Python3.0有什么新变化

Print 是函数¶

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:printx,# 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

• 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\nB\n".

• Initially, you'll be finding yourself typing the old printx

  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 and the input sequences are all of equal length, a quick

  fix is to wrap map() in list(), 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).

  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(), e.g. 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.

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).)

整数¶

• PEP 237: 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 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 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 built-in 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.

• The initial values of 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

  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. 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：现在默认的源码编码格式是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 指南, which was updated for Python 3.0.

新语法¶

• 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 nonlocalx

  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:vfork,vinstuff} 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., {xforxinstuff} 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().

修改的语法¶

• PEP 3109 and PEP 3134: new raise statement syntax:

  raise[expr[fromexpr]]. 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 exceptexc, var to

  exceptexcasvar. See PEP 3110.

• PEP 3115: New Metaclass Syntax. Instead of:

  classC:
  __metaclass__=M
  ...
  

  你现在需要使用:

  classC(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

  [...forvarinitem1,item2,...]. Use

  [...forvarin(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.)

移除的语法¶

• PEP 3113: Tuple parameter unpacking removed. You can no longer

  write deffoo(a,(b,c)):....

  Use deffoo(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 frommoduleimport* syntax is only

  allowed at the module level, no longer inside functions.

• The only acceptable syntax for relative imports is from.[module]

  importname. All import forms not starting with . are

  interpreted as absolute imports. (PEP 328)

• Classic classes are gone.

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 built-in function next() was added to call the

  __next__() method on an object.

• The 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 intern() to sys.intern().

• Removed: apply(). Instead of apply(f,args) use

  f(*args).

• Removed callable(). Instead of callable(f) you can use

  isinstance(f,collections.Callable). 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 the file type. Use open(). There are now several

  different kinds of streams that open can return in the io module.

• 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.