Python 2.2 有什么新变化
***********************

作者:
   A.M. Kuchling


概述
====

本文本介绍了 Python 2.2.2 的新增特性，该版本发布于 2002 年 10 月 14日
。 Python 2.2.2 是 Python 2.2 的问题修正发布版，最初发布于 2001 年 12
月 21 日。

Python 2.2 可以被看作是 "清理发布版"。 有一些特性如生成器和迭代器等是
全新的，但大多数变化，尽管可能是重大而深远的，都是为了清理语言设计中的
不规范和阴暗角落。

本文并不试图提供对新特性的完整规范说明，而是提供一个便捷的概览。 要获
取全部细节，你应该参阅 Python 2.2 的文档，比如 Python 库参考 和 Python
参考指南。 如果你想要了解某项更改的完整实现和设计理念，请参阅特定新特
性的 PEP。


PEP 252 和 253：类型和类的修改
==============================

Python 2.2 中最大且影响最深远的改变是针对 Python 的对象和类模型。 这些
变化应该是向下兼容的，因此你的代码将能继续运行而无需修改，但这些变化提
供了一些很棒的新功能。 在开始本文最长和最复杂的部分之前，我提供对这些
变化的概览并附带一些注释。

很久以前，我写过一个网页，列出了 Python 设计中的一些缺陷。其中一个最显
著的缺陷是无法对子类化用 C 实现的 Python 类型。特别是，无法对子类化内
置类型，因此你不能简单地对子类化列表，以便向其添加一个有用的方法。虽然
"UserList" 模块提供了一个支持所有列表方法的类并且可以进一步子类化，但
有很多 C 代码期望的是一个普通的 Python 列表，不能接受 "UserList" 实例
。

Python 2.2 修正了此问题，并在此过程中添加了一些令人激动的新功能。 简明
概述如下:

* 你可以继承内置类型，例如列表和整数，并且你的子类应该在任何需要原始类
  型的地方正常工作。这使得 Python 的面向对象编程更加灵活和强大。

* 现在，除了之前版本的 Python 中可用的实例方法外，还可以定义静态方法和
  类方法。这使得你可以更灵活地组织类的行为。

* 还可以通过使用一种称为 *属性* 的新机制，在访问或设置实例属性时自动调
  用方法。许多 "__getattr__()" 的用法可以重写为使用属性，从而使代码更
  简单、更快速。作为一个小的额外好处，属性现在也可以有文档字符串。

* 可以使用 __slots__ 限制实例的合法属性列表，从而防止拼写错误，并且在
  未来的 Python 版本中可能进行更多的优化。

一些用户对这些变化表示担忧。确实，他们说，新功能很棒，可以实现以前版本
的 Python 无法做到的各种技巧，但它们也使语言变得更加复杂。一些人表示，
他们一直推荐 Python 是因为它的简单性，现在感觉这种简单性正在丧失。

个人而言，我认为没有必要担心。许多新功能相当深奥，你可以编写大量
Python 代码而不需要了解它们。编写一个简单的类并不比以前更难，因此除非
确实需要，否则你不必费心去学习或教授这些新功能。一些以前只有在 C 语言
中才能实现的非常复杂的任务，现在可以用纯 Python 实现，在我看来，这一切
都更好了。

我不会尝试涵盖所有为了使新功能生效而需要的每一个边缘情况和小改动。相反
，本节将只勾勒出大致的轮廓。有关 Python 2.2 新对象模型的更多信息，请参
见 相关链接 的“相关链接”部分。


旧式类和新式类
--------------

首先，你应该知道 Python 2.2 实际上有两种类型的类：经典类（或旧式类）和
新式类。旧式类模型与早期版本的 Python 中的类模型完全相同。本节描述的所
有新功能仅适用于新式类。这种分歧并不是永久的；最终，旧式类将被淘汰，可
能在 Python 3.0 中被移除。

那么如何定义一个新式类呢？你可以通过继承一个现有的新式类来实现。大多数
Python 内置类型，如整数、列表、字典，甚至文件，现在都是新式类。此外，
还添加了一个名为 "object" 的新式类，它是所有内置类型的基类，因此如果没
有合适的内置类型，你可以直接继承 "object" 类：

   class C(object):
       def __init__ (self):
           ...
       ...

这意味着在 Python 2.2 中，不带任何基类的 "class" 声明总是经典类。（实
际上，你也可以通过设置一个名为 "__metaclass__" 的模块级变量来改变这一
点——详见 **PEP 253** ——但更简单的方法是直接继承 "object"。）

内置类型的类型对象在 Python 2.2 中作为内置对象提供，使用了一种巧妙的技
巧命名。Python 一直有名为 "int()"、"float()" 和 "str()" 的内置函数。在
Python 2.2 中，它们不再是函数，而是作为被调用时表现为工厂的类型对象。

   >>> int
   <type 'int'>
   >>> int('123')
   123

为了使类型集合完整，Python 2.2 中还添加了新的类型对象，例如 "dict()"
和 "file()"。以下是一个更有趣的例子，向文件对象添加一个 "lock()" 方法
：

   class LockableFile(file):
       def lock (self, operation, length=0, start=0, whence=0):
           import fcntl
           return fcntl.lockf(self.fileno(), operation,
                              length, start, whence)

现在已经过时的 "posixfile" 模块包含一个类，该类模仿了文件对象的所有方
法，并添加了一个 "lock()" 方法，但这个类不能传递给期望内置文件对象的内
部函数，而这在我们的新 "LockableFile" 实现中是可能的。


描述器
------

在以前的 Python 版本中，没有一致的方法来发现对象支持的属性和方法。有一
些非正式的约定，例如定义 "__members__" 和 "__methods__" 属性，这些属性
是名称列表，但扩展类型或类的作者往往不会去定义它们。你可以退而求其次，
检查对象的 "__dict__" 属性，但在使用类继承或任意的 "__getattr__()" 钩
子时，这仍然可能是不准确的。

新类模型的一个核心理念是正式化了使用描述符来描述对象属性的 API。描述符
指定属性的值，说明它是方法还是字段。通过描述符 API，静态方法和类方法成
为可能，以及其他更复杂的构造。

