PEP 705 – TypedMapping

PEPs
40

Indeed I am remembering incorrectly. According to mypy, you cannot write the value of an unknown key:

# td_setitem_unknown_key.py
from typing import TypedDict

class Point2D(TypedDict):
    x: int
    y: int
    # unknown keys must not be ReadOnly because this is a TypedDict

class Example(TypedDict, Point2D):
    w: str

w: Example = { "x": 1, "y": 2, "w": "foo" }
p: Point2D = w
p['w'] = 0  # error: TypedDict "Point2D" has no key "w"  [typeddict-unknown-key]

…however you can read the value of an unknown key with get, since there might actually be a value (from when previously manipulated as a subclass):

# td_get_unknown_key.py
from typing import TypedDict

class Point2D(TypedDict):
    x: int
    y: int
    # unknown keys must not be ReadOnly because this is a TypedDict

class Example(TypedDict, Point2D):
    w: str

w: Example = { "x": 1, "y": 2, "w": "foo" }
p: Point2D = w
print(p.get('w'))  # OK

I’ll edit away blur my discussion which relies on this misremembering.