Incorrect "Unused 'type: ignore' comment" on top-level ignore[error-code]

[jonathanslenders]

Bug Report

When there's a top-level type-ignore with error code, like # type: ignore[no-untyped-defs], and --warn-unused-ignores is passed, this type comment always shows up as an unused ignore, even though the type checking would fail without that comment.

To Reproduce

Create this file:

# type: ignore[no-untyped-def]

def untyped(): pass

Run using mypy test.py --strict. (which includes --warn-unused-ignores)

Expected Behavior

This should pass, because removing the type comment causes the no-untyped-def error to appear.

Actual Behavior

 $ mypy test.py   --warn-unused-ignores
test.py:1: error: Unused "type: ignore" comment
Found 1 error in 1 file (checked 1 source file)

Your Environment

Mypy 0.931, Python 3.9.9, Ubuntu 20.04

[KotlinIsland]

You have a small typo:

# type: ignore[no-untyped-defs]

Should be: # type: ignore[no-untyped-def]

But the issue is still completely valid.

[jonathanslenders]

Thanks @KotlinIsland, I fixed the code snippet.

[kreathon]

I am not sure if this is a bug or a missing features.

The documentation states only to use type: ignore to ignore the whole module (without any specific error code). That also matches with the code (I just looked it up):

# A "# type: ignore" comment before the first statement of a module
# ignores the whole module:
if (ismodule and stmts and self.type_ignores
        and min(self.type_ignores) < self.get_lineno(stmts[0])):
    self.errors.used_ignored_lines[self.errors.file][min(self.type_ignores)].append(
        codes.MISC.code)
    block = Block(self.fix_function_overloads(self.translate_stmt_list(stmts)))
    mark_block_unreachable(block)
    return [block]

(Note that I am not a mypy developer, so I might be wrong)

[Zeckie]

It is ignoring the supplied error code, and treating it as though it is just # type: ignore, pretty much the opposite of what the error message says.

Example showing that it ignores errors with a different code:
https://mypy-play.net/?mypy=latest&python=3.10&flags=show-error-codes%2Cstrict%2Cwarn-unused-ignores&gist=a864a4660d66062af67f85f49e89096f

The documentation currently says:

... you can also ignore all mypy checks in a file, by adding a # type: ignore at the top of the file:

And on a different page:

You can use a special comment # type: ignore[code, ...] to only ignore errors with a specific error code (or codes) on a particular line. This can be used even if you have not configured mypy to show error codes. Currently it’s only possible to disable arbitrary error codes on individual lines using this comment.

The best solution would be to add support for error codes on ignores at the top of a file, to ignore all errors of that code in the whole file.

If that isn't possible, I think it should give an error more like (whether used with --warn-unused-ignores or not):

test.py:1: error: Error code not supported on file level ignore

[KotlinIsland]

Put this at the top of the file and you will get the desired behavior:

# mypy: allow-untyped-defs

playground
It's using a feature called inline configuration

The following doesn't work (invalid module level config), but is silently ignored due to #12342:

# mypy: disable-error-code=no-untyped-def