Question

>>> class Test(object):
...   test = {}
... 
>>> class Test2(Test):
...   pass
... 
>>> Test2.test.update({1:2})
>>> Test.test
{1: 2}
>>> 

I was expecting {}. Happens also with old style classes.

Answer 1

There’s nothing odd with how update works. The point is that test is a class attribute, and class attributes are shared between classes, (untill someone rebinds test to something else).

Take a look at this IDE session:

>>> class Test(object):
...     test = {}
... 
>>> class Test2(Test):
...     pass
... 
>>> id(Test.test)
32424144
>>> id(Test2.test)
32424144
>>> Test.test.update({1:2})
>>> Test2.test
{1: 2}
>>> Test2.test = {}
>>> id(Test2.test)
32424480
>>> Test.test
{1: 2}
>>> Test2.test
{}
>>> del Test2.test
>>> Test2.test
{1: 2}

For further informations on how class attributes works look at Data Model Reference under Classes.