Attempting to assign an infinte generator to an extended slice hangs indefinitely instead of raising ValueError

Bug report

Bug description:

For builtin types which support assignment to extended slices (i.e. slices with a step other than 1), the length of the iterable being assigned from must be equal to the length of the slice being assigned to. However, Python appears to determine the length of the iterable by repeatedly calling next() until StopIteration is raised.

In the event that an object never raises StopIteration, this creates an infinite evaluation loop, even though it should be apparent that the two sizes are not equal once the size of the slice has been exceeded. See the example below:

class MyGen:
    def __iter__(self):
        while True:
            yield "foo"

l = [0,1,2,3]

l[::2] = range(10)  # raises ValueError: attempt to assign sequence of size 10 to extended slice of size 2
l[::2] = MyGen()  # hangs indefinitely despite returning more than 2 values

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-146272

Reproduce the hang with the extended-slice assignment examples in the issue, then trace the builtin extended-slice assignment path that consumes the iterable. Add a regression test covering an infinite generator and verify that assignment raises ValueError once the slice length is exceeded.