Пример 3.18. Introducing class attributes

class MP3FileInfo(FileInfo):
    "хранит ID3v1.0 MP3 теги"
    tagDataMap = {"title"   : (  3,  33, stripnulls),
                  "artist"  : ( 33,  63, stripnulls),
                  "album"   : ( 63,  93, stripnulls),
                  "year"    : ( 93,  97, stripnulls),
                  "comment" : ( 97, 126, stripnulls),
                  "genre"   : (127, 128, ord)}

>>> import fileinfo
>>> fileinfo.MP3FileInfo
<class fileinfo.MP3FileInfo at 01257FDC>
>>> fileinfo.MP3FileInfo.tagDataMap
{'title': (3, 33, <function stripnulls at 0260C8D4>), 
'genre': (127, 128, <built-in function ord>), 
'artist': (33, 63, <function stripnulls at 0260C8D4>), 
'year': (93, 97, <function stripnulls at 0260C8D4>), 
'comment': (97, 126, <function stripnulls at 0260C8D4>), 
'album': (63, 93, <function stripnulls at 0260C8D4>)}
>>> m = fileinfo.MP3FileInfo()
>>> m.tagDataMap
{'title': (3, 33, <function stripnulls at 0260C8D4>), 
'genre': (127, 128, <built-in function ord>), 
'artist': (33, 63, <function stripnulls at 0260C8D4>), 
'year': (93, 97, <function stripnulls at 0260C8D4>), 
'comment': (97, 126, <function stripnulls at 0260C8D4>), 
'album': (63, 93, <function stripnulls at 0260C8D4>)}

	MP3FileInfo is the class itself, not any particular instance of the class.
	tagDataMap is a class attribute: literally, an attribute of the class. It is available before creating any instances of the class.
	Class attributes are available both through direct reference to the class and through any instance of the class.

Пример 3.19. Modifying class attributes

>>> class counter:
...     count = 0
...     def __init__(self)
...         self.__class__.count += 1
...     
>>> counter
<class __main__.counter at 010EAECC>
>>> counter.count
0
>>> c = counter()
>>> c.count
1
>>> counter.count
1
>>> d = counter()
>>> d.count
2
>>> c.count
2
>>> counter.count
2

	count is a class attribute of the counter class.
	__class__ is a built-in attribute of every class instance (of every class). It is a reference to the class that self is an instance of (in this case, the counter class).
	Because count is a class attribute, it is available through direct reference to the class, before we have created any instances of the class.
	Creating an instance of the class calls the __init__ method, which increments the class attribute count by 1. This affects the class itself, not just the newly created instance.
	Creating a second instance will increment the class attribute count again. Notice how the class attribute is shared by the class and all instances of the class.