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

使用 issue 中擴充切片指派的範例重現掛起問題，然後追蹤會消耗 iterable 的內建擴充切片指派路徑。新增一個涵蓋無限產生器的回歸測試，並驗證切片長度超出後指派會引發 ValueError。