What is the difference between class and instance attributes?

Question

Is there any meaningful distinction between:

class A(object):
    foo = 5   # some default value

vs.

class B(object):
    d         


        

Answer 1

Beyond performance considerations, there is a significant semantic difference. In the class attribute case, there is just one object referred to. In the instance-attribute-set-at-instantiation, there can be multiple objects referred to. For instance

>>> class A: foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[5]
>>> class A:
...  def __init__(self): self.foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo    
[]