How can I use mypy to type check a date that cannot be a datetime?

Question

Let's say I have this function:

def subtract_dates(date1: datetime.date, date2: datetime.date):
    return date1 - date2

If I try to call subtract_dates(datetime.date.today(), datetime.datetime.now()) this will raise a TypeError because I'm trying to subtract a datetime object from a date object.

However, because datetime inherits from date (see for instance this issue), MyPy does not raise an error on the above call to subtract_dates.

Is there a way to add a MyPy type hint that allows a date object but not a datetime object?

Answer 1

I'd solve it by function overloading. Example:

import datetime
from typing import NoReturn, overload


@overload
def subtract_dates(date1: datetime.date, date2: datetime.datetime) -> NoReturn: ...

@overload
def subtract_dates(date1: datetime.datetime, date2: datetime.date) -> NoReturn: ...

@overload
def subtract_dates(date1: datetime.date, date2: datetime.date) -> datetime.timedelta: ...


# this is your original function implementation    
def subtract_dates(date1: datetime.date, date2: datetime.date):
    return date1 - date2

Now, subtracting dates is still allowed, while using datetime objects will be caught:

subtract_dates(datetime.date.today(), datetime.datetime.now()).days

will give you an error

<string>:1: error: "NoReturn" has no attribute "days"
Found 1 error in 1 file (checked 1 source file)

Answer 2

The only way to really do this definitively is to recreate all the stubs for datetime in such a way that it pretends not to be a subclass of date. This is really tedious though.

That's why I did it for you and put it into an open source library.

You can see some examples of how to use it here.