属性描述符是存在于类对象内部的对象，它们自身具有一些属性。描述符协议由
三个主要方法组成：

* "__name__" 是属性的名称。

* "__doc__" 是属性的文档字符串。

* "__get__(object)" 是一个从 *object* 中提取属性值的方法。

* "__set__(object, value)" 将 *object* 上的属性设为 *value*。

* "__delete__(object, value)" 将删除 *object* 的 *value* 属性。

例如，当你写下 "obj.x"，Python 实际要执行的步骤是:

   descriptor = obj.__class__.x
   descriptor.__get__(obj)

对于方法，"descriptor.__get__()" 返回一个可调用的临时对象，它将实例和
要调用的方法封装在一起。这也是为什么现在可以实现静态方法和类方法的原因
；它们有描述符，可以只封装方法，或者封装方法和类。作为对这些新方法类型
的简要说明，静态方法不传递实例，因此类似于常规函数。类方法传递对象的类
，但不传递对象本身。静态方法和类方法的定义如下：

   class C(object):
       def f(arg1, arg2):
           ...
       f = staticmethod(f)

       def g(cls, arg1, arg2):
           ...
       g = classmethod(g)

The "staticmethod()" function takes the function "f()", and returns it
wrapped up in a descriptor so it can be stored in the class object.
You might expect there to be special syntax for creating such methods
("def static f", "defstatic f()", or something like that) but no such
syntax has been defined yet; that's been left for future versions of
Python.

更多的新功能，如 __slots__ 和属性，也作为新类型的描述符实现。编写一个
实现新功能的描述符类并不困难。例如，可以编写一个描述符类，使其能够为方
法编写类似 Eiffel 风格的前置条件和后置条件。使用该功能的类可能定义如下
：

   from eiffel import eiffelmethod

   class C(object):
       def f(self, arg1, arg2):
           # The actual function
           ...
       def pre_f(self):
           # Check preconditions
           ...
       def post_f(self):
           # Check postconditions
           ...

       f = eiffelmethod(f, pre_f, post_f)

Note that a person using the new "eiffelmethod()" doesn't have to
understand anything about descriptors.  This is why I think the new
features don't increase the basic complexity of the language. There
will be a few wizards who need to know about it in order to write
"eiffelmethod()" or the ZODB or whatever, but most users will just
write code on top of the resulting libraries and ignore the
implementation details.


多重继承：钻石规则
------------------

Multiple inheritance has also been made more useful through changing
the rules under which names are resolved.  Consider this set of
classes (diagram taken from **PEP 253** by Guido van Rossum):

         class A:
           ^ ^  def save(self): ...
          /   \
         /     \
        /       \
       /         \
   class B     class C:
       ^         ^  def save(self): ...
        \       /
         \     /
          \   /
           \ /
         class D

The lookup rule for classic classes is simple but not very smart; the
base classes are searched depth-first, going from left to right.  A
reference to "D.save()" will search the classes "D", "B", and then
"A", where "save()" would be found and returned.  "C.save()" would
never be found at all.  This is bad, because if "C"'s "save()" method
is saving some internal state specific to "C", not calling it will
result in that state never getting saved.

新式类遵循一种不同的算法，虽然解释起来有点复杂，但在这种情况下能做正确
的事情。（请注意，Python 2.3 改变了这个算法，在大多数情况下会产生相同
的结果，但对于非常复杂的继承图会产生更有用的结果。）

1. List all the base classes, following the classic lookup rule and
   include a class multiple times if it's visited repeatedly.  In the
   above example, the list of visited classes is ["D", "B", "A", "C",
   "A"].

2. Scan the list for duplicated classes.  If any are found, remove all
   but one occurrence, leaving the *last* one in the list.  In the
   above example, the list becomes ["D", "B", "C", "A"] after dropping
   duplicates.

Following this rule, referring to "D.save()" will return "C.save()",
which is the behaviour we're after.  This lookup rule is the same as
the one followed by Common Lisp.  A new built-in function, "super()",
provides a way to get at a class's superclasses without having to
reimplement Python's algorithm. The most commonly used form will be
"super(class, obj)", which returns  a bound superclass object (not the
actual class object).  This form will be used in methods to call a
method in the superclass; for example, "D"'s "save()" method would
look like this:

   class D (B,C):
       def save (self):
           # Call superclass .save()
           super(D, self).save()
           # Save D's private information here
           ...

"super()" 在以 "super(class)" 或 "super(class1, class2)" 形式调用时也
可以返回未绑定的超类对象，但这可能并不常用。


属性访问
--------

A fair number of sophisticated Python classes define hooks for
attribute access using "__getattr__()"; most commonly this is done for
convenience, to make code more readable by automatically mapping an
attribute access such as "obj.parent" into a method call such as
"obj.get_parent".  Python 2.2 adds some new ways of controlling
attribute access.

First, "__getattr__(attr_name)" is still supported by new-style
classes, and nothing about it has changed.  As before, it will be
called when an attempt is made to access "obj.foo" and no attribute
named "foo" is found in the instance's dictionary.

New-style classes also support a new method,
"__getattribute__(attr_name)".  The difference between the two methods
is that "__getattribute__()" is *always* called whenever any attribute
is accessed, while the old "__getattr__()" is only called if "foo"
isn't found in the instance's dictionary.

However, Python 2.2's support for *properties* will often be a simpler
way to trap attribute references.  Writing a "__getattr__()" method is
complicated because to avoid recursion you can't use regular attribute
accesses inside them, and instead have to mess around with the
contents of "__dict__". "__getattr__()" methods also end up being
called by Python when it checks for other methods such as "__repr__()"
or "__coerce__()", and so have to be written with this in mind.
Finally, calling a function on every attribute access results in a
sizable performance loss.

"property" is a new built-in type that packages up three functions
that get, set, or delete an attribute, and a docstring.  For example,
if you want to define a "size" attribute that's computed, but also
settable, you could write:

   class C(object):
       def get_size (self):
           result = ... computation ...
           return result
       def set_size (self, size):
           ... compute something based on the size
           and set internal state appropriately ...

       # Define a property.  The 'delete this attribute'
       # method is defined as None, so the attribute
       # can't be deleted.
       size = property(get_size, set_size,
                       None,
                       "Storage size of this instance")

That is certainly clearer and easier to write than a pair of
"__getattr__()"/"__setattr__()" methods that check for the "size"
attribute and handle it specially while retrieving all other
attributes from the instance's "__dict__".  Accesses to "size" are
also the only ones which have to perform the work of calling a
function, so references to other attributes run at their usual speed.

