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

Tái hiện tình trạng treo bằng các ví dụ về phép gán extended-slice trong issue, sau đó lần theo đường dẫn gán extended-slice builtin sử dụng iterable. Thêm một bài kiểm thử hồi quy bao quát một trình tạo vô hạn và xác minh rằng phép gán phát sinh ValueError ngay khi độ dài của slice bị vượt quá.