How to type a function that takes either a class, or an instance of the class, and returns an instance of that class?

user_2233608 0
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I want to type a function that can take either a class type, or an instance of the class, and then return an instance of that same class. For example:

from typing import Type, TypeVar, Union

DT = TypeVar("DT")

def tmap(dest_type_or_obj: Union[DT, Type[DT]]) -> DT:
    if isinstance(dest_type_or_obj, type):
        dest_obj = dest_type_or_obj()
    else:
        dest_obj = dest_type_or_obj
    return dest_obj

class Dest:
    pass

instance_dest = tmap(Dest())  # Works fine

type_dest = tmap(Dest)  # [arg-type] mypy(error)
# Argument 2 to "map" of "ObjectMapper" has incompatible type "Type[Dest]"; expected "Type[<nothing>]"
python-typing mypy python
2 Answers

You should bound the Dest class to TypeVar, to prevent the Type[<nothing>] error:

from typing import Type, TypeVar, Union


class Dest:
    pass


DT = TypeVar("DT", bound=Dest)


def tmap(dest_type_or_obj: Union[DT, Type[DT]]) -> DT:
    if isinstance(dest_type_or_obj, type):
        dest_obj = dest_type_or_obj()
    else:
        dest_obj = dest_type_or_obj
    return dest_obj


instance_dest = tmap(Dest())
print((type(instance_dest)))
type_dest = tmap(Dest) 
print((type(type_dest)))
# output
<class '__main__.Dest'>
<class '__main__.Dest'>
user_95555050

While technically equivalent, MyPy requires to separate both cases using overload:

@overload
def tmap(dest_type_or_obj: Type[DT]) -> DT:
    ...
@overload
def tmap(dest_type_or_obj: DT) -> DT:
    ...

This enables MyPy to infer the type variable properly in both the instance and type case.

reveal_type(tmap(Dest()))  # note: Revealed type is "__main__.Dest*"

reveal_type(tmap(Dest))    # note: Revealed type is "__main__.Dest*"
user_53499160
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