Finally, it's possible to constrain the list of attributes that can be
referenced on an object using the new "__slots__" class attribute.
Python objects are usually very dynamic; at any time it's possible to
define a new attribute on an instance by just doing "obj.new_attr=1".
A new-style class can define a class attribute named "__slots__" to
limit the legal attributes  to a particular set of names.  An example
will make this clear:

   >>> class C(object):
   ...     __slots__ = ('template', 'name')
   ...
   >>> obj = C()
   >>> print obj.template
   None
   >>> obj.template = 'Test'
   >>> print obj.template
   Test
   >>> obj.newattr = None
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   AttributeError: 'C' object has no attribute 'newattr'

Note how you get an "AttributeError" on the attempt to assign to an
attribute not listed in "__slots__".


相关链接
--------

本节只是对新特性进行了简要概述，提供了足够的解释以帮助你开始编程，但许
多细节已被简化或忽略。想要获得更全面的了解，你可以去哪里呢？

描述器指南 是由 Guido van Rossum 编写的一篇详细教程，介绍了描述符功能
。如果我的描述激起了你的兴趣，请继续阅读这篇教程，因为它更加详细地介绍
了这些新功能，同时仍然保持了相当的易读性。

Next, there are two relevant PEPs, **PEP 252** and **PEP 253**.  **PEP
252** is titled "Making Types Look More Like Classes", and covers the
descriptor API. **PEP 253** is titled "Subtyping Built-in Types", and
describes the changes to type objects that make it possible to subtype
built-in objects.  **PEP 253** is the more complicated PEP of the two,
and at a few points the necessary explanations of types and meta-types
may cause your head to explode.  Both PEPs were written and
implemented by Guido van Rossum, with substantial assistance from the
rest of the Zope Corp. team.

Finally, there's the ultimate authority: the source code.  Most of the
machinery for the type handling is in "Objects/typeobject.c", but you
should only resort to it after all other avenues have been exhausted,
including posting a question to python-list or python-dev.


PEP 234: 迭代器
===============

Python 2.2 的另一个重要新增功能是在 C 和 Python 两个层面上引入了迭代接
口。对象可以定义如何被调用者循环遍历。

In Python versions up to 2.1, the usual way to make "for item in obj"
work is to define a "__getitem__()" method that looks something like
this:

   def __getitem__(self, index):
       return <next item>

"__getitem__()" is more properly used to define an indexing operation
on an object so that you can write "obj[5]" to retrieve the sixth
element.  It's a bit misleading when you're using this only to support
"for" loops. Consider some file-like object that wants to be looped
over; the *index* parameter is essentially meaningless, as the class
probably assumes that a series of "__getitem__()" calls will be made
with *index* incrementing by one each time.  In other words, the
presence of the "__getitem__()" method doesn't mean that using
"file[5]"  to randomly access the sixth element will work, though it
really should.

In Python 2.2, iteration can be implemented separately, and
"__getitem__()" methods can be limited to classes that really do
support random access.  The basic idea of iterators is  simple.  A new
built-in function, "iter(obj)" or "iter(C, sentinel)", is used to get
an iterator. "iter(obj)" returns an iterator for the object *obj*,
while "iter(C, sentinel)" returns an iterator that will invoke the
callable object *C* until it returns *sentinel* to signal that the
iterator is done.

Python classes can define an "__iter__()" method, which should create
and return a new iterator for the object; if the object is its own
iterator, this method can just return "self".  In particular,
iterators will usually be their own iterators.  Extension types
implemented in C can implement a "tp_iter" function in order to return
an iterator, and extension types that want to behave as iterators can
define a "tp_iternext" function.

So, after all this, what do iterators actually do?  They have one
required method, "next()", which takes no arguments and returns the
next value.  When there are no more values to be returned, calling
"next()" should raise the "StopIteration" exception.

   >>> L = [1,2,3]
   >>> i = iter(L)
   >>> print i
   <iterator object at 0x8116870>
   >>> i.next()
   1
   >>> i.next()
   2
   >>> i.next()
   3
   >>> i.next()
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   StopIteration
   >>>

In 2.2, Python's "for" statement no longer expects a sequence; it
expects something for which "iter()" will return an iterator. For
backward compatibility and convenience, an iterator is automatically
constructed for sequences that don't implement "__iter__()" or a
"tp_iter" slot, so "for i in [1,2,3]" will still work.  Wherever the
Python interpreter loops over a sequence, it's been changed to use the
iterator protocol.  This means you can do things like this:

   >>> L = [1,2,3]
   >>> i = iter(L)
   >>> a,b,c = i
   >>> a,b,c
   (1, 2, 3)

Iterator support has been added to some of Python's basic types.
Calling "iter()" on a dictionary will return an iterator which loops
over its keys:

   >>> m = {'Jan': 1, 'Feb': 2, 'Mar': 3, 'Apr': 4, 'May': 5, 'Jun': 6,
   ...      'Jul': 7, 'Aug': 8, 'Sep': 9, 'Oct': 10, 'Nov': 11, 'Dec': 12}
   >>> for key in m: print key, m[key]
   ...
   Mar 3
   Feb 2
   Aug 8
   Sep 9
   May 5
   Jun 6
   Jul 7
   Jan 1
   Apr 4
   Nov 11
   Dec 12
   Oct 10

That's just the default behaviour.  If you want to iterate over keys,
values, or key/value pairs, you can explicitly call the "iterkeys()",
"itervalues()", or "iteritems()" methods to get an appropriate
iterator. In a minor related change, the "in" operator now works on
dictionaries, so "key in dict" is now equivalent to
"dict.has_key(key)".

Files also provide an iterator, which calls the "readline()" method
until there are no more lines in the file.  This means you can now
read each line of a file using code like this:

   for line in file:
       # do something for each line
       ...

Note that you can only go forward in an iterator; there's no way to
get the previous element, reset the iterator, or make a copy of it. An
iterator object could provide such additional capabilities, but the
iterator protocol only requires a "next()" method.

