mypy claims compatible types are incompatible when non-related mixin classes are present

[zzzeek]

as always, I apologize if this falls into one of the categories of "that's expected, because what if you did this?" and/or "this is a known mypy limitation / issue number", this one does not have a great workaround since I'd have to "type:ignore" the class def itself, or have to add redundant attributes everywhere, and I will likely just re-organize the code to not have this particular issue.

mypy --version
mypy 0.950+dev.0fecec8c72605ec7bae343b28d545c477c726e81 (compiled: no)

Here's the demo - ImplementsWOMixin and ImplementsWMixin are basically the same class except one of them has an extra entry in its __bases__. mypy then decides that a local attribute that returns str instead of Optional[str] is no longer compatible with the base class:

from typing import Optional


class Base:
    description: Optional[str]


class MixinWDesc:
    description: str


class SubClass(MixinWDesc, Base):
    description: str


class SomeOtherMixin:
    pass

# succeeds
class ImplementsWOMixin(SubClass):
    pass


# fails
class ImplementsWMixin(SomeOtherMixin, SubClass):
    pass

output - it complains about MixinWDesc, but the line number is that of the ImplementsWMixin class itself, meaning every class that subclasses MixinWDesc and uses other mixins would need workarounds or type:ignores.

$ mypy test3.py 
test3.py:26: error: Definition of "description" in base class "MixinWDesc" is incompatible with definition in base class "Base"  [misc]
Found 1 error in 1 file (checked 1 source file)

[zzzeek]

Easier workaround is I will change MixinWDesc alone:

class MixinWDesc:
    description: Optional[str]

[JelleZijlstra]

This error is correct. Your type annotations define a read-write attribute, so a function that gets an object b: Base is allowed to set b.description = None. But MixinWDesc requires that it's not None.

To prevent this error, make the attribute read-only by using a property.

[zzzeek]

why does ImplementsWMixin and ImplementsWOMixin behave differently?

[zzzeek]

also I generally cant use @property due to #4125. hopefully mypy can recognize other descriptor types that don't have __set__, will find out

[zzzeek]

nope , it doesn't work:

from typing import Optional, TypeVar, Generic, Any, Callable, overload, Union

_T_co = TypeVar("_T_co", covariant=True)



_GFDRO = TypeVar("_GFDRO", bound="generic_ro_descriptor[Any]")


class generic_ro_descriptor(Generic[_T_co]):
    fget: Callable[..., _T_co]
    __doc__: Optional[str]
    __name__: str

    def __init__(self, fget: Callable[..., _T_co], doc: Optional[str] = None):
        self.fget = fget  # type: ignore[assignment]
        self.__doc__ = doc or fget.__doc__
        self.__name__ = fget.__name__

    @overload
    def __get__(self: _GFDRO, obj: None, cls: Any) -> _GFDRO:
        ...

    @overload
    def __get__(self, obj: object, cls: Any) -> _T_co:
        ...

    def __get__(self: _GFDRO, obj: Any, cls: Any) -> Union[_GFDRO, _T_co]:
        raise NotImplementedError()

    # whether or not these are here
    def __set__(self, instance: Any, value: Any) -> NoReturn:
        ...

    def __delete__(self, instance: Any) -> NoReturn:
        ...



class Base:
    @generic_ro_descriptor
    def description(self) -> Optional[str]:
        ...


class MixinWDesc:
    @generic_ro_descriptor
    def description(self) -> str:
        ...


class SubClass(MixinWDesc, Base):
    @generic_ro_descriptor
    def description(self) -> str:
        ...


class SomeOtherMixin:
    pass

# succeeds
class ImplementsWOMixin(SubClass):
    pass


# fails:
#  Definition of "description" in base class "MixinWDesc" is incompatible with definition in base class "Base"
class ImplementsWMixin(SomeOtherMixin, SubClass):
    pass

is @property the only way Mypy sees something as "read only" ? a descriptor with no __set__ method, or __set__ method that returns NoReturn is not sufficient ?

[zzzeek]

OK, I guess I will try to use the "tell typing it's @property but use your own descriptor" trick...

[JelleZijlstra]

Good point, it should complain about Subclass but not the other ones.

[zzzeek]

still wondering if there's any way to make my own descriptor that is not @property that mypy sees as "read only". I would think a descriptor class that has __get__() but not __set__(), or perhaps a __set__() that returns NoReturn, would be seen as a read-only property, but at the moment I think this is not the case (going to test a little more today). (edit: I guess I already did all this above and none of it works. Some clear docs on this whole area would be helpful).

I still encounter inconvenient behaviors with @property that are difficult to debug because I dont know what's hardcoded with @property and is beyond the scope of making sense without reading mypy's source code.

[zzzeek]

or otherwise, if the whole "mypy has a special rule that @Property can be used to make a read only field" could be better documented, the only mention I can find for this concept is as a passing phrase here, "You can use @property to make an attribute read-only, but unlike Final, it doesn’t work with module attributes, and it doesn’t prevent overriding in subclasses." that whole concept ("you can use @property, and @property alone, not any other kind of descriptor, to make an attribute read-only") is not obvious to someone who doesnt know it already. the variety of hardcoded "property" behaviors should really be documented, there should be a section in mypy's docs with all the special rules that are present right now listed out ( as well as which are exclusive to @property alone and not user-defined descriptors).

I would gladly contribute this documentation if I myself knew the behaviors well, but I only have a vague concept right now.