| 作者： | Guido van Rossum |
|---|
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.
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
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!
注意：
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"
.
print x
a lot in interactive mode. Time to retrain your fingers to type
print(x)
instead!
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.
Some well-known APIs no longer return lists:
dict
方法
dict.keys()
,
dict.items()
and
dict.values()
return “views” instead of lists. For example, this no longer works:
k = d.keys(); k.sort()
。使用
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 and the input sequences are all of equal length, a quick fix is to wrap
map()
in
list()
，如
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).
If the input sequences are not of equal length,
map()
will stop at the termination of the shortest of the sequences. For full compatibility with
map()
from Python 2.x, also wrap the sequences in
itertools.zip_longest()
，如
map(func, *sequences)
becomes
list(map(func, itertools.zip_longest(*sequences)))
.
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.
Python 3.0 has simplified the rules for ordering comparisons:
<
,
<=
,
>=
,
>
)
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
引发
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”.
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
表达式
(a > b) - (a < b)
as the equivalent for
cmp(a, b)
)。
long
renamed to
int
.
That is, there is only one built-in integral type, named
int
; but it behaves mostly like the old
long
类型。
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.)
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).
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.)
0720
；使用
0o720
代替。
Everything you thought you knew about binary data and Unicode has changed.
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.
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.
u"..."
literals for Unicode text.
However, you must use
b"..."
literals for binary data.
str
and
bytes
types cannot be mixed, you
must always explicitly convert between them. Use
str.encode()
to go from
str
to
bytes
，和
bytes.decode()
to go from
bytes
to
str
。还可以使用
bytes(s, encoding=...)
and
str(b, encoding=...)
,
分别。
str
，
bytes
type is immutable. There is a
separate
可变
type to hold buffered binary data,
bytearray
. Nearly all APIs that accept
bytes
also
accept
bytearray
. The mutable API is based on
collections.MutableSequence
.
'\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.)
basestring
abstract type was removed. Use
str
instead. The
str
and
bytes
类型
don’t have functionality enough in common to warrant a shared base
类。
2to3
tool (see below) replaces every occurrence of
basestring
with
str
.
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
在
codecs
模块。
sys.stdin
,
sys.stdout
and
sys.stderr
are now unicode-only text files (i.e., they are
instances of
io.TextIOBase
). To read and write bytes data
with these streams, you need to use their
io.TextIOBase.buffer
属性。
open()
and many functions in the
os
module) that take filenames accept
bytes
对象
as well as strings, and a few APIs have a way to ask for a
bytes
return value. Thus,
os.listdir()
返回
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
.
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.
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.
StringIO
and
cStringIO
modules are gone. Instead,
import the
io
module and use
io.StringIO
or
io.BytesIO
for text and data respectively.
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
__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
。
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
，和
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}
。注意，
{}
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,
bin()
.
Bytes literals are introduced with a leading
b
or
B
, and there is a new corresponding built-in function,
bytes()
.
PEP 3109
and
PEP 3134
: new
raise
statement syntax:
raise [expr [from expr]]
. See below.
True
,
False
，和
None
are reserved words. (2.6 partially enforced the restrictions on
None
already.)
Change from
except
exc
,
var
to
except
exc
as
var
。见
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, ...]
。使用
[... 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.)
def foo(a, (b, c)): ...
.
使用
def foo(a, b_c): b, c = b_c
代替。
repr()
代替)。
<>
(使用
!=
代替)。
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;
而不是
exec(f)
you can use
exec(f.read())
.
l
or
L
.
u
or
U
.
from
module
import
*
syntax is only
allowed at the module level, no longer inside functions.
from .[module]
import name
. All
import
forms not starting with
.
are
interpreted as absolute imports. (
PEP 0328
)
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 Python 2.6 的新功能 should be consulted for longer descriptions.
with
statement is now a standard
feature and no longer needs to be imported from the
__future__
.
Also check out
Writing Context Managers
and
The contextlib module
.
-m
option when the referenced module lives in a package.
format()
method for both 8-bit and Unicode strings. In 3.0,
only the
str
type (text strings with Unicode support)
supports this method; the
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.
__future__
. More details were given above.
except
exc
as
var
syntax is now standard and
except
exc
,
var
is no
longer supported. (Of course, the
as
var
part is still
optional.)
b"..."
string literal notation (and its
variants like
b'...'