参见:

  **PEP 234** - 迭代器
     由 Ka-Ping Yee 和 GvR 撰写；由 Python Labs 团队（主要由 GvR 和
     Tim Peters）实现。


PEP 255: 简单的生成器
=====================

生成器是另一个新增特性，它是与迭代器的引入相互关联的。

You're doubtless familiar with how function calls work in Python or C.
When you call a function, it gets a private namespace where its local
variables are created.  When the function reaches a "return"
statement, the local variables are destroyed and the resulting value
is returned to the caller.  A later call to the same function will get
a fresh new set of local variables. But, what if the local variables
weren't thrown away on exiting a function? What if you could later
resume the function where it left off?  This is what generators
provide; they can be thought of as resumable functions.

这里是一个生成器函数的最简示例:

   def generate_ints(N):
       for i in range(N):
           yield i

A new keyword, "yield", was introduced for generators.  Any function
containing a "yield" statement is a generator function; this is
detected by Python's bytecode compiler which compiles the function
specially as a result.  Because a new keyword was introduced,
generators must be explicitly enabled in a module by including a "from
__future__ import generators" statement near the top of the module's
source code.  In Python 2.3 this statement will become unnecessary.

When you call a generator function, it doesn't return a single value;
instead it returns a generator object that supports the iterator
protocol.  On executing the "yield" statement, the generator outputs
the value of "i", similar to a "return" statement.  The big difference
between "yield" and a "return" statement is that on reaching a "yield"
the generator's state of execution is suspended and local variables
are preserved.  On the next call to the generator's "next()" method,
the function will resume executing immediately after the "yield"
statement.  (For complicated reasons, the "yield" statement isn't
allowed inside the "try" block of a "try"..."finally" statement; read
**PEP 255** for a full explanation of the interaction between "yield"
and exceptions.)

下面是 "generate_ints()" 生成器的用法示例:

   >>> gen = generate_ints(3)
   >>> gen
   <generator object at 0x8117f90>
   >>> gen.next()
   0
   >>> gen.next()
   1
   >>> gen.next()
   2
   >>> gen.next()
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
     File "<stdin>", line 2, in generate_ints
   StopIteration

你可以等价地写成 "for i in generate_ints(5)" 或 "a,b,c =
generate_ints(3)"。

Inside a generator function, the "return" statement can only be used
without a value, and signals the end of the procession of values;
afterwards the generator cannot return any further values. "return"
with a value, such as "return 5", is a syntax error inside a generator
function.  The end of the generator's results can also be indicated by
raising "StopIteration" manually, or by just letting the flow of
execution fall off the bottom of the function.

You could achieve the effect of generators manually by writing your
own class and storing all the local variables of the generator as
instance variables.  For example, returning a list of integers could
be done by setting "self.count" to 0, and having the "next()" method
increment "self.count" and return it. However, for a moderately
complicated generator, writing a corresponding class would be much
messier. "Lib/test/test_generators.py" contains a number of more
interesting examples.  The simplest one implements an in-order
traversal of a tree using generators recursively.

   # A recursive generator that generates Tree leaves in in-order.
   def inorder(t):
       if t:
           for x in inorder(t.left):
               yield x
           yield t.label
           for x in inorder(t.right):
               yield x

Two other examples in "Lib/test/test_generators.py" produce solutions
for the N-Queens problem (placing $N$ queens on an $NxN$ chess board
so that no queen threatens another) and the Knight's Tour (a route
that takes a knight to every square of an $NxN$ chessboard without
visiting any square twice).

生成器的概念源自其他编程语言，尤其是 Icon（
https://www2.cs.arizona.edu/icon/ ），在 Icon 语言中，生成器的概念是核
心。在 Icon 中，每个表达式和函数调用生成器的概念源自其他编程语言，尤其
是 Icon。在Icon中，每个表达式和函数调用都可以表现得像一个生成器。以下
是来自“Icon 编程语言概述”中的一个示例，展示了生成器的用法
https://www2.cs.arizona.edu/icon/docs/ipd266.htm ：

   sentence := "Store it in the neighboring harbor"
   if (i := find("or", sentence)) > 5 then write(i)

In Icon the "find()" function returns the indexes at which the
substring "or" is found: 3, 23, 33.  In the "if" statement, "i" is
first assigned a value of 3, but 3 is less than 5, so the comparison
fails, and Icon retries it with the second value of 23.  23 is greater
than 5, so the comparison now succeeds, and the code prints the value
23 to the screen.

Python 并没有像 Icon 那样将生成器作为核心概念来采纳。生成器被认为是
Python 核心语言的一部分，但学习或使用它们并不是强制性的；如果它们不能
解决你的问题，可以完全忽略它们。与 Icon 相比，Python 的一个新颖特性是
生成器的状态表示为一个具体对象（迭代器），该对象可以传递给其他函数或存
储在数据结构中。

参见:

  **PEP 255** - 简单生成器
     由 Neil Schemenauer, Tim Peters, Magnus Lie Hetland 撰写。 主要由
     Neil Schemenauer 和 Tim Peters 实现，并包含来自 Python Labs 团队
     的修正。


PEP 237: 统一长整数和整数
=========================

In recent versions, the distinction between regular integers, which
are 32-bit values on most machines, and long integers, which can be of
arbitrary size, was becoming an annoyance.  For example, on platforms
that support files larger than "2**32" bytes, the "tell()" method of
file objects has to return a long integer. However, there were various
bits of Python that expected plain integers and would raise an error
if a long integer was provided instead.  For example, in Python 1.5,
only regular integers could be used as a slice index, and "'abc'[1L:]"
would raise a "TypeError" exception with the message 'slice index must
be int'.

Python 2.2 将根据需要将数值从短整数转换为长整数。'L' 后缀不再需要用于
表示长整数字面量，因为现在编译器会自动选择适当的类型。（在未来的 2.x
版本的 Python 中，使用 'L' 后缀将被不鼓励，并在 Python 2.4 中触发警告
，可能在 Python 3.0 中被移除。）许多以前会引发 "OverflowError" 的操作
现在会返回一个长整数作为结果。例如：

   >>> 1234567890123
   1234567890123L
   >>> 2 ** 64
   18446744073709551616L

In most cases, integers and long integers will now be treated
identically.  You can still distinguish them with the "type()" built-
in function, but that's rarely needed.

