TypeError: Cannot create a consistent method resolution order (MRO)

Question:

This is the code which I plan to use for my game, but it complains about an MRO error:

class Player:
    pass

class Enemy(Player):
    pass

class GameObject(Player, Enemy):
    pass

g = GameObject()
Asked By: user188276

||
Source

Answers:

Your GameObject is inheriting from Player and Enemy. Because Enemy already inherits from Player Python now cannot determine what class to look methods up on first; either Player, or on Enemy, which would override things defined in Player.

You don’t need to name all base classes of Enemy here; just inherit from that one class:

class GameObject(Enemy):
    pass

Enemy already includes Player, you don’t need to include it again.

Answered By: Martijn Pieters

What you wrote is you want a GameObject to be both a Player and an Enemy. But an Enemy is already a Player. The MRO issue just states that if you had a field a in Player, asking for this field in a GameObject instance would be ambiguous: should it be the a from the first Player you inherit or the one from the Player you inherit through your Enemy inheritance?

But are you sure you don’t want to use composition instead of inheritance, here?

class GameObject(object):
    def __init__(self):
        self.player = Player()
        self.enemy = Enemy()
Answered By: Francis Colas

I’ll explain the reason the original code doesn’t work.

Python needs to decide in which order to search through (direct and indirect) base classes when looking up an instance attribute / method. It does this by linearizing the inheritance graph, that is by converting the graph of base classes into a sequence, using an algorithm called C3 or MRO. The MRO algorithm is the unique algorithm that achieves several desirable properties:

  1. each ancestor class appears exactly once
  2. a class always appears before its ancestor (“monotonicity”)
  3. direct parents of the same class should appear in the same order as they are listed in class definition (“consistent local precedence order”)
  4. if children of class A always appear before children of class B, then A should appear before B (“consistent extended precedence order”)

With your code, the second constraint requires that Enemy appears first; the third constraint requires that Player appears first. Since there’s no way to satisfy all constraints, python reports that your inheritance hierarchy is illegal.

Your code will work if you switch the order of base classes in GameObject like so:

class GameObject(Enemy, Player):
    pass

This is not just a technical detail. In some (hopefully rare) cases, you might want to think about which class should be used to grab the method you called if the method is defined in multiple classes. The order in which you define base classes affects this choice.

Answered By: max