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Specify return type of a wrapper function that calls an abstract method in Python

January 7, 2023

For this example, consider the simplified scenario where a Solver will return a Solution.

We have Solutions:

class Solution(ABC):
    pass


class AnalyticalSolution(Solution):
    pass


class NumericalSolution(Solution):
    def get_mesh_size(self) -> float:
        return 0.12345

And Solvers:

class Solver(ABC):
    def solve(self, task: int) -> Solution:
        # Do some pre-processing with task
        # ...
        return self._solve(task)

    @abstractmethod
    def _solve(self, task: int) -> Solution:
        pass


class NumericalSolver(Solver):
    def _solve(self, task: int) -> NumericalSolution:
        return NumericalSolution()


class AnalyticalSolver(Solver):
    def _solve(self, task: int) -> AnalyticalSolution:
        return AnalyticalSolution()

The problem I encounter results from the implementation of the wrapper method solve that then calls the abstract method _solve.
I often encounter a situation like this where I want to do some preprocessing in the solve method that is the same for all solver, but then the actual implementation of _solve might differ.

If I now call the numerical solver and call the get_mesh_size() method, Pylance (correctly) tells me that a Solution object has no get_mesh_sizemember.

if __name__ == "__main__":
    solver = NumericalSolver()
    solution = solver.solve(1)
    print(solution.get_mesh_size())

I understand that Pylance only sees the interface of solve which indicates that the return type is a Solution object that does not need to have a get_mesh_size method.
I am also aware that this example works at runtime.

I tried to use TypeVar like this (actually, because ChatGPT suggested it):

class Solution(ABC):
    pass
T = TypeVar("T", bound=Solution)

and then rewrite the Solver class:

class Solver(ABC):
    def solve(self, task: int) -> T:
        # Do some pre-processing with task
        # ...
        return self._solve(task)

    @abstractmethod
    def _solve(self, task: int) -> T:
        pass

But Pylance now tells me TypeVar "T" appears only once in generic function signature. So this can’t be the solution.

How do I get typing to work with this example?

>Solution :

You may use Generic[T] as a base for Solver and then extend it as follows

from abc import ABC, abstractmethod
from typing import TypeVar, Generic


class Solution(ABC):
    pass


class AnalyticalSolution(Solution):
    pass


class NumericalSolution(Solution):
    def get_mesh_size(self) -> float:
        return 0.12345


SolutionGeneric = TypeVar("SolutionGeneric", bound=Solution)


class Solver(ABC, Generic[SolutionGeneric]):
    def solve(self, task: int) -> SolutionGeneric:
        # Do some pre-processing with task
        # ...
        return self._solve(task)

    @abstractmethod
    def _solve(self, task: int) -> SolutionGeneric:
        pass


class NumericalSolver(Solver[NumericalSolution]):
    def _solve(self, task: int) -> NumericalSolution:
        return NumericalSolution()


class AnalyticalSolver(Solver):
    def _solve(self, task: int) -> AnalyticalSolution:
        return AnalyticalSolution()


if __name__ == "__main__":
    solver = NumericalSolver()
    solution = solver.solve(1)
    print(solution.get_mesh_size())