@property on subclass raises error

[gibiansky]

Defining a property in a superclass and then using a @property decorator in a subclass does not detect that the signatures match.

I think this is a bug, but I'm not sure if there are some edge cases I haven't thought through that make this error valid.

Here's code:

class Superclass:                    
    prop: int                   
                                     
class Subclass(Superclass):          
    @property                        
    def prop(self) -> int: ...

The mypy error:

test.py:10: error: Signature of "prop" incompatible with supertype "Superclass"

Mypy --version is mypy 0.770.

I tried to look through the list of issues with @property but couldn't determine if this is a duplicate; the others (that seem largely resolved) have to do with @property whereas this has to do with @property when in a subclass with a superclass that has a type annotation.

This might be a duplicate of #5936; if so, please close this. I wasn't sure because that seems to have to do with setters, which this does not.

[JelleZijlstra]

This is because prop is assignable on the superclass but not the subclass.

Example:

def f(x: Superclass):
    x.prop = 42

f(Subclass())  # fails at runtime, but mypy is OK with it

[gibiansky]

Ah, thank you. Could there be a clearer error message for this?

Also, should this work then:

class Superclass:                    
    prop: int                        
                                     
                                     
class Subclass(Superclass):          
    @property                        
    def prop(self) -> int:           
        ...                          
                                     
    @prop.setter                     
    def prop(self, val: int) -> None:
        ...                          

Because I get the same errors there, but that addresses this concern.

[JelleZijlstra]

Hm, yes. That should be allowed unless I'm missing something.

Technically it's unsafe because the base class would allow del obj.prop and the subclass wouldn't, but I don't think mypy is that pedantic.

[rik]

I think this is a duplicate of #4125

[JukkaL]

Yeah, looks like a duplicate of #4125.

[gibiansky]

Thanks for figuring this out!