Python Class Inheritance And __dict__ Lookup

Let's say I define class A: >>> class A: ... a = 1 ... class SubA: ... sub_a = { 'a': 1, 'b': 1} Then I define class B that inherits from A: >>>

Solution 1:

@bgporter has given a good explanation of the behaviour, I'll just go into why a little:

If your class variable was in B.__dict__, how would it function? Each subclass would have its own value for a, independent of the value for A.a - this is not what you would expect. A class variable should exist once - in that class.

Instead, Python does a lookup on the class and if it doesn't exist, then looks up to its base classes - note that means it is possible to shadow a class variable in a subclass.

Solution 2:

That's how Python's object model works:

A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., C.x is translated to C.__dict__["x"] (although for new-style classes in particular there are a number of hooks which allow for other means of locating attributes). When the attribute name is not found there, the attribute search continues in the base classes.