Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
+ steam/http.py:180: error: The type "type[SimpleCookie]" is not generic and not indexable  [misc]
+ steam/http.py:181: error: The type "type[SimpleCookie]" is not generic and not indexable  [misc]

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/helpers.py:942:24: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/helpers.py:942:24: note: Error code "type-arg" not covered by "type: ignore" comment
+ aiohttp/helpers.py:942:24: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-type-arg for more info
+ aiohttp/helpers.py:945:26: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/helpers.py:1028:43: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/cookiejar.py:66:53: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/cookiejar.py:169:22: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/cookiejar.py:235:35: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/cookiejar.py:246:25: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/connector.py:228:23: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/client_reqrep.py:377:12: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/client_reqrep.py:710:23: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]
+ aiohttp/web_request.py:574:17: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]

starlette (https://github.com/encode/starlette)
+ tests/test_responses.py:347: error: "SimpleCookie" expects no type arguments, but 1 given  [type-arg]

I'm not too familiar with this module, so this may be a dumb question, but should even BaseCookie be generic? I can't find any uses on grep.app where BaseCookie is subscripted with anything other than str, and the docs don't shine any light on a situation where you might have a Morsel[bytes] or whatever as the value in a BaseCookie dictionary.

>>> from http.cookies import BaseCookie
>>> b = BaseCookie()
>>> b['foo'] = b'bar'
>>> b['foo'
<Morsel: foo=b'bar'>
>>> _.coded_value
"b'bar'"
>>> type(b['foo'].coded_value)
<class 'str'>

Yeah, I don't know either. I'm getting a feeling it might be about conversion between formats rather than conversion to e.g. bytes. But I'm only guessing here

Chances are it's a Python 2 remnant: maybe in Python 2 you could have both str and unicode cookies.

Looks like both classes were originally made generic in:

• Improve http #248

But there's no explanation in that PR thread as to why

Documentation for BaseCookie.value_encode says that coded_value will always "be converted to a string". Where it is set as Morsel.coded_value. So I suppose it should be valid to declare BaseCookie[int] and then implement a value_decode that parses an int from a str?

I'm not too familiar with this module, so this may be a dumb question, but should even BaseCookie be generic? I can't find any uses on grep.app where BaseCookie is subscripted with anything other than str, and the docs don't shine any light on a situation where you might have a Morsel[bytes] or whatever as the value in a BaseCookie dictionary.

>>> from http.cookies import BaseCookie
>>> b = BaseCookie()
>>> b['foo'] = b'bar'
>>> b['foo'
<Morsel: foo=b'bar'>
>>> _.coded_value
"b'bar'"
>>> type(b['foo'].coded_value)
<class 'str'>

I think an additional thing to check up on here is this

>>> from http.cookies import BaseCookie
>>> class C(BaseCookie):
...     def value_encode(self, value):
...         return value, value
...
>>> c = C()
>>> c["foo"] = b"bar"
>>> c["foo"]
<Morsel: foo=b'bar'>
>>> c["foo"].coded_value
b'bar'
>>> type(c["foo"].coded_value)
<class 'bytes'>
>>> c["foo"].output()
"Set-Cookie: foo=b'bar'"

Morsel.coded_value is always stringified before it's sent. At least that's what I interpret from "coded_value will always be converted to a string".

Funny thing is though, just because you've declared BaseCookie[int] it won't make .coded_value return int. And the only control over that is to enforce usage of Morsel[int] as set value.

Consider this program, which passes, but coded_value is <class 'str'>

from http.cookies import BaseCookie, Morsel
class C(BaseCookie[int]):
    ...

c = C()
c["foo"] = "1"
coded_value = c["foo"].coded_value
reveal_type(coded_value)  # Revealed type is "builtins.int"

But if you'd instead use below, types align.

m = Morsel[int]()
m.set("foo", "1", 1)
c["foo"] = m

OK cool, sounds like BaseCookie should remain generic, then, in order to allow flexibility for hypothetical user subclasses of BaseCookie. Thanks for the research and the thorough explanation @flaeppe!!