Method that Defines and Returns Class Instance

[adam-grant-hendry]

I am using a third-party library that defines a method like so:

# some_module.py

def func(cls_to_wrap):
    class A(cls_to_wrap):
        ...
    return A()

The function, class, and class methods are all not type-hinted, so the above is an accurate representation. My problem is when using this method:

wrapped = func(some_class)

mypy rightly is unable to determine that wrapped has any attributes defined by A and gives an attr-defined error. I cannot use something like assert isinstance(wrapped, A) because A is not exposed in some_module.py.

Being that this is a third-party library that I cannot change, is my only option at this point to use # typing: ignore to disable the warning message?

[TeamSpen210]

You could declare it as def func(cls: Type[T]) -> T:, but that doesn't include the additional attributes A offers. What you need is -> Intersection[T, ProtocolOfA] (#213), where you define a (possibly generic) protocol outside the function with the methods contained. But we don't have intersections. Perhaps you could do def func(cls: Type[ProtocolA[T1, T2]]) -> ProtocolB[T1, T2]:, with one to match the parent class and extract the defined types, then a second to declare the combination...

[srittau]

You could define your own @runtime_checkable protocol with the fields of A and use isinstance against that.

from typing import runtime_checkable, Protocol

@runtime_checkable
class A(Protocol):
   ...

wrapped = func(some_class)
assert isinstance(wrapped, A)