参见:

  **PEP 237** - 统一长整数和整数
     由 Moshe Zadka 和 Guido van Rossum 撰写 ; 大部分由 Guido van
     Rossum 实现。


PEP 238：修改除法运算符
=======================

The most controversial change in Python 2.2 heralds the start of an
effort to fix an old design flaw that's been in Python from the
beginning. Currently Python's division operator, "/", behaves like C's
division operator when presented with two integer arguments: it
returns an integer result that's truncated down when there would be a
fractional part.  For example, "3/2" is 1, not 1.5, and "(-1)/2" is
-1, not -0.5.  This means that the results of division can vary
unexpectedly depending on the type of the two operands and because
Python is dynamically typed, it can be difficult to determine the
possible types of the operands.

(The controversy is over whether this is *really* a design flaw, and
whether it's worth breaking existing code to fix this.  It's caused
endless discussions on python-dev, and in July 2001 erupted into a
storm of acidly sarcastic postings on *comp.lang.python*. I won't
argue for either side here and will stick to describing what's
implemented in 2.2.  Read **PEP 238** for a summary of arguments and
counter-arguments.)

由于这一变化可能会破坏现有代码，因此它正在非常逐步地引入。Python 2.2
开始了这一过渡，但直到 Python 3.0 这一转换才会完全完成。

First, I'll borrow some terminology from **PEP 238**.  "True division"
is the division that most non-programmers are familiar with: 3/2 is
1.5, 1/4 is 0.25, and so forth.  "Floor division" is what Python's "/"
operator currently does when given integer operands; the result is the
floor of the value returned by true division.  "Classic division" is
the current mixed behaviour of "/"; it returns the result of floor
division when the operands are integers, and returns the result of
true division when one of the operands is a floating-point number.

Python 2.2 引入了以下变化：

* A new operator, "//", is the floor division operator. (Yes, we know
  it looks like C++'s comment symbol.)  "//" *always* performs floor
  division no matter what the types of its operands are, so "1 // 2"
  is 0 and "1.0 // 2.0" is also 0.0.

  "//" is always available in Python 2.2; you don't need to enable it
  using a "__future__" statement.

* By including a "from __future__ import division" in a module, the
  "/" operator will be changed to return the result of true division,
  so "1/2" is 0.5.  Without the "__future__" statement, "/" still
  means classic division. The default meaning of "/" will not change
  until Python 3.0.

* Classes can define methods called "__truediv__()" and
  "__floordiv__()" to overload the two division operators.  At the C
  level, there are also slots in the "PyNumberMethods" structure so
  extension types can define the two operators.

* Python 2.2 支持一些命令行参数，用于测试代码是否能在除法语义改变的情
  况下正常工作。运行 Python 并使用 -Q warn 选项时，当对两个整数应用除
  法时会发出警告。你可以利用这个功能找到受影响的代码并进行修复。默认情
  况下，Python 2.2 会执行经典除法而不会发出警告；在 Python 2.3 中，警
  告将默认开启。

参见:

  **PEP 238**：改变除法运算符
     由 Moshe Zadka 和 Guido van Rossum 撰写 ; 由 Guido van Rossum 实
     现。


Unicode 的改变
==============

Python's Unicode support has been enhanced a bit in 2.2.  Unicode
strings are usually stored as UCS-2, as 16-bit unsigned integers.
Python 2.2 can also be compiled to use UCS-4, 32-bit unsigned
integers, as its internal encoding by supplying "--enable-
unicode=ucs4" to the configure script.   (It's also possible to
specify "--disable-unicode" to completely disable Unicode support.)

When built to use UCS-4 (a "wide Python"), the interpreter can
natively handle Unicode characters from U+000000 to U+110000, so the
range of legal values for the "unichr()" function is expanded
accordingly.  Using an interpreter compiled to use UCS-2 (a "narrow
Python"), values greater than 65535 will still cause "unichr()" to
raise a "ValueError" exception. This is all described in **PEP 261**,
"Support for 'wide' Unicode characters"; consult it for further
details.

Another change is simpler to explain. Since their introduction,
Unicode strings have supported an "encode()" method to convert the
string to a selected encoding such as UTF-8 or Latin-1.  A symmetric
"decode([*encoding*])" method has been added to 8-bit strings (though
not to Unicode strings) in 2.2. "decode()" assumes that the string is
in the specified encoding and decodes it, returning whatever is
returned by the codec.

Using this new feature, codecs have been added for tasks not directly
related to Unicode.  For example, codecs have been added for uu-
encoding, MIME's base64 encoding, and compression with the "zlib"
module:

   >>> s = """Here is a lengthy piece of redundant, overly verbose,
   ... and repetitive text.
   ... """
   >>> data = s.encode('zlib')
   >>> data
   'x\x9c\r\xc9\xc1\r\x80 \x10\x04\xc0?Ul...'
   >>> data.decode('zlib')
   'Here is a lengthy piece of redundant, overly verbose,\nand repetitive text.\n'
   >>> print s.encode('uu')
   begin 666 <data>
   M2&5R92!I<R!A(&QE;F=T:'D@<&EE8V4@;V8@<F5D=6YD86YT+"!O=F5R;'D@
   >=F5R8F]S92P*86YD(')E<&5T:71I=F4@=&5X="X*

   end
   >>> "sheesh".encode('rot-13')
   'furrfu'

To convert a class instance to Unicode, a "__unicode__()" method can
be defined by a class, analogous to "__str__()".

"encode()", "decode()", and "__unicode__()" were implemented by Marc-
André Lemburg.  The changes to support using UCS-4 internally were
implemented by Fredrik Lundh and Martin von Löwis.

参见:

  **PEP 261** - 对 '宽' Unicode 字符的支持
     由 Paul Prescod 编写。


PEP 227: 嵌套的作用域
=====================

In Python 2.1, statically nested scopes were added as an optional
feature, to be enabled by a "from __future__ import nested_scopes"
directive.  In 2.2 nested scopes no longer need to be specially
enabled, and are now always present.  The rest of this section is a
copy of the description of nested scopes from my "What's New in Python
2.1" document; if you read it when 2.1 came out, you can skip the rest
of this section.

