why python associative map member variable is shared between objects

Question

For class A, why is aMap member variable being shared between object and object b?

>>> class A:
...     aMap = {}

>>> a = A()
>>> a.aMap["hello"] = 1

>>> b = A()
>>> b.aMap["world"] = 2

>>> c = []
>>> c.append(a)
>>> c.append(b)

>>> for i in c:
...     for j in i.aMap.items():
...         print j
('world', 2)  
('hello', 1)  
('world', 2)  
('hello', 1)  

Answer 1

Because you defined it as a class attribute, not instance attribute.

If you wish to have it as instance attribute and not be shared between instances, you have to define it like this:

class A(object):
    def __init__(self):
        self.aMap = {}

Answer 2

Because its a class attribute, not an instance attribute ("member variable").

To make it a instance attribute, assign it on an instance, e.g. in the constructor:

class A:
    def __init__(self):
        self.aMap = {}

But you could also do:

a = A()
a.aMap = {}

Answer 3

Because you are effectively defining a class variable instead of an instance variable.

To define an instance variable, you should do it on a method referring the object itself (generally self):

class A(object):
    class_variable = 0

    def __init__(self, a, b):
        self.instance_variable = a
        class_variable = b

>>> a=A(1,2)
>>> print a.instance_variable
1
>>> print a.class_variable
2
>>> b=A(2,1)
>>> print b.instance_variable
2
>>> print b.class_variable
1
>>> print a.instance_variable
1
>>> print a.class_variable
1