Mypy Gives `Incompatible Definitions` Error When Dynamically Typed

Bug Report

Discussed in https://github.com/python/typing/discussions/1235

^{Originally posted by adam-grant-hendry August 4, 2022}
Related to this SO question, I'm trying to understand why

• this doesn't yield errors

but

• adding **kwargs to the end of the run functions does

I would have assumed since nothing is statically typed, that mypy would not produced any errors. In fact, the same code on the pyright playground shows no errors.

I though perhaps the error could be due to the potential for a keyword collision, which I gleaned from PEP 692. e.g. If I made the name of the 4th arguments the same, each could fail the same way if I tried to pass foo as a keyword argument:

>>> a = Foo()
>>> a.run(1, 2, 'baz', foo='bar')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: func() got multiple values for argument 'foo'

but if I keep the argument names different, then the above would work if I tried to call Bar.run(1, 2, 'baz', foo='bar'), but break for Foo.

However, I still would originally assume mypy would do nothing because nothing has been statically typed:

By default, mypy will not type check dynamically typed functions. This means that with a few exceptions, mypy will not report any errors with regular unannotated Python.

To Reproduce

Please see the mypy playground (and pyright playground) gists above.

Expected Behavior

mypy issues no errors.

Actual Behavior

mypy issues the error:

Definition of "run" in base class "Foo" is incompatible with definition in base class "Bar"

Your Environment

• Mypy version used: mypy 0.971 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.10, x64-bit
• Operating system and version: Windows 10, version 20H2

Start with the two linked mypy-play reproductions and compare their behavior with the linked Pyright playground, focusing on the dynamically typed method definitions and added **kwargs. Check the related typing discussion and existing mypy tests for method-definition compatibility; done means the behavior is explained and, if confirmed as a bug, covered by a regression test.