Two-level abstract class hierarchy without 'consistent method resolution' error

Question

I need two levels of abstract classes and a third level concrete class:

from abc import ABC

class Shape(ABC):
...

class Quad(ABC, Shape):
...

class Square(Quadrilateral)
...

This code generates TypeError: Cannot create a consistent method resolution against the declaration of Quad. I realize that the problem is a multiple inheritance ambiguity. Quad doesn't know whether to derive certain functions directly from ABC or from Shape, but how do I fix it? I can't drop ABC from Quad's declaration because then it won't be abstract anymore.

Where does `ABC` come from? Is it `abc.ABC`? Could you give a [mcve]? — jonrsharpe, Dec 21 '16 at 21:07
It's the standard Pythonic way of creating an abstract class: from abc import ABC — Keith Wiley, Dec 21 '16 at 21:24
Then *put that in the question*. Please [edit], and give less abstract (and more internally consistent) examples. — jonrsharpe, Dec 21 '16 at 21:24
Okay jonrsharpe. I added it. I thought ABC could be assumed in a Python question about abstract classes. Sorry. — Keith Wiley, Dec 21 '16 at 21:26
Note that `class Quad(Shape, ABC):` does not have this error - see e.g. http://stackoverflow.com/a/3003284/3001761 — jonrsharpe, Dec 21 '16 at 21:27
so... why not just do `class Quad(Shape):`? It will inherit any `ABC` methods and variables that are not overridden in `Shape`. — drevicko, Mar 23 '20 at 23:54

score 5 · Answer 1 · edited Jul 26 '22 at 12:19

5

Apparently, simply reversing the order of the parent classes to Quad fixes the problem. I'm still unclear on the underlying theory (although it can sort of be inferred), but at least my code is running now.

class Quad(Shape, ABC)

edited Jul 26 '22 at 12:19

Farvardin

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answered Dec 21 '16 at 21:29

Keith Wiley

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The underlying theory is python's Method Resolution Order algorithm; see here: python.org/download/releases/2.3/mro Long story short, the MRO algorithm enforces that left classes appear before right classes in the MRO, and also that, if X appears before Y in the MRO of an ancestor class, X will appear before Y in the MRO of the descendant class. There is no way to meet both conditions for `Quad` in the original post, but there is a way in this answer (namely: `Quad`, `Shape`, `ABC`). – Damien Jan 20 '23 at 17:27

score 1 · Answer 2 · answered May 04 '21 at 20:29

This is explained in abc documentation:

Note that the type of ABC is still ABCMeta, therefore inheriting from ABC requires the usual precautions regarding metaclass usage, as multiple inheritance may lead to metaclass conflicts. There is also an example how to use ABCMeta directly.

With that your code would look like this:

from abc import ABC

class Shape(metaclass=ABCMeta):
...

class Quad(Shape, metaclass=ABCMeta):
...

class Square(Quadrilateral)
...

I don't know why it worked when ABC is the last class in the list, but the benefit of using ABCMeta is that you cannot put it at the beginning of the list.

Two-level abstract class hierarchy without 'consistent method resolution' error

2 Answers2