I can't figure out why `isinstance()` returns `True` for these subclasses

I'm using the Python package ariadne, v0.23.0.

I wrote a utility to scan my code for instances of ariadne.types.SchemaBindable, but it's also unintentionally picking up the SchemaBindable subclasses that I've imported:

ariadne.input.InputType( SchemaBindable )
ariadne.objects.ObjectType( SchemaBindable )
ariadne.scalars.ScalarType( SchemaBindable )
ariadne.unions.UnionType( SchemaBindable )

I ran a test in a Python shell, and sure enough, isinstance() is returning True when comparing those classes to SchemaBindable:

isinstance( ObjectType, SchemaBindable )  ->  True
...etc...

SchemaBindable even appears to be an instance of itself:

isinstance( SchemaBindable, SchemaBindable )  ->  True

Meanwhile, issubclass() continues to also return True:

issubclass( ObjectType, SchemaBindable )  ->  True

Make it make sense.

Solution

ariadne.types.SchemaBindable is documented as a regular class that you're supposed to extend to create bindables, but it's actually implemented as a protocol, and marked runtime-checkable:

@runtime_checkable
class SchemaBindable(Protocol):
    # docstring omitted because it's long

    def bind_to_schema(self, schema: GraphQLSchema) -> None:
        """Binds this `SchemaBindable` instance to the instance of GraphQL schema."""

Runtime-checkable protocols use a metaclass __instancecheck__ method to customize isinstance checks. Any object that has the attributes specified by the protocol will be considered an instance of the protocol.

But if you write a subclass of SchemaBindable:

class YourBindable(SchemaBindable):
    def bind_to_schema(self, schema):
        # implementation here

then that subclass has a bind_to_schema attribute, and the checker logic doesn't care that that attribute is meant to be a method implementation for instances of YourBindable. It just checks that the attribute exists, and if the attribute is supposed to be callable, it checks that the attribute isn't None:

        for attr in cls.__protocol_attrs__:
            try:
                val = getattr_static(instance, attr)
            except AttributeError:
                break
            # this attribute is set by @runtime_checkable:
            if val is None and attr not in cls.__non_callable_proto_members__:
                break
        else:
            return True

        return False

So that means that subclasses of SchemaBindable get treated as instances of SchemaBindable, even though they probably shouldn't be.