Vision: python_type_check CI is broken

[fmassa]

This looks like an upstream change that broke CI, see https://app.circleci.com/pipelines/github/pytorch/vision/4305/workflows/9ae30f70-6fa0-419f-966b-8db2e249a12b/jobs/254769 for an example

The error we get is

torchvision/datasets/mnist.py:446: error:
Incompatible
return
value
type
(got
"GzipFile",
expected
"IO[Any]")

[return-value]
            return gzip.open(path, 'rb')
                   ^
Found 1 error in 1 file (checked 97 source files)

cc @seemethere @pmeier

[pmeier]

I'm on it.

[pmeier]

This is caused by mypy==0.790 released two days ago. It includes python/typeshed@846d922df238d307a99347f74d91ccb362f876ad which amongst others makes the return types of the gzip module more precise.

[fmassa]

@pmeier so does this mean that using an older mypy will give type checking errors now?

[pmeier]

It shouldn't, but I'm not sure. I'll check and get back to you.

[vfdev-5]

Just checked with mypy 0.780 on the master -> no issues

$ mypy --config-file mypy.ini
Success: no issues found in 97 source files
$ pip list | grep mypy
mypy                   0.780
$ git log -1
commit 610c9d2a063bf0695d4f1c3cc396f0c1f2c3a4af (HEAD -> master, origin/master)
Author: Adeel Hassan <[email protected]>
Date:   Mon Oct 12 13:53:17 2020 +0500

    remove hard coded value (#2793)

[fmassa]

awesome, thanks for checking!