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
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Python tracker <[email protected]>
<http://bugs.python.org/issue27402>
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