Python 2.1 中的最大改变是 Python 的作用域规则，在Python 2.2中得到完善
。 在 Python 2.0 中，任意给定的时刻至多使用三个命名空间来查找变量名称
：局部、模块和内置命名空间。 这往往会导致令人吃惊的结果因为它与人们直
觉上的预期不相匹配。 例如，一个嵌套的递归函数将不起作用:

   def f():
       ...
       def g(value):
           ...
           return g(value-1) + 1
       ...

The function "g()" will always raise a "NameError" exception, because
the binding of the name "g" isn't in either its local namespace or in
the module-level namespace.  This isn't much of a problem in practice
(how often do you recursively define interior functions like this?),
but this also made using the "lambda" expression clumsier, and this
was a problem in practice. In code which uses "lambda" you can often
find local variables being copied by passing them as the default
values of arguments.

   def find(self, name):
       "Return list of any entries equal to 'name'"
       L = filter(lambda x, name=name: x == name,
                  self.list_attribute)
       return L

结果将会严重损害以高度函数式风格编写的 Python 代码的可读性。

Python 2.2 最显著的改变是增加了静态作用域这一语言特征来解决此问题。 作
为它的第一项影响，在上述示例中的 "name=name" 默认参数现在将不再必要。
简单地说，当一个函数内部的给定变量名没有被赋值时（通过赋值语句，或者
"def", "class" 或 "import" 语句），对该变量的引用将在外层作用域的局部
命名空间中查找。 对于该规则的更详细解释，以及具体实现的分析，请参阅相
应的 PEP。

对于同时在模块层级和包含下层函数定义的函数内部局部变量使用了相同变量名
的代码来说这项改变可能会导致一些兼容性问题。 不过这看来不太可能发生，
因为阅读这样的代码本来就会相当令人困惑。

此项改变的一个附带影响是在特定条件下函数作用域内部 "from module import
*" 和 "exec" 语句将不允许使用。 Python 参考手册已经写明 "from module
import *" 仅在模块最高层级上是可用的，但此前 CPython 解释器从未强制实
施此规则。 作为嵌套作用域具体实现的一部分，将 Python 源码转为字节码的
编译器会生成不同的代码来访问某个包含作用域内的变量。 "from module
import *" 和 "exec" 会使得编译器无法正确执行，因为它们会向局部命名空间
添加在编译时还不存在的名称。 为此，如果一个函数包含带有自由变量的函数
定义或 "lambda" 表达式，编译器将通过引发 "SyntaxError" 异常来提示。

为了使前面的解释更清楚，下面是一个例子:

   x = 1
   def f():
       # The next line is a syntax error
       exec 'x=2'
       def g():
           return x

包含 "exec" 语句的第 4 行有语法错误，因为 "exec" 会定义一个名为 "x" 的
新局部变量，它的值应当被 "g()" 所访问。

这应该不会是太大的限制，因为 "exec" 在多数 Python 代码中都极少被使用（
而当它被使用时，往往也是个存在糟糕设计的信号）。

参见:

  **PEP 227** - 静态嵌套作用域
     由 Jeremy Hylton 撰写并实现。


新增和改进的模块
================

* The "xmlrpclib" module was contributed to the standard library by
  Fredrik Lundh, providing support for writing XML-RPC clients.  XML-
  RPC is a simple remote procedure call protocol built on top of HTTP
  and XML. For example, the following snippet retrieves a list of RSS
  channels from the O'Reilly Network, and then  lists the recent
  headlines for one channel:

     import xmlrpclib
     s = xmlrpclib.Server(
           'http://www.oreillynet.com/meerkat/xml-rpc/server.php')
     channels = s.meerkat.getChannels()
     # channels is a list of dictionaries, like this:
     # [{'id': 4, 'title': 'Freshmeat Daily News'}
     #  {'id': 190, 'title': '32Bits Online'},
     #  {'id': 4549, 'title': '3DGamers'}, ... ]

     # Get the items for one channel
     items = s.meerkat.getItems( {'channel': 4} )

     # 'items' is another list of dictionaries, like this:
     # [{'link': 'http://freshmeat.net/releases/52719/',
     #   'description': 'A utility which converts HTML to XSL FO.',
     #   'title': 'html2fo 0.3 (Default)'}, ... ]

  The "SimpleXMLRPCServer" module makes it easy to create
  straightforward XML-RPC servers.  See http://xmlrpc.scripting.com/
  for more information about XML-RPC.

* The new "hmac" module implements the HMAC algorithm described by
  **RFC 2104**. (Contributed by Gerhard Häring.)

* Several functions that originally returned lengthy tuples now return
  pseudo-sequences that still behave like tuples but also have
  mnemonic attributes such as "memberst_mtime" or "tm_year". The
  enhanced functions include "stat()", "fstat()", "statvfs()", and
  "fstatvfs()" in the "os" module, and "localtime()", "gmtime()", and
  "strptime()" in the "time" module.

  For example, to obtain a file's size using the old tuples, you'd end
  up writing something like "file_size =
  os.stat(filename)[stat.ST_SIZE]", but now this can be written more
  clearly as "file_size = os.stat(filename).st_size".

  此特性的初始补丁由 Nick Mathewson 贡献。

* The Python profiler has been extensively reworked and various errors
  in its output have been corrected.  (Contributed by Fred L. Drake,
  Jr. and Tim Peters.)

* The "socket" module can be compiled to support IPv6; specify the "--
  enable-ipv6" option to Python's configure script.  (Contributed by
  Jun-ichiro "itojun" Hagino.)

* Two new format characters were added to the "struct" module for
  64-bit integers on platforms that support the C long long type.  "q"
  is for a signed 64-bit integer, and "Q" is for an unsigned one.  The
  value is returned in Python's long integer type.  (Contributed by
  Tim Peters.)

* In the interpreter's interactive mode, there's a new built-in
  function "help()" that uses the "pydoc" module introduced in Python
  2.1 to provide interactive help. "help(object)" displays any
  available help text about *object*.  "help()" with no argument puts
  you in an online help utility, where you can enter the names of
  functions, classes, or modules to read their help text. (Contributed
  by Guido van Rossum, using Ka-Ping Yee's "pydoc" module.)

