The Meaning of Class Definitions

we have to keep in mind that the meaning of class definitions are simply
code sections that are executed !

>>>class C:
print 'Class C being defined...'
Class C being defined...
>>>class MemberCounter:
members = 0
def init(self):
MemberCounter.members += 1
>>>m1 = MemberCounter()
>>>m1.init()
>>>MemberCounter.members
1
>>>m2 = MemberCounter()
>>>m2.init()
>>>MemberCounter.members
2

In the preceding code, a variable is defined in the class scope, which can be accessed by all the members (instances), in this case to count the number of class members.
This class scope variable is accessible from every instance as well, just as methods are:

>>>m1.members
2
>>>m2.members
2

What happens when you rebind the members attribute in an instance?

>>> m1.members = 'Two'
>>> m1.members
'Two'
>>> m2.members
2

The new members value has been written into an attribute in m1, shadowing the class-wide variable. This mirrors the behavior of local and global variables in functions.

>>>MemberCounter.members = 10
10
>>>m1.members
'Two'
>>>m2.members
10

from above, we see, once you rebind the members attribute in an instance, a instance-local variable was create. Otherwise the m2.members refer to (or point to) the MemberCounter.members,
that is, instance.memberAttribute are bound to the class.memberAttribute.