TypedDict does not allow .get("missing_key", ...) to pass

[henryiii]

Bug Report

MyPy does not allow .get("key") on a TypedDict if the key does not exist. This nullifies the usefulness of .get, and seems to be in direct contradiction to PEP 589:

"Operations with arbitrary str keys (instead of string literals or other expressions with known string values) should generally be rejected. This involves both destructive operations such as setting an item and read-only operations such as subscription expressions. As an exception to the above rule, d.get(e) and e in d should be allowed for TypedDict objects, for an arbitrary expression e with type str. The motivation is that these are safe and can be useful for introspecting TypedDict objects."

(Credit to @freundTech on python/typing gitter)

An example of where this is broken in a practical example is shown below.

To Reproduce

from typing import TypedDict

class BaseDict(TypedDict):
    a: str

class CustomDict(BaseDict):
    b: str

def my_general_fun(x: BaseDict) -> None:
    print(x["a"], x.get("b"))

Expected Behavior

This should pass as far as I can tell from the PEP; a literal expression is an arbitrary expression e with type str, just a special case of it. For [] non-literals are thrown out, but for .get, non-literals are included; I don't think this implies literals should be thrown out, but rather that the point of .get is for generic access. The type of .get should be inferred if a literal is passed and that literal is known, but it should it should the value of the second argument (.get("b", "") could be str if the TypedDict did not have a "b", for example, normally None). This would be much more useful than simply not allowing this to pass at all.

From looking at this function:

mypy/mypy/plugins/default.py

Line 227 in 6e99a2d

def typed_dict_get_callback(ctx: MethodContext) -> Type:

The check only happens for literals, so a workaround is to use the following instead:

def my_general_fun(x: BaseDict) -> None:
    b = "b"
    print(x["a"], x.get(b))

Btu this should not be required, and could even lose some knowledge about the type.

I would expect the type of x.get("b") to be None.

Actual Behavior

tmp.py:11: error: TypedDict "BaseDict" has no key 'b'
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 7.900
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9
• Operating system and version: macOS 11