,
b"""..."""
，和
br"..."
) now
produces a literal of type
bytes
.
io
module is now the standard way of
doing file I/O. The built-in
open()
function is now an
alias for
io.open()
and has additional keyword arguments
encoding
,
errors
,
newline
and
closefd
. Also note that an
invalid
mode
argument now raises
ValueError
, not
IOError
. 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).
buffer()
is now really gone;
the new builtin
memoryview()
provides (mostly) similar
功能。
abc
module and the ABCs defined in the
collections
module plays a somewhat more prominent role in
the language now, and built-in collection types like
dict
and
list
conform to the
collections.MutableMapping
and
collections.MutableSequence
ABCs, respectively.
numbers
module is another new use of
ABCs, defining Python’s “numeric tower”. Also note the new
fractions
module which implements
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
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 |
| 队列 | 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
模块。
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 :
sets
. Use the built-in
set()
类。
sys
module: removed
sys.exitfunc()
,
sys.exc_clear()
,
sys.exc_type
,
sys.exc_value
,
sys.exc_traceback
。(注意，
sys.last_type
etc. remain.)
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
字符。
operator
module: removed
sequenceIncludes()
and
isCallable()
.
thread
模块：
acquire_lock()
and
release_lock()
are gone; use
acquire()
and
release()
代替。
random
module: removed the
jumpahead()
API。
new
module is gone.
os.tmpnam()
,
os.tempnam()
and
os.tmpfile()
have been removed in favor of the
tempfile
模块。
tokenize
module has been changed to work with bytes. The
main entry point is now
tokenize.tokenize()
，而不是
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”.)
__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__
!
%
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 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.
Exceptions no longer behave as sequences. Use the
args
attribute instead.
PEP 3109
: Raising exceptions. You must now use
raise
Exception(args)
而不是
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
而不是
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
(在哪里
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
现为
%1 is
not
a
valid
Win32
application
. Strings now deal with non-English locales.
!=
now returns the opposite of
==
，除非
==
返回
NotImplemented
.
__getslice__()
,
__setslice__()
and
__delslice__()
were killed. The syntax
a[i:j]
now translates to
a.__getitem__(slice(i, j))
(或
__setitem__()
or
__delitem__()
, when used as an assignment or deletion target,
分别)。
next()
method has been renamed to
__next__()
.
__oct__()
and
__hex__()
special methods are removed
–
oct()
and
hex()
使用
__index__()
now to convert
the argument to an integer.
__members__
and
__methods__
.
func_X
have been renamed to
使用
__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__
,
分别。
__nonzero__()
现为
__bool__()
.
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.
raw_input()
被重命名为
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()
，使用
eval(input())
.
next()
was added to call the
__next__()
method on an object.
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
返回
2
而不是
3
)。
round(x[, n])
now
委托
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
自变量。
intern()
to
sys.intern()
.
apply()
. Instead of
apply(f, args)
使用
f(*args)
.
callable()
. Instead of
callable(f)
you can use
isinstance(f, collections.Callable)
。
operator.isCallable()
function is also gone.
coerce()
. This function no longer serves a purpose
now that classic classes are gone.
execfile()
. Instead of
execfile(fn)
使用
exec(open(fn).read())
.
file
type. Use
open()
. There are now several
different kinds of streams that open can return in the
io
模块。
reduce()
。使用
functools.reduce()
if you really
need it; however, 99 percent of the time an explicit
for
loop is more readable.
reload()
。使用
imp.reload()
.
dict.has_key()
– use the
in
operator
代替。
Due to time constraints, here is a very incomplete list of changes to the C API.
PyObject_HEAD
conform to standard C.
PyNumber_Coerce()
,
PyNumber_CoerceEx()
,
PyMember_Get()
，和
PyMember_Set()
C APIs are removed.
PyImport_ImportModuleNoBlock()
, works like
PyImport_ImportModule()
but won’t block on the import lock
(returning an error instead).
nb_nonzero
现为
nb_bool
.
METH_OLDARGS
and
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:
-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.
2to3
source-to-source translator over your source code
tree. (See
2to3 - 自动 Python 2 到 3 代码翻译
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 将扩展模块移植到 Python 3 .