[Python] mypy error:Incompatible types in assignment (expression has type "List[str]",...

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!

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