python type annotations: use Mapping instead of Dict?

[lheckemann]

The following code fails mypy type checking:

from lsprotocol.types import WorkspaceEdit, TextEdit, Range, Position
pos = Position(0, 0)
changes = {"file:///tmp/foo.txt": [TextEdit(Range(pos, pos), new_text="hello world")]}
WorkspaceEdit(changes=changes)

Because changes is a dict[str, list[TextEdit]] while WorkspaceEdit's changes expects a dict[str, Sequence[TextEdit]] and dict is invariant -- but we can't actually create a Sequence!

I think the type annotations should probably use Mapping instead of Dict?

[karthiknadig]

Isn't this a bug in mypy. list is a Sequence.

[lheckemann]

No, mypy's behaviour is correct -- dict is invariant (see https://mypy.readthedocs.io/en/stable/generics.html#variance-of-generic-types ).

Consider:

pos = Position(0, 0)
range = Range(pos, pos)
change = TextEdit(range, new_text='foo')

def add_change(changes: dict[str, Sequence[TextEdit]]):
    # dict is mutable, so we can add something in.
    # a tuple is a Sequence so that should be fine, right?
    changes["file:///foo"] = (change,)

changes: dict[str, list[TextEdit]] = {}
# But this doesn't make sense: we'd be adding a tuple value into the dict, making its type no longer correct!
add_change(changes)

That's why dict is invariant, i.e. its keys and values have to have the exact type that's given and a dict[str, list[TextEdit]] is not a dict[str, Sequence[TextEdit]] (even though a list[TextEdit] is a Sequence[TextEdit]).

Mapping on the other hand doesn't entail mutability, so it doesn't have this problem and is covariant, so a dict[str, list[TextEdit]] is a Mapping[str, Sequence[TextEdit].

[karthiknadig]

Thanks for explaining. We are happy to take a PR for this.