TypedDict keys reuse?

[jstasiak]

I may be in a need of the following - two TypeDicts with the same keys, one of them created with total=False:

from mypy_extensions import TypedDict

Details = TypedDict('Details', {'name': str, 'age': int, 'address': str})
DetailsSubset = TypedDict('DetailsSubset', {'name': str, 'age': int, 'address': str}, total=False)

Is there a way to reuse the keys instead of having to repeat the {'name': str, 'age': int, address: str} fragment twice?

I can't do

from mypy_extensions import TypedDict

keys = {'name': str, 'age': int, 'address': str}
Details = TypedDict('Details', keys)
DetailsSubset = TypedDict('DetailsSubset', keys, total=False)

Because

% mypy code.py
code.py:4: error: TypedDict() expects a dictionary literal as the second argument
code.py:5: error: TypedDict() expects a dictionary literal as the second argument

(which is quite reasonable, tried that on the off chance it'd work)

[ilevkivskyi]

This is currently not possible. We will probably need the Const type modifier for this to work, see #1214

[JukkaL]

We now have a Final modified but that isn't enough, since we never implicitly infer TypedDict types from dict expressions.

[ilevkivskyi]

Another4 request for this appeared, see #8186, so raising priority to normal.

[ilevkivskyi]

(Just to clarify, we can start from only supporting final names for individual keys, which should be pretty easy.)

[cwhy]

@ilevkivskyi I agree that we can just support final names for keys first, at least for NamedTuple, as stated at the end of PEP-591:

Type checkers should treat uses of a final name that was initialized with a literal as if it was replaced by the literal. For example, the following should be allowed:

X: Final = "x"
Y: Final = "y"
N = NamedTuple("N", [(X, int), (Y, int)])

It is sometimes useful to have different types on the same label as well:

X: Final = "x"
Y: Final = "y"
Position = NamedTuple("Position", [(X, int), (Y, int)])
DimensionName = NamedTuple("DimensionName", [(X, str), (Y, str)])