Typehint method as returning return type of other method in python?

Question

I have a base class:

from abc import abstractmethod

class Thing:
    @abstractmethod
    def _process(self):
        ...

    def process(self, x: int):
        self.pre_process(x)
        return self._process()

How do I typehint process as returning the return type of _process? My first thought was something like:

from abc import abstractmethod
from typing import TypeVar

class Thing:
    T = TypeVar("T")

    @abstractmethod
    def _process(self) -> T:
        ...

    def process(self, x: int) -> T:
        ...

But mypy 1.3.0 complains quite rightly that T is only present once in the function signature:

> mypy /tmp/t.py
...
error: A function returning TypeVar should receive at least one argument containing the same TypeVar
...

Answer 1

You can make Thing inherit Generic[T].

from typing import TypeVar
from typing import Generic
from abc import abstractmethod

T = TypeVar("T")

class Thing(Generic[T]):
    @abstractmethod
    def _process(self) -> T:
        ...

    def process(self, x: int) -> T:
        return self._process()

> mypy /tmp/t.py
Success: no issues found in 1 source file

Now you can inherit from Thing like this:

class A(Thing[list]):
    def _process(self) -> list:
        return []