* Various bugfixes and performance improvements have been made to the
  SRE engine underlying the "re" module.  For example, the "re.sub()"
  and "re.split()" functions have been rewritten in C.  Another
  contributed patch speeds up certain Unicode character ranges by a
  factor of two, and a new "finditer()"  method that returns an
  iterator over all the non-overlapping matches in  a given string.
  (SRE is maintained by Fredrik Lundh.  The BIGCHARSET patch was
  contributed by Martin von Löwis.)

* The "smtplib" module now supports **RFC 2487**, "Secure SMTP over
  TLS", so it's now possible to encrypt the SMTP traffic between a
  Python program and the mail transport agent being handed a message.
  "smtplib" also supports SMTP authentication.  (Contributed by
  Gerhard Häring.)

* The "imaplib" module, maintained by Piers Lauder, has support for
  several new extensions: the NAMESPACE extension defined in **RFC
  2342**, SORT, GETACL and SETACL.  (Contributed by Anthony Baxter and
  Michel Pelletier.)

* The "rfc822" module's parsing of email addresses is now compliant
  with **RFC 2822**, an update to **RFC 822**.  (The module's name is
  *not* going to be changed to "rfc2822".)  A new package, "email",
  has also been added for parsing and generating e-mail messages.
  (Contributed by Barry Warsaw, and arising out of his work on
  Mailman.)

* The "difflib" module now contains a new "Differ" class for producing
  human-readable lists of changes (a "delta") between two sequences of
  lines of text.  There are also two generator functions, "ndiff()"
  and "restore()", which respectively return a delta from two
  sequences, or one of the original sequences from a delta. (Grunt
  work contributed by David Goodger, from ndiff.py code by Tim Peters
  who then did the generatorization.)

* New constants "ascii_letters", "ascii_lowercase", and
  "ascii_uppercase" were added to the "string" module.  There were
  several modules in the standard library that used "string.letters"
  to mean the ranges A-Za-z, but that assumption is incorrect when
  locales are in use, because "string.letters" varies depending on the
  set of legal characters defined by the current locale.  The buggy
  modules have all been fixed to use "ascii_letters" instead.
  (Reported by an unknown person; fixed by Fred L. Drake, Jr.)

* The "mimetypes" module now makes it easier to use alternative MIME-
  type databases by the addition of a "MimeTypes" class, which takes a
  list of filenames to be parsed.  (Contributed by Fred L. Drake, Jr.)

* A "Timer" class was added to the "threading" module that allows
  scheduling an activity to happen at some future time.  (Contributed
  by Itamar Shtull-Trauring.)


解释器的改变和修正
==================

有些变化只会影响那些在 C 级别处理 Python 解释器的人，因为他们正在编写
Python 扩展模块、嵌入解释器或仅仅是在修改解释器本身。如果你只编写
Python 代码，这里描述的变化对你几乎没有影响。

* Profiling and tracing functions can now be implemented in C, which
  can operate at much higher speeds than Python-based functions and
  should reduce the overhead of profiling and tracing.  This  will be
  of interest to authors of development environments for Python.  Two
  new C functions were added to Python's API, "PyEval_SetProfile()"
  and "PyEval_SetTrace()". The existing "sys.setprofile()" and
  "sys.settrace()" functions still exist, and have simply been changed
  to use the new C-level interface.  (Contributed by Fred L. Drake,
  Jr.)

* Another low-level API, primarily of interest to implementers of
  Python debuggers and development tools, was added.
  "PyInterpreterState_Head()" and "PyInterpreterState_Next()" let a
  caller walk through all the existing interpreter objects;
  "PyInterpreterState_ThreadHead()" and "PyThreadState_Next()" allow
  looping over all the thread states for a given interpreter.
  (Contributed by David Beazley.)

* 垃圾收集器的 C 级接口已经发生了变化，使得编写支持垃圾收集的扩展类型
  和调试函数误用变得更容易。各种函数的语义略有不同，因此需要重命名一系
  列函数。使用旧 API 的扩展仍然可以编译，但不会参与垃圾收集，因此应优
  先考虑将它们更新为 2.2 版本。

  要将一个扩展模块升级至新 API，请执行下列步骤:

* 将 "Py_TPFLAGS_GC" 重命名为 "Py_TPFLAGS_HAVE_GC"。

* 使用 "PyObject_GC_New()" 或 "PyObject_GC_NewVar()" 来分配
     对象，并使用 "PyObject_GC_Del()" 来释放它们。

* Rename "PyObject_GC_Init()" to "PyObject_GC_Track()" and
  "PyObject_GC_Fini()" to "PyObject_GC_UnTrack()".

* 从对象大小计算中移除 "PyGC_HEAD_SIZE"。

* Remove calls to "PyObject_AS_GC()" and "PyObject_FROM_GC()".

* A new "et" format sequence was added to "PyArg_ParseTuple()"; "et"
  takes both a parameter and an encoding name, and converts the
  parameter to the given encoding if the parameter turns out to be a
  Unicode string, or leaves it alone if it's an 8-bit string, assuming
  it to already be in the desired encoding.  This differs from the
  "es" format character, which assumes that 8-bit strings are in
  Python's default ASCII encoding and converts them to the specified
  new encoding. (Contributed by M.-A. Lemburg, and used for the MBCS
  support on Windows described in the following section.)

* A different argument parsing function, "PyArg_UnpackTuple()", has
  been added that's simpler and presumably faster.  Instead of
  specifying a format string, the caller simply gives the minimum and
  maximum number of arguments expected, and a set of pointers to
  PyObject* variables that will be filled in with argument values.

* Two new flags "METH_NOARGS" and "METH_O" are available in method
  definition tables to simplify implementation of methods with no
  arguments or a single untyped argument. Calling such methods is more
  efficient than calling a corresponding method that uses
  "METH_VARARGS".  Also, the old "METH_OLDARGS" style of writing C
  methods is  now officially deprecated.

* Two new wrapper functions, "PyOS_snprintf()" and "PyOS_vsnprintf()"
  were added to provide  cross-platform implementations for the
  relatively new "snprintf()" and "vsnprintf()" C lib APIs. In
  contrast to the standard "sprintf()" and "vsprintf()" functions, the
  Python versions check the bounds of the buffer used to protect
  against buffer overruns. (Contributed by M.-A. Lemburg.)

* The "_PyTuple_Resize()" function has lost an unused parameter, so
  now it takes 2 parameters instead of 3.  The third argument was
  never used, and can simply be discarded when porting code from
  earlier versions to Python 2.2.


