Enhanced Literal Detection: Allow string type narrowing to literals

[CarliJoy]

Feature

It would be great if mypy could detect and type narrow strings to Literals if this is statically detectable.
I.e. if accessing TypedDict or Enum values

Pitch

from typing import TypedDict

class Part(TypedDict, total=False):
    a: int
    b: int

class Full(Part, total=False):
    c: int

def make_part(full: Full) -> Part:
    part: Part = {}
    for k, v in full.items():
        if k in Part.__annotations__:
            part[k]=v  #  error: TypedDict key must be a string literal; expected one of ("a", "b")  [literal-required]
    return part 

MyPy Play

Currently there is no way to type this kind of expression without a # type: ignore.

It would be nice if mypy could detect that if k in Part.__annotations__ will narrow k to the Literal of all key of Part.
Also if k == "a" isn't working at the moment, but at least for this I am able to write a TypeGuard.

Also if Literals are explicitly defined, we loose this once we are with a loop

Even if the literal is defined explictly, it gets lost in a for loop

def make_part_two(full: Full) -> Part:
    part: Part = {}
    keys: Final = ("a", "b")
    reveal_type(keys)  # tuple[Literal['a']?, Literal['b']?]
    ok = full[keys[0]]
    for key in keys:
        reveal_type(key) # str
        part[key] = full[key]  # literal-required

If explicitly given as keys: tuple[Literal["a", "b"], ...] it works also in the for loop but I don't want to duplicate these values...

[CarliJoy]

related to #13767

[JelleZijlstra]

This is a lot of different feature requests in one. You should probably open multiple issues. The one about __annotations__ might be a step too far, the others are more likely to be OK.

[CarliJoy]

I will create different issues an link them here. After this I will close this issue.

[CarliJoy]

Opened three different Issues that should tackle this.

[CarliJoy]

Note this already works in pyright

Correction: Pyright just doesn't check this correctly

[bcmills]

• Narrowing types to Literal using in syntax #12535 is another specific case pertaining to this request.