python - How can I type-hint a function where the return type depends on the input type of an argument?

Question

Assume that I have a function which converts Python data-types to Postgres data-types like this:

def map_type(input):
    if isinstance(input, int):
        return MyEnum(input)
    elif isinstance(input, str):
        return MyCustomClass(str)

I could type-hint this as:

def map_type(input: Union[int, str]) -> Union[MyEnum, MyCustomClass]: ...

But then code like the following would fail to type-check even though it is correct:

myvar = map_type('foobar')
print(myvar.property_of_my_custom_class)

Complete example (working code, but errors in type-hinting):

from typing import Union
from enum import Enum


class MyEnum(Enum):
    VALUE_1 = 1
    VALUE_2 = 2


class MyCustomClass:

    def __init__(self, value: str) -> None:
        self.value = value

    @property
    def myproperty(self) -> str:
        return 2 * self.value


def map_type(value: Union[int, str]) -> Union[MyEnum, MyCustomClass]:

    if isinstance(value, int):
        return MyEnum(value)
    elif isinstance(value, str):
        return MyCustomClass(value)
    raise TypeError('Invalid input type')


myvar1 = map_type(1)
print(myvar1.value, myvar1.name)

myvar2 = map_type('foobar')
print(myvar2.myproperty)

I'm aware that I could split up the mapping into two functions, but the aim is to have a generic type-mapping function.

I was also thinking about working with classes and polymorphism, but then how would I type-hint the topmost class methods? Because their output type would depend on the concrete instance type.

Answer 1

This is exactly what function overloads are for.

In short, you do the following:

from typing import overload

# ...snip...

@overload
def map_type(value: int) -> MyEnum: ...

@overload
def map_type(value: str) -> MyCustomClass: ...

def map_type(value: Union[int, str]) -> Union[MyEnum, MyCustomClass]:
    if isinstance(value, int):
        return MyEnum(value)
    elif isinstance(value, str):
        return MyCustomClass(value)
    raise TypeError('Invalid input type')

Now, when you do map_type(3), mypy will understand that the return type is MyEnum.

And at runtime, the only function to actually run is the final one -- the first two are completely overridden and ignored.