Stricter settings in mypy_test

[Avasam]

I've noticed some blank # type: ignore comments. For the sake of being consistent, explicit and strict, I've added their missing error codes and added "--enable-error-code ignore-without-code" & --warn-unused-ignores.

In the same vein, mypy_test.py can be made stricter, I've used --strict in favor of:

--warn-unused-ignores
--no-implicit-optional
--disallow-untyped-decorators
--disallow-any-generics
--strict-equality

(source: https://mypy.readthedocs.io/en/stable/existing_code.html#introduce-stricter-options)

Once #5768 is completed, we should be able to drop --allow-subclassing-any as well.

[AlexWaygood]

Ah, removing mypy error codes from the test cases was a deliberate policy decision we made a few months ago; see the justification laid out in

• Remove mypy error codes from the test_cases directory #8083

I suppose we could document this, but the README for the test_cases directory is already a bit too long for my liking :/

[Avasam]

No problem! Thanks for the source. I had a feeling about it. Maybe a simple comment in the mypy config (well, in the args list, it's not really a config file) would be sufficient.

Thoughts on stricter mypy_test?

[AlexWaygood]

This might not make our tests that much stricter. The issue is that mypy does some pretty cursèd special-casing for files that it detects are in a "typeshed directory". A side effect of that special-casing is, unfortunately, that a bunch of the stricter mypy settings just don't work on files in a "typeshed directory" (and it's obviously pretty hard to escape being in a typeshed directory when you're working on files in this repo). For example, if I apply this diff:

diff --git a/stdlib/builtins.pyi b/stdlib/builtins.pyi
index df74a00a4..62dd12145 100644
--- a/stdlib/builtins.pyi
+++ b/stdlib/builtins.pyi
@@ -67,7 +67,7 @@ _KT = TypeVar("_KT")
 _VT = TypeVar("_VT")
 _S = TypeVar("_S")
 _T1 = TypeVar("_T1")
-_T2 = TypeVar("_T2")
+_T2 = TypeVar("_T2")  # type: ignore
 _T3 = TypeVar("_T3")
 _T4 = TypeVar("_T4")
 _T5 = TypeVar("_T5")
diff --git a/tests/mypy_test.py b/tests/mypy_test.py
index a0069ae69..986ac8f82 100644
--- a/tests/mypy_test.py
+++ b/tests/mypy_test.py
@@ -259,6 +259,7 @@ def get_mypy_flags(args: TestConfig, temp_name: str, *, testing_stdlib: bool) ->
         "ignore-without-code",
         "--config-file",
         temp_name,
+        "--warn-unused-ignores",
     ]
     if not testing_stdlib:
         flags.append("--explicit-package-bases")

Then, sadly, mypy_test.py still passes just fine:

(.venv) C:\Users\alexw\coding\typeshed>python tests/mypy_test.py -p3.11 stdlib
*** Testing Python 3.11 on win32
Testing stdlib (489 files)...
Running mypy --python-version 3.11 --show-traceback --warn-incomplete-stub --show-error-codes --no-error-summary --platform win32 --no-site-packages --custom-typeshed-dir C:\Users\alexw\coding\typeshed --no-implicit-optional --disallow-untyped-decorators --disallow-any-generics --strict-equality --enable-error-code ignore-without-code --config-file /tmp/... --warn-unused-ignores
success

--- success, 489 files checked ---

Having said that, I'm not sure there's any reason not to do this. It might mean that mypy_test.py suddenly starts failing with a future mypy upgrade, if mypy improves the way it special-cases "typeshed directories" at some point in the future. But we can probably cross that bridge when we come to it.

[Avasam]

I didn't know about that special casing!
In any case this change is basically just the same thing as #9293, but for mypy.

[AlexWaygood]

The "typeshed_dir" special-casing is why we have to copy the whole test_cases directory into a tempdir in regr_test.py before running mypy on the test cases — --warn-unused-ignores is crucial for that test, and there's no other way I could think of to get it to work in this repo 🙃