"copyreg" --- 注册配合 "pickle" 模块使用的函数
**********************************************

**源代码:** Lib/copyreg.py

======================================================================

"copyreg" 模块提供了可在封存特定对象时使用的一种定义函数方式。
"pickle" 和 "copy" 模块会在封存/拷贝特定对象时使用这些函数。 此模块提
供了非类对象构造器的相关配置信息。 这样的构造器可以是工厂函数或类实例
。

copyreg.constructor(object)

   将 *object* 声明为一个有效的构造器。 如果 *object* 是不可调用的（因
   而不是一个有效的构造器）则会引发 "TypeError"。

copyreg.pickle(type, function, constructor_ob=None)

   Declares that *function* should be used as a "reduction" function
   for objects of type *type*.  *function* must return either a string
   or a tuple containing between two and six elements. See the
   "dispatch_table" for more details on the interface of *function*.

   The *constructor_ob* parameter is a legacy feature and is now
   ignored, but if passed it must be a callable.

   Note that the "dispatch_table" attribute of a pickler object or
   subclass of "pickle.Pickler" can also be used for declaring
   reduction functions.


示例
====

以下示例将会显示如何注册一个封存函数，以及如何来使用它：

>>> import copyreg, copy, pickle
>>> class C:
...     def __init__(self, a):
...         self.a = a
...
>>> def pickle_c(c):
...     print("pickling a C instance...")
...     return C, (c.a,)
...
>>> copyreg.pickle(C, pickle_c)
>>> c = C(1)
>>> d = copy.copy(c)  
pickling a C instance...
>>> p = pickle.dumps(c)  
pickling a C instance...
