Python ABC Multiple Inheritance

Question

I think the code will explain the problem better than I can do with words. Here is the code in my_abc.py:

from abc import ABCMeta, abstractmethod

class MyABC(object):
    __metaclass__ = ABCMeta

    @abstractmethod
    def print(self):
        pass

Here is the code in my_class.py

from my_abc import MyABC
from third_party_package import SomeClass

class MyClass(MyABC, SomeClass):
    def __init__(self):
        super(MyClass, self).__init__()

    def print(self):
        print('Hello ABC')

When I try to run my_class.py I get:

TypeError: Error when calling the metaclass bases metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

I understand that I can create a class the inherits directly from my interface MyABC, and then create another class which then inherits from both this class which I created and my third party module class.

My question is: Is there another better and proper way of doing this directly without having to create an intermediate class for my purpose?

Does you suggested workaround actually work? I'd expect you to have exactly the same issue if you put another class in between the `MyABC` and `MyClass`. The issue is that `SomeClass` has a different and incompatible metaclass. Without knowing what that is, I'm not sure if there's any way of answering the question. Can you add some concrete details about `SomeClass`? — Blckknght, Mar 01 '15 at 22:05
I'm using a third-party library called urwid. It's an ncurses replacement for python which has a much cleaner API. According to the example I provided above the SomeClass corresponds to an urwid.ListBox class which is a widget for creating list-box like UI component. I'm not sure how to find out the metaclass of urwid.ListBox, but when I run it with the code above I get the same TypeError. — moeabdol, Mar 02 '15 at 09:59
Try creating an intermediate metaclass: `class MyMeta(ABCMeta, type(SomeClass)): pass`, and then using that as the `__metaclass__` for `MyClass`. — user4815162342, Mar 02 '15 at 15:25

score 11 · Accepted Answer · answered Mar 02 '15 at 15:23

11

The SomeClass class has a custom metaclass. You will need to create a metaclass which inherits from both ABCMeta and this custom metaclass, then use it as the metaclass for MyClass. Without knowing more about this custom metaclass, I cannot determine a correct way to do this in the general case, but it will probably look like one of these possibilities:

class DerivedMeta(ABCMeta, type(SomeClass)):
    pass

class DerivedMeta(type(SomeClass), ABCMeta):
    pass

It's unlikely but possible you will also need to override one or more methods to ensure correct metaclass interactions.

answered Mar 02 '15 at 15:23

Kevin

28,963
9
62
81

1

@moeabdol: There is some discussion of it in [this essay](https://www.python.org/download/releases/2.2/descrintro/#metaclasses), but it talks about a lot of other stuff too. – Kevin Mar 02 '15 at 15:41
for the unaware, I have found a great explanation of metaclasses in stackoverflow checkout e-satis's answer here http://stackoverflow.com/questions/100003/what-is-a-metaclass-in-python thank you stackoverflow – moeabdol Mar 08 '15 at 00:01

score 6 · Answer 2 · answered Nov 09 '20 at 10:57

Thread is still at the top in search result, so I wanted to share my complete solution.
I ran into this problem when trying to create an abstract template class meant to PyQt5 widgets, in Python 3.8. I applied @Kevin's solution, creating a new meta class first. The working code:

from abc import ABC, ABCMeta
from PyQt5.QtWidgets import QWidget, QLabel


class QABCMeta(ABCMeta, type(QWidget)):
    """Create a meta class that combines ABC and the Qt meta class"""
    pass


class TcWidget(ABC, metaclass=QABCMeta):
    """Abstract class, to be multi-inherited together with a Qt item"""
    pass


class TcLabel(QLabel, TcWidget):
    """Label that shows a value"""
    pass


# ...
label = TcLabel()
# ...

score 0 · Answer 3 · edited Apr 03 '19 at 13:03

You could also set the other class's metaclass to ABCMeta so that all the multiple base classes's metaclass are ABCMeta.

I had a similar problem when using multiple inheritance with:

a class having a custom metaclass
a class having ABCMeta as metaclass

what I did to solve this was make my custom metaclass derive from ABCMeta. This seems to work in all cases.

This worked for me in all cases.

class MyMeta(ABCMeta):
  pass

instead of

class MyMeta(type):
  pass

Python ABC Multiple Inheritance

3 Answers3

Linked