I have this very simple function:

import datetime

def create_url(check_in: datetime.date) -> str:
"""take date such as '2018-06-05' and transform to format '06%2F05%2F2018'"""
    _check_in = check_in.strftime("%Y-%m-%d")
    _check_in = _check_in.split("-")
    _check_in = _check_in[1] + "%2F" + _check_in[2] + "%2F" + _check_in[0]

    return f"https://www.website.com/?arrival={_check_in}"

mypy throws the following error: error:Incompatible types in assignment (expression has type "List[str]", variable has type "str") for line 6 _check_in = _check_in.split("-"). I've tried renaming _check_inon line 6 but that makes no difference. This function works fine.

Is this the expected behavior? How do I fix the error.

Thanks!

In the first line _check_in = check_in.strftime("%Y-%m-%d"), _check_in is a string (or str as mypy like to think), then in _check_in = _check_in.split("-") _check_in becomes a list of string (List[str]), since mypy already think this should be a str, it will complain (or rather warn you about it as it's not a particularly nice practice).

As for how you should fix it, just rename the variable appropriately, or you can do _check_in = _check_in.split("-") # type: List[str] (and also _check_in = _check_in[1] + "%2F" + _check_in[2] + "%2F" + _check_in[0] # type: str the line below) if you are dead set on using _check_in as the variable name.

EDIT

Maybe you want to do this instead

import datetime

def create_url(check_in: datetime.datetime) -> str:
    return "https://www.website.com/?arrival={0}".format(
        check_in.strftime('%d%%2F%m%%2F%Y'),
    )