Type annotations

🚀 Feature

Type annotations.

Motivation

Right now, if a project depends on torchvision and you want to static type check it, you have no choice but either ignore it or write your own stubs. torch already has partial support for type annotations.

Pitch

We could add type annotations for torchvision as well.

Additional context

If we want this, I would take that up.

[pmeier]

@NicolasHug and I had a similar offline discussion a while back and I want to just add my two cents to the examples:

def _g(a):
    return a + 0.5

def f(a : int) -> int:
    return _g(a)

The problem stems from the fact that you are mixing typed and untyped definitions here. In these cases, mypy is very lenient by default. Multiple options out here:

1. Run with --disallow-untyped-calls:

lol.py:6: error: Call to untyped function "_g" in typed context
Found 1 error in 1 file (checked 1 source file)

1. Run with --warn-return-any

lol.py:6: error: Returning Any from function declared to return "int"
Found 1 error in 1 file (checked 1 source file)

So if you want this strictness, it is only one flag away.

---

def return_4() -> int:
    l = [2, 4, 6]
    assert 4 in l   # basically something that you *know* is True for at least one element in l

    def cond(x):
        return x == 4

    for x in l:
        if cond(x):
            return x

This example looks quite non-sensical to me. Could you link the original code so I can have a look at the “real” example?

In general, if you hit such an obscure case this either warrants a refactor of the current implementation or simply a # type: ignore[foo] comment.

---

Speaking of #type: ignore comments, I’m not sure why you basically insist on mypy being perfect. If the implementation is logically correct and you are happy with it but mypy is not, just silence it. I mean for other static checkers such as flake8 we also flat-out ignore some error codes and this doesn’t seem to bother you.

https://github.com/pytorch/vision/blob/8e2bd0e070dbac1011134b0a71e88313a406c2b6/setup.cfg#L12

Plus git grep '# noqa' | wc -l reveals 14 instances where we silence flake8 locally.

[NicolasHug]

@frgfm @oke-aditya thanks a lot for your recent PRs

I just wanted to point out something: we should only add types to files that we can effectively check types for. Otherwise, we risk having having incorrect type annotation, which is much worse than not having annotations in the first place.

So, when submitting a PR, please make sure that this file is properly covered by mypy.ini (which you may need to edit) so that it can be checked by the CI