其他的改变和修正
================

As usual there were a bunch of other improvements and bugfixes
scattered throughout the source tree.  A search through the CVS change
logs finds there were 527 patches applied and 683 bugs fixed between
Python 2.1 and 2.2; 2.2.1 applied 139 patches and fixed 143 bugs;
2.2.2 applied 106 patches and fixed 82 bugs.  These figures are likely
to be underestimates.

一些较为重要的改变:

* The code for the MacOS port for Python, maintained by Jack Jansen,
  is now kept in the main Python CVS tree, and many changes have been
  made to support MacOS X.

  The most significant change is the ability to build Python as a
  framework, enabled by supplying the "--enable-framework" option to
  the configure script when compiling Python.  According to Jack
  Jansen, "This installs a self-contained Python installation plus the
  OS X framework "glue" into "/Library/Frameworks/Python.framework"
  (or another location of choice). For now there is little immediate
  added benefit to this (actually, there is the disadvantage that you
  have to change your PATH to be able to find Python), but it is the
  basis for creating a full-blown Python application, porting the
  MacPython IDE, possibly using Python as a standard OSA scripting
  language and much more."

  Most of the MacPython toolbox modules, which interface to MacOS APIs
  such as windowing, QuickTime, scripting, etc. have been ported to OS
  X, but they've been left commented out in "setup.py".  People who
  want to experiment with these modules can uncomment them manually.

* Keyword arguments passed to built-in functions that don't take them
  now cause a "TypeError" exception to be raised, with the message
  "*function* takes no keyword arguments".

* Weak references, added in Python 2.1 as an extension module, are now
  part of the core because they're used in the implementation of new-
  style classes.  The "ReferenceError" exception has therefore moved
  from the "weakref" module to become a built-in exception.

* A new script, "Tools/scripts/cleanfuture.py" by Tim Peters,
  automatically removes obsolete "__future__" statements from Python
  source code.

* An additional *flags* argument has been added to the built-in
  function "compile()", so the behaviour of "__future__" statements
  can now be correctly observed in simulated shells, such as those
  presented by IDLE and other development environments.  This is
  described in **PEP 264**. (Contributed by Michael Hudson.)

* The new license introduced with Python 1.6 wasn't GPL-compatible.
  This is fixed by some minor textual changes to the 2.2 license, so
  it's now legal to embed Python inside a GPLed program again.  Note
  that Python itself is not GPLed, but instead is under a license
  that's essentially equivalent to the BSD license, same as it always
  was.  The license changes were also applied to the Python 2.0.1 and
  2.1.1 releases.

* When presented with a Unicode filename on Windows, Python will now
  convert it to an MBCS encoded string, as used by the Microsoft file
  APIs.  As MBCS is explicitly used by the file APIs, Python's choice
  of ASCII as the default encoding turns out to be an annoyance.  On
  Unix, the locale's character set is used if
  "locale.nl_langinfo(CODESET)" is available.  (Windows support was
  contributed by Mark Hammond with assistance from Marc-André Lemburg.
  Unix support was added by Martin von Löwis.)

* 大文件支持目前已在 Windows 上启用。 （由 Tim Peters 贡献。）

* "Tools/scripts/ftpmirror.py" 脚本现在会解析 ".netrc" 文件，如果存在
  的话。 （由 Mike Romberg 贡献。）

* Some features of the object returned by the "xrange()" function are
  now deprecated, and trigger warnings when they're accessed; they'll
  disappear in Python 2.3. "xrange" objects tried to pretend they were
  full sequence types by supporting slicing, sequence multiplication,
  and the "in" operator, but these features were rarely used and
  therefore buggy.  The "tolist()" method and the "start", "stop", and
  "step" attributes are also being deprecated.  At the C level, the
  fourth argument to the "PyRange_New()" function, "repeat", has also
  been deprecated.

* There were a bunch of patches to the dictionary implementation,
  mostly to fix potential core dumps if a dictionary contains objects
  that sneakily changed their hash value, or mutated the dictionary
  they were contained in. For a while python-dev fell into a gentle
  rhythm of Michael Hudson finding a case that dumped core, Tim Peters
  fixing the bug, Michael finding another case, and round and round it
  went.

* On Windows, Python can now be compiled with Borland C thanks to a
  number of patches contributed by Stephen Hansen, though the result
  isn't fully functional yet.  (But this *is* progress...)

* Another Windows enhancement: Wise Solutions generously offered
  PythonLabs use of their InstallerMaster 8.1 system.  Earlier
  PythonLabs Windows installers used Wise 5.0a, which was beginning to
  show its age.  (Packaged up by Tim Peters.)

* Files ending in ".pyw" can now be imported on Windows. ".pyw" is a
  Windows-only thing, used to indicate that a script needs to be run
  using PYTHONW.EXE instead of PYTHON.EXE in order to prevent a DOS
  console from popping up to display the output.  This patch makes it
  possible to import such scripts, in case they're also usable as
  modules.  (Implemented by David Bolen.)

* On platforms where Python uses the C "dlopen()" function  to load
  extension modules, it's now possible to set the flags used  by
  "dlopen()" using the "sys.getdlopenflags()" and
  "sys.setdlopenflags()" functions. (Contributed by Bram Stolk.)

* The "pow()" built-in function no longer supports 3 arguments when
  floating-point numbers are supplied. "pow(x, y, z)" returns "(x**y)
  % z", but this is never useful for floating point numbers, and the
  final result varies unpredictably depending on the platform.  A call
  such as "pow(2.0, 8.0, 7.0)" will now raise a "TypeError" exception.


致谢
====

作者感谢以下人员为本文的各种草案提供建议，更正和帮助： Fred Bremmer,
Keith Briggs, Andrew Dalke, Fred L. Drake, Jr., Carel Fellinger, David
Goodger, Mark Hammond, Stephen Hansen, Michael Hudson, Jack Jansen,
Marc-André Lemburg, Martin von Löwis, Fredrik Lundh, Michael McLay,
Nick Mathewson, Paul Moore, Gustavo Niemeyer, Don O'Donnell, Joonas
Paalasma, Tim Peters, Jens Quade, Tom Reinhardt, Neil Schemenauer,
Guido van Rossum, Greg Ward, Edward Welbourne.
