class Foo:
    a = 5

    def print_a(self):
        print(self.a)

class Foo:
    def __init__(self):
        self.a = 5

    def print_a(self):
        print(self.a)

class Foo:
    a = [100]                # Atributo de clase

    def __init__(self, n):
        self.b = [n]         # Atributo de instancia


>>> inst1 = Foo(1)
>>> inst2 = Foo(2)

>>> inst1.a
[100]
>>> inst2.a
[100]
>>> inst1.a.append(5)
>>> inst1.a
[100, 5]
>>> inst2.a
[100, 5]

>>> inst1.b
[1]
>>> inst2.b
[2]
>>> inst1.b.append(5)
>>> inst1.b
[1, 5]
>>> inst2.b
[2]

pi@rp1:~ $ python3
Python 3.4.2 (default, Oct 19 2014, 13:31:11)
[GCC 4.9.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class Perro:
...     numero_patas = 4
...
>>> pancho = Perro()
>>> fido = Perro()
>>> pancho.numero_patas
4
>>> fido.numero_patas
4
>>> Perro.numero_patas
4

>>> fido.numero_patas = 3
>>> fido.numero_patas
3
>>>

>>> fido.cola = 'cortada'
>>> fido.cola
'cortada'

>>> pancho.__dict__
{}
>>> fido.__dict__
{'cola': 'cortada', 'numero_patas': 3}
>>>

>>> pancho.__class__.__dict__
mappingproxy({'__weakref__': <attribute '__weakref__' of 'Perro' objects>, '__dict__': <attribute '__dict__' of 'Perro' objects>, 'numero_patas': 4, '__module__': '__main__', '__doc__': None})
>>> fido.__class__.__dict__
mappingproxy({'__weakref__': <attribute '__weakref__' of 'Perro' objects>, '__dict__': <attribute '__dict__' of 'Perro' objects>, 'numero_patas': 4, '__module__': '__main__', '__doc__': None})
>>>