Multiple inheritance using typing module and mypy

Question

Consider the following code:

from typing import Union

class A:
  def function_in_a(self) -> str:
    return 'a'

class B:
  def function_in_b(self) -> str:
    return "b"

class C(A, B):
  pass

def call_functions(c: Union[A, B]) -> None:
  print(c.function_in_a())
  print(c.function_in_b())

if __name__=="__main__":
  c = C()
  call_functions(c)

Note that the function call_functions relies on definitions contained in both classes A and B. It expects objects that inherit from both of these classes.

This code will compile when run using python test.py. But mypy --strict test.py throws an error:

test.py:15: note: Revealed type is "Union[test.A, test.B]"
test.py:16: error: Item "B" of "Union[A, B]" has no attribute "function_in_a"
test.py:17: error: Item "A" of "Union[A, B]" has no attribute "function_in_b"
Found 2 errors in 1 file (checked 1 source file)

This makes sense to me. Union means that c can be a subclass of either A or B, but not both. I saw mention of an Intersection type in PEP483 but a quick perusal of the typing module docs showed that this type was never implemented.

How can I get mypy to recognize that parameters of call_functions are objects which inherit from both A and B using type hinting?

It's currently not possible, there is an open issue to support it, the suggest workaround is to use Protocol - github.com/python/mypy/issues/8355 — daniel
– daniel, Commented Jan 23, 2022 at 8:05

rkachach · Accepted Answer · 2022-10-17 13:40:56Z

2

Use typing.Protocol (New in version 3.8.) to define a type that must implement both methods invoked in the function.

from typing import Protocol


class A:
    def function_in_a(self) -> str:
        return 'a'


class B:
    def function_in_b(self) -> str:
        return "b"


class C(A, B):
    pass


class D(B):
    pass


class ProtoAB(Protocol):
    def function_in_a(self) -> str:
        ...

    def function_in_b(self) -> str:
        ...


def call_functions(obj: ProtoAB) -> None:
    print(obj.function_in_a())
    print(obj.function_in_b())


def main() -> None:

    c = C()
    call_functions(c)
    d = D()
    call_functions(d)


if __name__ == "__main__":
    main()

edited Oct 17, 2022 at 13:40

rkachach

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answered Jan 22, 2022 at 19:23

user3515670

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2 Comments

user32882 Over a year ago

This is so much more complicated than the original code. Also, I feel like this is improper use of protocols.... I'm mainly concerned with type hinting a simple case of inheriting from multiple types. There has got to be an easier way.

user32882 Over a year ago

After digressing into the issue indicated by @daniel and looking a bit more deeply into protocols, I'm convinced this is the correct answer. Intersections are not yet implemented in the typing module, and the fact that Protocols can help alleviate this problem is stated explicitly in PEP544: One can use multiple inheritance to define an intersection of protocols. If this will prove to be a widely used scenario, then a special intersection type construct could be added in future as specified by PEP 483, see rejected ideas for more details.

hussic · Accepted Answer · 2022-01-23 09:09:46Z

0

Another solution is to make A and B Protocol. Protocols can can be used as normal class:

from typing import Protocol
        
class A(Protocol):
    def function_in_a(self) -> str:
        return 'a'
class B(Protocol):
    def function_in_b(self) -> str:
        return "b"
class AB(A, B, Protocol):
    pass
    
def call_functions(c: AB) -> None:
    print(c.function_in_a())
    print(c.function_in_b())

class C(A, B):
    pass
call_functions(C())

answered Jan 23, 2022 at 9:09

hussic

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