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?

@PadraicCunningham on an instance of the first type in the list — Alvaro, Oct 15 '15 at 17:18
Are their metaclasses involved? Are parent implementations called via `super`? There are a few variables that can change the default MRO. — Josh J, Oct 15 '15 at 17:20
I was probably unclear, I mean are you using super() in a class or Class.method or simply inheriting? — Padraic Cunningham, Oct 15 '15 at 17:20
@PadraicCunningham well, no metaclass issues for sure, but there are 19 classes, some used super, others didn't. `qualname` solved the issue in a second :) — Alvaro, Oct 15 '15 at 17:30
You can do it without any package http://stackoverflow.com/questions/961048/get-class-that-defined-method — Padraic Cunningham, Oct 15 '15 at 17:38

score 3 · Accepted Answer · edited Oct 15 '15 at 17:27

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)

This. This worked like a charm, don't get the downvote – Alvaro Oct 15 '15 at 17:24 — Alvaro, Oct 15 '15 at 17:24

Python: How to determine attribute MRO

1 Answers1