8.12. "UserDict" — Class wrapper for dictionary objects
*******************************************************

**Source code:** Lib/UserDict.py

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

The module defines a mixin,  "DictMixin", defining all dictionary
methods for classes that already have a minimum mapping interface.
This greatly simplifies writing classes that need to be substitutable
for dictionaries (such as the shelve module).

This module also defines a class, "UserDict", that acts as a wrapper
around dictionary objects.  The need for this class has been largely
supplanted by the ability to subclass directly from "dict" (a feature
that became available starting with Python version 2.2).  Prior to the
introduction of "dict", the "UserDict" class was used to create
dictionary-like sub-classes that obtained new behaviors by overriding
existing methods or adding new ones.

The "UserDict" module defines the "UserDict" class and "DictMixin":

class UserDict.UserDict([initialdata])

   Class that simulates a dictionary.  The instance’s contents are
   kept in a regular dictionary, which is accessible via the "data"
   attribute of "UserDict" instances.  If *initialdata* is provided,
   "data" is initialized with its contents; note that a reference to
   *initialdata* will not be kept, allowing it be used for other
   purposes.

   注解: For backward compatibility, instances of "UserDict" are not
     iterable.

class UserDict.IterableUserDict([initialdata])

   Subclass of "UserDict" that supports direct iteration (e.g.  "for
   key in myDict").

In addition to supporting the methods and operations of mappings (see
section Mapping Types — dict), "UserDict" and "IterableUserDict"
instances provide the following attribute:

IterableUserDict.data

   A real dictionary used to store the contents of the "UserDict"
   class.

class UserDict.DictMixin

   Mixin defining all dictionary methods for classes that already have
   a minimum dictionary interface including "__getitem__()",
   "__setitem__()", "__delitem__()", and "keys()".

   This mixin should be used as a superclass.  Adding each of the
   above methods adds progressively more functionality.  For instance,
   defining all but "__delitem__()" will preclude only "pop()" and
   "popitem()" from the full interface.

   In addition to the four base methods, progressively more efficiency
   comes with defining "__contains__()", "__iter__()", and
   "iteritems()".

   Since the mixin has no knowledge of the subclass constructor, it
   does not define "__init__()" or "copy()".

   Starting with Python version 2.6, it is recommended to use
   "collections.MutableMapping" instead of "DictMixin".

   Note that DictMixin does not implement the "viewkeys()",
   "viewvalues()", or "viewitems()" methods.


8.13. "UserList" — Class wrapper for list objects
*************************************************

注解: When Python 2.2 was released, many of the use cases for this
  class were subsumed by the ability to subclass "list" directly.
  However, a handful of use cases remain.This module provides a list-
  interface around an underlying data store.  By default, that data
  store is a "list"; however, it can be used to wrap a list-like
  interface around other objects (such as persistent storage).In
  addition, this class can be mixed-in with built-in classes using
  multiple inheritance. This can sometimes be useful.  For example,
  you can inherit from "UserList" and "str" at the same time.  That
  would not be possible with both a real "list" and a real "str".

This module defines a class that acts as a wrapper around list
objects.  It is a useful base class for your own list-like classes,
which can inherit from them and override existing methods or add new
ones.  In this way one can add new behaviors to lists.

The "UserList" module defines the "UserList" class:

class UserList.UserList([list])

   Class that simulates a list.  The instance’s contents are kept in a
   regular list, which is accessible via the "data" attribute of
   "UserList" instances.  The instance’s contents are initially set to
   a copy of *list*, defaulting to the empty list "[]".  *list* can be
   any iterable, e.g. a real Python list or a "UserList" object.

   注解: The "UserList" class has been moved to the "collections"
     module in Python 3. The *2to3* tool will automatically adapt
     imports when converting your sources to Python 3.

In addition to supporting the methods and operations of mutable
sequences (see section Sequence Types — str, unicode, list, tuple,
bytearray, buffer, xrange), "UserList" instances provide the following
attribute:

UserList.data

   A real Python list object used to store the contents of the
   "UserList" class.

**Subclassing requirements:** Subclasses of "UserList" are expected to
offer a constructor which can be called with either no arguments or
one argument.  List operations which return a new sequence attempt to
create an instance of the actual implementation class.  To do so, it
assumes that the constructor can be called with a single parameter,
which is a sequence object used as a data source.

If a derived class does not wish to comply with this requirement, all
of the special methods supported by this class will need to be
overridden; please consult the sources for information about the
methods which need to be provided in that case.

在 2.0 版更改: Python versions 1.5.2 and 1.6 also required that the
constructor be callable with no parameters, and offer a mutable "data"
attribute.  Earlier versions of Python did not attempt to create
instances of the derived class.


8.14. "UserString" — Class wrapper for string objects
*****************************************************

注解: This "UserString" class from this module is available for
  backward compatibility only.  If you are writing code that does not
  need to work with versions of Python earlier than Python 2.2, please
  consider subclassing directly from the built-in "str" type instead
  of using "UserString" (there is no built-in equivalent to
  "MutableString").

This module defines a class that acts as a wrapper around string
objects.  It is a useful base class for your own string-like classes,
which can inherit from them and override existing methods or add new
ones.  In this way one can add new behaviors to strings.

It should be noted that these classes are highly inefficient compared
to real string or Unicode objects; this is especially the case for
"MutableString".

The "UserString" module defines the following classes:

class UserString.UserString([sequence])

   Class that simulates a string or a Unicode string object.  The
   instance’s content is kept in a regular string or Unicode string
   object, which is accessible via the "data" attribute of
   "UserString" instances.  The instance’s contents are initially set
   to a copy of *sequence*.  *sequence* can be either a regular Python
   string or Unicode string, an instance of "UserString" (or a
   subclass) or an arbitrary sequence which can be converted into a
   string using the built-in "str()" function.

   注解: The "UserString" class has been moved to the "collections"
     module in Python 3. The *2to3* tool will automatically adapt
     imports when converting your sources to Python 3.

class UserString.MutableString([sequence])

   This class is derived from the "UserString" above and redefines
   strings to be *mutable*.  Mutable strings can’t be used as
   dictionary keys, because dictionaries require *immutable* objects
   as keys.  The main intention of this class is to serve as an
   educational example for inheritance and necessity to remove
   (override) the "__hash__()" method in order to trap attempts to use
   a mutable object as dictionary key, which would be otherwise very
   error prone and hard to track down.

   2.6 版后已移除: The "MutableString" class has been removed in
   Python 3.

In addition to supporting the methods and operations of string and
Unicode objects (see section String Methods), "UserString" instances
provide the following attribute:

MutableString.data

   A real Python string or Unicode object used to store the content of
   the "UserString" class.
