Behaviour of not annotated __init__ on inheritance when child class has no parameters

[kprzybyla]

After introduction of #604 you are not longer obligated to specify that __init__ returns None type. However, if you dealing with inheritance and the child class has no parameters, thus it is not annotated if you don't specify the __init__ return type, you unknowingly disable mypy inspection in that class. To make those inspections work you have to explicitly pass --check-untyped-defs argument or simply add return type annotation to __init__.

Now, I'm not saying that it is a bug but this behavior is not so obvious. Especially that after introduction of #604 most people probably removed those explicit annotations from their code or stopped writing them after that point.

Because of that, you can unknowingly make mypy validation pass even thought it shouldn't:

class Base:
    def __init__(self, value: str):
        self.value = value
        reveal_type(self.value)

class Bad(Base):
    def __init__(self):
        super().__init__(value=None)
        reveal_type(self.value)

example_1.py:4: note: Revealed type is 'builtins.str'
example_1.py:10: note: Revealed type is 'Any'
example_1.py:10: note: 'reveal_type' always outputs 'Any' in unchecked functions

However, after adding the annotation (-> None), everything will work as expected:

class Base:
    def __init__(self, value: str):
        self.value = value
        reveal_type(self.value)

class Bad(Base):
    def __init__(self) -> None:
        super().__init__(value=None)
        reveal_type(self.value)

example_2.py:4: note: Revealed type is 'builtins.str'
example_2.py:9: error: Argument "value" to "__init__" of "Base" has incompatible type "None"; expected "str"
example_2.py:10: note: Revealed type is 'builtins.str'
Found 1 error in 1 file (checked 1 source file)

You can also get the same result with the code from example_1.py if you pass --check-untyped-defs parameter to mypy.

Maybe it is worth adding this special case in mypy documentation? Maybe inside the No errors reported for obviously wrong code section in Common issues and solutions chapter?

Versions

mypy 0.761
Python 3.7.4

[JukkaL]

Yeah, this is a known gotcha and it would be useful to document it clearly.

Another idea (though a bit ad hoc) would be to generate a warning if class is otherwise annotated but a no-argument __init__ is missing an annotation.

[ilevkivskyi]

Fixed by #8303