Type error when creating a tuple from a Generator

[jatoben]

Describe the bug

Creating a tuple[tuple[float, float]] from a Generator[tuple[float, float], None, None] fails to type-check, unless a temporary variable is used.

To Reproduce

I have an existing API which is typed like this:

class Point:
    x: float
    y: float
    # additional members here


def get_points() -> Iterable[Point]:
    ...


def convert_point(point: Point) -> tuple[float, float]:
    ...


# Can draw either a single point represented as a tuple[float, float] or a sequence of points
def draw_path(
    points: tuple[float, float] | Sequence[tuple[float, float]], *, width: float
) -> None:
    ...

If I use tuple() to turn a generator of converted points into an immutable sequence, I get the following error:

draw_path(
    tuple(convert_point(p) for p in get_points()),
    width=0.25,
)

# error: Argument of type "Generator[tuple[float, float], None, None]" cannot be assigned to parameter "__iterable" of type "Iterable[_T_co@tuple]" in function "__new__"
#    "Generator[tuple[float, float], None, None]" is incompatible with "Iterable[float]"
#      TypeVar "_T_co@Iterable" is covariant
#        "tuple[float, float]" is incompatible with "float" (reportGeneralTypeIssues)

However, capturing the created tuple in a temporary variable first prevents the error.

points = tuple(convert_point(p) for p in get_points())
draw_path(
    points,
    width=0.25,
)

Expected behavior

I would expect equivalent type-checking results from draw_path() calls.

VS Code extension or command-line

Command-line version 1.1.314

Additional context

#5281 could perhaps be related.

[erictraut]

Thanks for the bug report. This is related to a heuristic that pyright uses when performing bidirectional type inference in the case where the "expected type" is a union. It currently assumes that if return type of the call (in this case the tuple constructor) matches one of the subtypes in the union (in this case tuple[float, float]), it should prefer this as the "expected type" over the other subtypes. This heuristic works 99.99% of the time, but you've identified a case where it fails.

I've adjusted the heuristic so it works with this code. This will be addressed in the next release.

[jatoben]

Great -- thank you!

[erictraut]

This is included in pyright 1.1.315, which I just published. It will also be included in this week's insiders release of pylance.