Wrong warning when assigning __str__

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

Start with the provided generate_str reproducer and inspect how mypy checks assignments to str. Done means the reproducer no longer emits the reported incompatible-types warning while preserving the callable's self argument.

Written by the indexing model from the issue text.

Description

bug topic-descriptors topic-runtime-semantics

I have the following decorator:

from typing import Type, Any

def generate_str(cls: Type) -> Type:
    """
    Class decorator that auto generates __str__().
    :param cls: class to modify
    :return: modified class
    """
    def __str__(self) -> str:
        return _pretty_print_obj(self)

    cls.__str__ = __str__
    return cls

Mypy 0.521 on Python 3.6.2 emits this warning:

error: Incompatible types in assignment (expression has type Callable[[Any], str], variable has type Callable[[], str])
Dominant language
Python
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