SimpleCookie generic type

Question

SimpleCookie is apparently a generic type and thus the following code (test.py) gives an error when checked with mypy:

from http.cookies import SimpleCookie

cookie = SimpleCookie()

test.py:3: error: Need type annotation for 'cookie'

Now if I change test.py line 3 to:

cookie: SimpleCookie = SimpleCookie()

I get the following error:

test.py:3: error: Missing type parameters for generic type "SimpleCookie"

SimpleCookie inherits from dict, has str keys and Morsel values, so I'd assume that the correct generic type annotation is something like this:

from http.cookies import Morsel, SimpleCookie

cookie: SimpleCookie[str, Morsel] = SimpleCookie()

But now the error is:

test.py:3: error: "SimpleCookie" expects 1 type argument, but 2 given

Changing line 3 to

cookie: SimpleCookie[str] = SimpleCookie()

suddenly makes mypy happy, but leaves me very confused why this is the correct solution, so I have two questions:

1. Why is SimpleCookie a generic type with one argument?
2. What's the best way to handle this in my code? Should I annotate SimpleCookie variables with SimpleCookie[str] (which seems like a lie to me) or should I just annotate them with Any and hope this will be cleaned up in future Python versions?

mypy version 0.750 and Python 3.8.0

Answer 1

Explanation

str in SimpleCookie[str] actually refers to the type _T of coded_value in Morsel.

mypy uses https://github.com/python/typeshed/blob/master/stdlib/3/http/cookies.pyi:

class Morsel(Dict[str, Any], Generic[_T]):
    value: str
    coded_value: _T
    key: str
    def set(self, key: str, val: str, coded_val: _T) -> None: ...
    # ...

class BaseCookie(Dict[str, Morsel[_T]], Generic[_T]):
    # ...
    def value_decode(self, val: str) -> _T: ...
    def value_encode(self, val: _T) -> str: ...
    # ...
    def __setitem__(self, key: str, value: Union[str, Morsel[_T]]) -> None: ...

class SimpleCookie(BaseCookie[_T], Generic[_T]): ...

Correct typing

_T should be Any, i.e. SimpleCookie[Any], as explained in python/typeshed#3060:

Morsel does cast any value to string ... max-age can take an integer (unix time) and http-only a boolean.

Actually, I could not reproduce the error you get with this:

from http.cookies import SimpleCookie

cookie: SimpleCookie = SimpleCookie()