Python Type Hinting: Output Type Based on Input Type

According to Suggested syntax for Python 2.7 and straddling code in PEP 484 which defined type hinting, there is an alternative syntax for compatibility with Python 2.7. It is however not mandatory so I don't know how well supported it is, but quoting the PEP:

Some tools may want to support type annotations in code that must be compatible with Python 2.7. For this purpose this PEP has a suggested (but not mandatory) extension where function annotations are placed in a # type: comment. Such a comment must be placed immediately following the function header (before the docstring). An example: the following Python 3 code:

def embezzle(self, account: str, funds: int = 1000000, *fake_receipts: str) -> None:
    """Embezzle funds from account using fake receipts."""
    <code goes here>

is equivalent to the following:

def embezzle(self, account, funds=1000000, *fake_receipts):
    # type: (str, int, *str) -> None
    """Embezzle funds from account using fake receipts."""
    <code goes here>

For mypy support, see Type checking Python 2 code.

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.