Python: How to determine attribute MRO

Question

We can easily determine an object's mro, by accessing its __mro__ attribute.

In my case I have a really complex hierarchy that I'm trying to untangle to avoid the current mro hell we are undergoing.

I have a very long mro chain like this one:

(<class 'CompanyUserViewSet'>, <class 'CompanyGenericViewSet'>, 
 <class 'CompanyDispatchMixin'>, <class 'CompanyCorsLiteMixin'>, 
 <class 'CorsLiteMixin'>, <class 'ErrorHandlingMixin'>, ..., <type 'object'>)

This is shortened for the sake of the question, but in this case it sums up to 19 classes.

I currently have a problem: I need to know from which one of these classes python is resolving the as_view method.

I know I can check these in order, but I fail to see what I'm doing wrong, since the as_view that is getting called is not the correct one.

The call to the method is pretty simple, we have a CompanyUserViewSet that Rest Framework is using to build the urls for a router:

view = viewset.as_view(mapping, **route.initkwargs)

How can I determine, given an object and one of its attributes (or methods) from what class is python resolving it?

Answer 1

To get where a method is defined use, method.__func__.__qualname__. That requires Python >= 3.3. In older pythons you can use the qualname package.

See the example below:

class A:
  def f1():
    return 'f1A'
  def f2():
    return 'f2A'

class B(A):
  def f2():
    return 'f2B'

a = A()
b = B()
print(a.f1.__func__.__qualname__)
print(a.f2.__func__.__qualname__)
print(b.f1.__func__.__qualname__)
print(b.f2.__func__.__qualname__)

which produces the output:

A.f1
A.f2
A.f1
B.f2

If you are using the qualname package:

import qualname
print qualname.qualname(a.f1)
print qualname.qualname(a.f2)
print qualname.qualname(b.f1)
print qualname.qualname(b.f2)