mpy gives keeps raising error because of Protocol

Question

Im using pytest framework with mypy and when writing fixtures I created Protocols to mimic return types. (I could not import it directly because of env instantiation)

This example greatly simplifies my troubles:

 from typing_extensions import Protocol

class LoadServiceType(Protocol):
    z: int

class LoadService:
    z: int

#This works as expected
load_serivce: LoadServiceType = LoadService()

# But now I dont know why this does not work
class FooType(Protocol):
    load_service: LoadServiceType
    
class Boo:
    x:int
    load_service:LoadService
    bb: int
    
    
cc: FooType = Boo()

Basically I want to use my FooType as a return type because of import issues. Is there any solution ?

Answer 1

This problem can be represented by the following minimal example:

from dataclasses import dataclass
from typing import Protocol


@dataclass
class Fooish:
    bar: int


class FooLike(Protocol):
    bar: float


foo: FooLike = Fooish(bar=321)

mypy raises the following error:

foo.py:18: error: Incompatible types in assignment (expression has type "Fooish", variable has type "FooLike")  [assignment]
foo.py:18: note: Following member(s) of "Fooish" have conflicts:
foo.py:18: note:     bar: expected "float", got "int"

The FooLike protocol represents a set of objects which have a writable float member named bar. The Fooish class' bar member is an int and does not accept float values to be assigned, thus is incompatible.

According to foo: FooLike, the following should be valid, but isn't:

foo: FooLike = Fooish(bar=321)
foo.bar = 21.37

---

To fix the compatibility issue, one should define the variable as read-only via a @property:

from dataclasses import dataclass
from typing import Protocol


@dataclass
class Fooish:
    bar: int


class FooLike(Protocol):
    @property
    def bar(self) -> float:
        ...


foo: FooLike = Fooish(bar=321)

---

So, going back to your original example, the following snippet works as expected:

from typing_extensions import Protocol


class LoadServiceType(Protocol):
    z: int


class LoadService:
    z: int


load_serivce: LoadServiceType = LoadService()


class FooType(Protocol):
    @property
    def load_service(self) -> LoadServiceType:
        ...


class Boo:
    x: int
    load_service: LoadService
    bb: int


cc: FooType = Boo()