPCServer
  
  ).

Some other changes to standard library modules, not covered by
PEP 3108:

Killed
sets

. Use the builtin

set()

function.
Cleanup of the
sys

module: removed

sys.exitfunc()

,

sys.exc_clear()

,

sys.exc_type

,

sys.exc_value

,

sys.exc_traceback

. (Note that

sys.last_type

etc. remain.)
Cleanup of the
array.array

type: the

read()

and

write()

methods are gone; 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
operator

module: removed

sequenceIncludes()

and

isCallable()

.
Cleanup of the
thread

module:

acquire_lock()

and

release_lock()

are gone; use

acquire()

and

release()

instead.
Cleanup of the
random

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

tempfile

module.
The
tokenize

module has been changed to work with bytes. The
main entry point is now

tokenize.tokenize()

, instead of
generate_tokens.
string.letters

and its friends (

string.lowercase

and

string.uppercase

) are gone. Use

string.ascii_letters

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

builtins

(removing the
underscores, adding an ‘s’). The

__builtins__

variable
found in most global namespaces is unchanged. To modify a builtin,
you should use

builtins

, not

__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
scoop.

Changes To Exceptions¶

The APIs for raising and catching exception have been cleaned up and
new powerful features added:

PEP 0352: All exceptions must be derived (directly or indirectly)
from BaseException. This is the root of the exception
hierarchy. This is not new as a recommendation, but the
requirement to inherit from 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 Exception;
BaseException should only be used as a base class for
exceptions that should only be handled at the top level, such as
SystemExit or KeyboardInterrupt. The recommended
idiom for handling all exceptions except for this latter category is
to use except Exception.
StandardError was removed (in 2.6 already).
Exceptions no longer behave as sequences. Use the args
attribute instead.
PEP 3109: Raising exceptions. You must now use raise Exception(args) instead of 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
__traceback__ attribute (see below).
PEP 3110: Catching exceptions. You must now use
except SomeException as variable instead
of except SomeException, variable. Moreover, the
variable is explicitly deleted when the 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 except or 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
__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. The
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.)
PEP 3134: 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 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.

Miscellaneous Other Changes¶

Operators And Special Methods¶

!=

now returns the opposite of

==

, unless

==

returns

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.
__getslice__()

,

__setslice__()

and

__delslice__()

were killed. The syntax

a[i:j]

now translates to

a.__getitem__(slice(i,

j))

(or

__setitem__()

or

__delitem__()

, when used as an assignment or deletion target,
respectively).
PEP 3114: the standard

next()

method has been renamed to

__next__()

.
The
__oct__()

and

__hex__()

special methods are removed
–

oct()

and

hex()

use

__index__()

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. To wit,

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

__bool__()

.

Builtins¶

PEP 3135: New

super()

. You can now invoke

super()

without arguments and (assuming this is in a regular instance method
defined inside a

class

statement) the right class and
instance will automatically be chosen. With arguments, the behavior
of

super()

is unchanged.
PEP 3111:

raw_input()

was renamed to

input()

. That
is, the new

input()

function reads a line from

sys.stdin

and returns it with the trailing newline stripped.
It raises

EOFError

if the input is terminated prematurely.
To get the old behavior of

input()

, use

eval(input())

.
A new builtin
next()

was added to call the

__next__()

method on an object.
Moved
intern()

to

sys.intern()

.
Removed:
apply()

. Instead of

apply(f,

args)

use

f(*args)

.
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

. Use

open()

.
Removed
reduce()

. Use

functools.reduce()

if you really
need it; however, 99 percent of the time an explicit

for

loop is more readable.
Removed
reload()

. Use

imp.reload()

.
Removed.
dict.has_key()

– use the

in

operator
instead.

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

PyObject_HEAD

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
PyImport_ImportModuleNoBlock

, works like

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
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
released!

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
pass.
(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
tree. (See 2to3 – Automated Python 2 to 3 code translation for more on this tool.) 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 Porting Extension Modules to 3.0.

What’s New In Python 3.0 — Python v3.0.1 documentation