“Duplicate base class” blocks checking

[xiaxinmeng]

The duplicate base class causes the checking blocked.

example.py

class N(type):
    pass

class B(N, metaclass=N):
    pass

class D(B, B):
    pass

The expected behavior：

The error can be ignored and the checking will not be blocked.

The actual behavior：

>> mypy example.py
Desktop/example.py:7: error: Duplicate base class "B"
Found 1 error in 1 file (errors prevented further checking)

Environment

Mypy version: 1.3.0
Python version: 3.11.3
Operating System: Ubuntu 18.04
How to install MyPy: "python3.11 -m pip install mypy"

[gregorysantosa]

@xiaxinmeng I believe mypy is correct in this case because when class D is constructed, since it inherits two instances of class B, when an attribute or method of class N is invoked from an instance of class D, Python does not know which instance of class N should be called. I believe this issue can be worked around by removing the "metaclass=N" argument from class B

[sobolevn]

I think that this is a correct thing to do:

1. It is a TypeError in runtime:

>>> class A: ...
... 
>>> class B(A, A): ...
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: duplicate base class A

So, you app won't work and mypy catches that.

2. It is not clear how to construct D type info for us, so we can continue type checking

[JelleZijlstra]

Can't we make it inherit from Any or ignore the second base? I feel we should strongly avoid blocking errors if possible.

[sobolevn]

We can still inherit from one of them, I guess. I will take a look and send a PR.