New submission from Michael Lee: In the documentation for the `typing` module, the [entry for the `List` type][0] uses the following example to help demonstrate when to use `Sequence` vs `List`: T = TypeVar('T', int, float) def vec2(x: T, y: T) -> List[T]: return [x, y] def slice__to_4(vector: Sequence[T]) -> List[T]: return vector[0:4] However, the `slice__to_4` function does not actually typecheck since there's no guarantee that a slice of a sequence will return a `List`. For example the vector could be a numpy array or a custom subclass of `collections.abc.Sequence` with an unusual `__getitem__`. (Mypy correctly catches this error and complains about an "Incompatible return value type"). The documentation should probably be updated to use an example that _does_ typecheck, though I'm not sure what exactly that example might look like? Maybe replace `slice__to_4` with something like this? def keep_positives(vector: Sequence[T]) -> List[T]: return [item for item in vector if item > 0] ---------- assignee: docs@python components: Documentation messages: 269389 nosy: docs@python, michael0x2a priority: normal severity: normal status: open title: Sequence example in typing module documentation does not typecheck versions: Python 3.5, Python 3.6 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue27402> _______________________________________