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]

Whoops, forgot the link to the error in the docs: 

https://docs.python.org/3/library/typing.html#typing.List

New changeset 0aec17c8f434 by Guido van Rossum in branch '3.5':
Fix issue #27402: example for typing did not type-check.
https://hg.python.org/cpython/rev/0aec17c8f434

New changeset 57f3514564f6 by Guido van Rossum in branch 'default':
Fix issue #27402: example for typing did not type-check. (Merge 3.5->3.6)
https://hg.python.org/cpython/rev/57f3514564f6

Thanks!