How to explicitly tell mypy the correct type of a variable?

I pick a value of a pandas dataframe and the type of the variable is string. In a function I return this value and I have annoted the value as str.

def get_item_name(code: str) -> str:
    item = df.loc[code, "item_name"]
    return item

However, mypy gives me the following warning:

Expression of type "Scalar" cannot be assigned to return type "str"
Type "Scalar" cannot be assigned to type "str" "bytes" is incompatible with "str"PylancereportGeneralTypeIssues

Is there a way to explicitly tell mypy that the correct type of variable item is string?

I get the warning to disappear if I use str(df.loc[code, "item_name"]) but this makes the code look like the value could be a number but we convert it to string, which is not the case as the value is already string.

Solution

If you are really sure, you can use cast. You should be careful with this, though.

from typing import cast

def get_item_name(code: str) -> str:
    item = df.loc[code, "item_name"]
    return cast(str, item)