simple example of Python MRO and class attributes

pythonmro.py

In [1]: class A(object):
   ...:     x = [1,2,3]
   ...: 

In [2]: class B(A): pass
   ...: 

In [3]: b = B()

In [4]: b.x.append(33)

In [5]: A.x
Out[5]: [1, 2, 3, 33]

In [6]: b.x = 'asdasd'

In [7]: A.x
Out[7]: [1, 2, 3, 33]