Summary

This PR adds context to the metaclass conflict error message along with clear terminology.

Motivation

Class inheritance in OOP has well-established terminology conventions across languages, none of which should be used to describe the custom metaclass introduced by Python. This PR proposes to distinguish between them by the following convention:

• A classis derived from itsmetaclass
• A classis based on itsbase classes

This convention is then reflected in the proposed error message raised by metaclass conflicts.

Example

In the example below,MetaFoo andMetaBar are both subclasses oftype and neither is a (non-strict) subclass of the other.Foo is a class derived fromMetaFoo, andBar attempts to be derived fromMetaBar while also based onFoo, revealing the incompatibility betweenMetaFoo andMetaBar:

classMetaFoo(type):passclassMetaBar(type):passclassFoo(metaclass=MetaFoo):passclassBar(Foo,metaclass=MetaBar):pass# raises

Behaviour Before

Previously, the resulting TypeError would show:

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

Behaviour After

This pull request attempts to improve with:

Metaclass conflict while defining class: 'Foo'- Declared metaclass: 'MetaFoo'- Incompatible base class: 'Bar'- That base is derived from metaclass: 'MetaBar'All base classes must be based on classes derived from the same metaclass or a subclass thereof.

Conclusion

This PR hopes to contribute to the ongoing effort towards better error messages, whilst reinforcing clear and consistent terminology.