Warn against user defined __magic__ methods

[jpy-git]

Current problem

I'm currently working on a PR (#5938) that warns users against manually invoking magic methods.

This led me to wonder what happens if a user defines their own method using the __dunder__ naming convention.

e.g.

class A:
    def __foo__():
        pass

According to the docs

__*__
System-defined names, informally known as “dunder” names. These names are defined by the interpreter and its implementation (including the standard library). Current system names are discussed in the Special method names section and elsewhere. More will likely be defined in future versions of Python. Any use of __*__ names, in any context, that does not follow explicitly documented use, is subject to breakage without warning.

So we should warn users against ever creating a method with that naming convention!

Desired solution

New lint checker that checks for any method definition with a __dunder__ style name that is not an official python __dunder__ method.

Additional context

No response

[Pierre-Sassoulas]

Closing as duplicate of #3038