coding style - Attributes initialization/declaration in Python class: where to place them?

Question

I was wondering what was the best practice for initializing object attributes in Python, in the body of the class or inside the __init__ function?

i.e.

class A(object):
    foo = None

vs

class A(object):
   def __init__(self):
       self.foo = None

Answer 1

If you want the attribute to be shared by all instances of the class, use a class attribute:

class A(object):
    foo = None

This causes ('foo',None) to be a (key,value) pair in A.__dict__.

If you want the attribute to be customizable on a per-instance basis, use an instance attribute:

class A(object):
   def __init__(self):
       self.foo = None

This causes ('foo',None) to be a (key,value) pair in a.__dict__ where a=A() is an instance of A.