How to create type hinting for a generic factory method?

Question

How can type hints be declared to indicate that a function returns an instance of the class reference that is passed as an argument?

Declaring it as follows does not seem right, as it indicates that the returned type is the same as the type of the argument:

from typing import TypeVar


T = TypeVar('T')

def my_factory(some_class: T) -> T:
    instance_of_some_class = some_class()
    return instance_of_some_class

Example usage:

class MyClass:
    pass

my_class = my_factory(MyClass)  # Inferred type should be MyClass

This doesn't really work with mypy it seems. github.com/python/mypy/issues/8946 — LtWorf
– LtWorf, Commented Jun 14, 2020 at 8:43

David Pärsson · Accepted Answer · 2017-02-14 13:03:06Z

41

According to PEP-484, the right way to do this is to use Type[T] for the argument:

from typing import TypeVar, Type


T = TypeVar('T')

def my_factory(some_class: Type[T]) -> T:
    instance_of_some_class = some_class()
    return instance_of_some_class

It however seems like my editor does not (yet) support this.

answered Feb 14, 2017 at 13:03

David Pärsson

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Unmitigated · Accepted Answer · 2024-12-14 21:24:39Z

7

Python 3.12 introduces type parameter lists, which allows generic functions to be defined without explicit TypeVars. Moreover, typing.Type was deprecated in Python 3.9 in favor of using type directly.

The function can be rewritten like so:

def my_factory[T](some_class: type[T]) -> T:
    return some_class()

answered Dec 14, 2024 at 21:24

Unmitigated

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