argument type that is constrained to a class and protocol

[shadycuz]

Not sure if I explained it correctly in the title, so here is some sample code.

from typing import Protocol, TypeVar


class HasJob(Protocol):

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


class Person:
    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age


class Engineer(Person):
    def __init__(self, name: str, age: int):
        super().__init__(name, age)

    def job(self) -> str:
        return "Engineer"


# T = TypeVar("T", bound=HasJob)


# A function that accepts a Person with a job method
def go_to_work(person: Person):
    print(f"{person.name} is going to work as a {person.job()}")


bob = Engineer("Bob", 30)

go_to_work(bob)

I would like my go_to_work function to take any Person or subclass of Person. But the class that is passed in must also implement HasJob.

I have not found a way to do it with TypeVar as is. I'm trying to avoid creating a new class like Employee.

[JelleZijlstra]

What you're looking for is called an intersection type. This is not currently part of the Python type system, but it's a popular idea: #213.

[shadycuz]

Thanks for linking to that. Since they are asking for people not to post there, then I guess I will post here =).

Even though you cant specify a intersection type today. If you take a class and then check if it fits a protocol using isinstance, Pylance can put together the correct "intersection" type.

Code:

if isinstance(compatiple_version, UpgradeableConfig):
  params = compatiple_version.upgrade(params)

Pylance says the type is:

It does seem like, at least in Pylance, the intersection is possible to have. It's just not possible to define one.

[JelleZijlstra]

Yes, type checkers may internally synthesize an intersection type in some cases. However, it is not possible to define such a type in an annotation.