Document when multiple inheritance is not allowed

[JelleZijlstra]

Python generally supports multiple inheritance, but only under certain conditions (the classes must have compatible memory layouts, compatible metaclasses, and a compatible MRO). This behavior is quite stable in practice but the documentation doesn't discuss it very much:

• https://docs.python.org/3/tutorial/classes.html#tut-multiple is a section of the tutorial covering multiple inheritance, but says nothing about the fact that multiple inhritance will sometimes fail
• https://docs.python.org/3/reference/datamodel.html#object.__slots__ mentions the effect of slots on multiple inheritance.
• https://docs.python.org/3/reference/datamodel.html#determining-the-appropriate-metaclass mentions that class creation will fail if no appropriate metaclass can be found.

I propose to add a section to the language reference (not the tutorial) about multiple inheritance, explaining how it works and when it doesn't.

Relatedly, I just posted a proposal to allow type checkers to better support this: https://discuss.python.org/t/solid-bases-for-detecting-incompatible-base-classes/99280

Linked PRs

• gh-136843: Document how multiple inheritance works #136844