Fix subgroups typing

[Henri-J-Norden]

Issue

With pyright, the subgroups function always produces a type error (reportArgumentType).

Example:

from dataclasses import dataclass
from simple_parsing import subgroups


@dataclass
class A:
    pass

subgroups({"a": A})
          ^^^^^^^^
          Argument of type "dict[str, type[A]]" cannot be assigned to parameter "subgroups" of type "dict[Key@subgroups, DataclassT@subgroups | type[DataclassT@subgroups] | partial[DataclassT@subgroups]]" in function "subgroups"

PS. In a different project, the error was reported as:

No overloads for "subgroups" match the provided arguments
subgroups.py: Overload 1 is the closest match

Solution

As noted in the pyright docs, the subgroups function argument should be typed as Mapping[...] instead of dict[...] due to type variance:

Use Sequence rather than list, Mapping rather than dict, etc. Immutable containers allow for more flexibility because their type parameters are covariant rather than invariant. A parameter that is typed as Sequence[str | int] can accept a list[str | int], list[int], Sequence[str], and a Sequence[int]. But a parameter typed as list[str | int] is much more restrictive and accepts only a list[str | int].

The only limitation is that Mapping is read-only, therefore the subgroups function must not modify the contents of its subgroups argument, but this appears to already be the case with the current implementation.