Subclassing a user-defined exception in a user-defined class in Python

Question

Why does this work:

class A:
    class BError(Exception):
        pass
    class CError(AError):
        pass

But this doesn't:

class A:
    class BError(Exception):
        pass
    class CError(A.BError):
        pass

If "method 1" works for calling class variables, why doesn't it work for "class classes"

Answer 1

When compiling, python builds the class and then assigns it to a named variable in the enclosing namespace. In the first example (assuming you really meant class CError(BError) ), BError has been compiled and assigned to the A class namespace so it is visible for the next class. But in the second example, the top class has not yet been assigned to the global A variable so A.BError cannot be found.

The problem can be demonstrated as:

>>> class A:
...     print('A' in globals())
... 
False
>>> print('A' in globals())
True
>>>