How to add hint to a factory method?

问题

I'm looking for a way to annotate return type of a factory function.

It returns random child of 'AlgorithmBase'.

class AlgorithmFactory:
    _algorithm_types = AlgorithmBase.__subclasses__()

    def select_random_algorithm(self) -> AlgorithmBase:
        # Select random algorithm
        algorithm_class = self._random_generator.choice(AlgorithmFactory._algorithm_types)
        algorithm = algorithm_class()
        return algorithm

I get error from mypy:

The error I'm getting is:

Cannot instantiate abstract class 'AlgorithmBase' with abstract attributes 'get_constraints' and 'satisfy_constraints'

There is no way to instantiate class 'AlgorithmBase' in this code, how to make mypy understand it?

I want to avoid specifying actual sub-classes with 'Union' in return type. Any suggestions?

回答1:

The problem here wasn't return type, but '_algorithm_types'. mypy has no way to understand what type it is, so it assumed that it is like return type and got error.

The following code fix the issue:

_algorithm_types: List[Type[AlgorithmBase]] = AlgorithmBase.__subclasses__()

回答2:

As far as I can tell this should work, but it seems like one or more of your AlgorithmBase subclasses doesn't implement these two abstract methods.

Running MyPy for

import abc

class AlgorithmBase(abc.ABC):
    @abc.abstractmethod
    def get_constraints(self):
        raise NotImplementedError

    @abc.abstractmethod
    def satisfy_constraints(self):
        raise NotImplementedError


class SomeAlgorithm(AlgorithmBase):
    pass


class AlgorithmFactory:
    def get(self) -> AlgorithmBase:
        algorithm = SomeAlgorithm()
        return algorithm

yields the same error you get, and it runs without any error once the methods are implemented.

import abc

class AlgorithmBase(abc.ABC):
    @abc.abstractmethod
    def get_constraints(self):
        raise NotImplementedError

    @abc.abstractmethod
    def satisfy_constraints(self):
        raise NotImplementedError


class SomeAlgorithm(AlgorithmBase):
    def get_constraints(self):
        pass

    def satisfy_constraints(self):
        pass


class AlgorithmFactory:
    def get(self) -> AlgorithmBase:
        algorithm = SomeAlgorithm()
        return algorithm

来源：https://stackoverflow.com/questions/62283480/how-to-add-hint-to-a-factory-method

标签

python

python-3.x

mypy