Type ignore comments erroneously marked as unused by dmypy (#15043)

There is currently a misbehaviour where "type: ignore" comments are
erroneously marked as unused in re-runs of dmypy. There are also cases
where errors disappear on the re-run.

As far as I can tell, this only happens in modules which contain an
import that we don't know how to type (such as a module which does not
exist), and a submodule which is unused.

There was a lot of commenting and investigation on this PR, but I hope
that the committed tests and fixes illustrate and address the issue.

Related to #9655

---------

Co-authored-by: David Seddon <david@seddonym.me>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 5 people

mypy/errors.py

@@ -848,6 +848,8 @@ def generate_unused_ignore_errors(self, file: str) -> None:
                 code=codes.UNUSED_IGNORE,
                 blocker=False,
                 only_once=False,
+                origin=(self.file, [line]),
+                target=self.target_module,
             )
             self._add_error_info(file, info)
 
@@ -899,6 +901,8 @@ def generate_ignore_without_code_errors(
                 code=codes.IGNORE_WITHOUT_CODE,
                 blocker=False,
                 only_once=False,
+                origin=(self.file, [line]),
+                target=self.target_module,
             )
             self._add_error_info(file, info)
 

mypy/server/update.py

@@ -668,6 +668,8 @@ def restore(ids: list[str]) -> None:
     state.type_check_first_pass()
     state.type_check_second_pass()
     state.detect_possibly_undefined_vars()
+    state.generate_unused_ignore_notes()
+    state.generate_ignore_without_code_notes()
     t2 = time.time()
     state.finish_passes()
     t3 = time.time()

test-data/unit/daemon.test

@@ -648,6 +648,143 @@ from demo.test import a
 [file demo/test.py]
 a: int
 
+[case testUnusedTypeIgnorePreservedOnRerun]
+-- Regression test for https://github.com/python/mypy/issues/9655
+$ dmypy start -- --warn-unused-ignores --no-error-summary --hide-error-codes
+Daemon started
+$ dmypy check -- bar.py
+bar.py:2: error: Unused "type: ignore" comment
+== Return code: 1
+$ dmypy check -- bar.py
+bar.py:2: error: Unused "type: ignore" comment
+== Return code: 1
+
+[file foo/__init__.py]
+[file foo/empty.py]
+[file bar.py]
+from foo.empty import *
+a = 1  # type: ignore
+
+[case testTypeIgnoreWithoutCodePreservedOnRerun]
+-- Regression test for https://github.com/python/mypy/issues/9655
+$ dmypy start -- --enable-error-code ignore-without-code --no-error-summary
+Daemon started
+$ dmypy check -- bar.py
+bar.py:2: error: "type: ignore" comment without error code  [ignore-without-code]
+== Return code: 1
+$ dmypy check -- bar.py
+bar.py:2: error: "type: ignore" comment without error code  [ignore-without-code]
+== Return code: 1
+
+[file foo/__init__.py]
+[file foo/empty.py]
+[file bar.py]
+from foo.empty import *
+a = 1  # type: ignore
+
+[case testPossiblyUndefinedVarsPreservedAfterRerun]
+-- Regression test for https://github.com/python/mypy/issues/9655
+$ dmypy start -- --enable-error-code possibly-undefined --no-error-summary
+Daemon started
+$ dmypy check -- bar.py
+bar.py:4: error: Name "a" may be undefined  [possibly-undefined]
+== Return code: 1
+$ dmypy check -- bar.py
+bar.py:4: error: Name "a" may be undefined  [possibly-undefined]
+== Return code: 1
+
+[file foo/__init__.py]
+[file foo/empty.py]
+[file bar.py]
+from foo.empty import *
+if False:
+    a = 1
+a
+
+[case testUnusedTypeIgnorePreservedOnRerunWithIgnoredMissingImports]
+$ dmypy start -- --no-error-summary --ignore-missing-imports --warn-unused-ignores
+Daemon started
+$ dmypy check foo
+foo/main.py:3: error: Unused "type: ignore" comment  [unused-ignore]
+== Return code: 1
+$ dmypy check foo
+foo/main.py:3: error: Unused "type: ignore" comment  [unused-ignore]
+== Return code: 1
+
+[file unused/__init__.py]
+[file unused/submodule.py]
+[file foo/empty.py]
+[file foo/__init__.py]
+from foo.main import *
+from unused.submodule import *
+[file foo/main.py]
+from foo import empty
+from foo.does_not_exist import *
+a = 1  # type: ignore
+
+[case testModuleDoesNotExistPreservedOnRerun]
+$ dmypy start -- --no-error-summary --ignore-missing-imports
+Daemon started
+$ dmypy check foo
+foo/main.py:1: error: Module "foo" has no attribute "does_not_exist"  [attr-defined]
+== Return code: 1
+$ dmypy check foo
+foo/main.py:1: error: Module "foo" has no attribute "does_not_exist"  [attr-defined]
+== Return code: 1
+
+[file unused/__init__.py]
+[file unused/submodule.py]
+[file foo/__init__.py]
+from foo.main import *
+[file foo/main.py]
+from foo import does_not_exist
+from unused.submodule import *
+
+[case testReturnTypeIgnoreAfterUnknownImport]
+-- Return type ignores after unknown imports and unused modules are respected on the second pass.
+$ dmypy start -- --warn-unused-ignores --no-error-summary
+Daemon started
+$ dmypy check -- foo.py
+foo.py:2: error: Cannot find implementation or library stub for module named "a_module_which_does_not_exist"  [import-not-found]
+foo.py:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+== Return code: 1
+$ dmypy check -- foo.py
+foo.py:2: error: Cannot find implementation or library stub for module named "a_module_which_does_not_exist"  [import-not-found]
+foo.py:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+== Return code: 1
+
+[file unused/__init__.py]
+[file unused/empty.py]
+[file foo.py]
+from unused.empty import *
+import a_module_which_does_not_exist
+def is_foo() -> str:
+    return True  # type: ignore
+
+[case testAttrsTypeIgnoreAfterUnknownImport]
+$ dmypy start -- --warn-unused-ignores --no-error-summary
+Daemon started
+$ dmypy check -- foo.py
+foo.py:3: error: Cannot find implementation or library stub for module named "a_module_which_does_not_exist"  [import-not-found]
+foo.py:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+== Return code: 1
+$ dmypy check -- foo.py
+foo.py:3: error: Cannot find implementation or library stub for module named "a_module_which_does_not_exist"  [import-not-found]
+foo.py:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+== Return code: 1
+
+[file unused/__init__.py]
+[file unused/empty.py]
+[file foo.py]
+import attr
+from unused.empty import *
+import a_module_which_does_not_exist
+
+@attr.frozen
+class A:
+    def __init__(self) -> None:
+        self.__attrs_init__()  # type: ignore[attr-defined]
+
 [case testDaemonImportAncestors]
 $ dmypy run test.py
 Daemon started

test-data/unit/fine-grained.test

@@ -10540,6 +10540,30 @@ from pkg.sub import modb
 [out]
 ==
 
+[case testUnusedTypeIgnorePreservedAfterChange]
+# flags: --warn-unused-ignores --no-error-summary
+[file main.py]
+a = 1  # type: ignore
+[file main.py.2]
+a = 1  # type: ignore
+# Comment to trigger reload.
+[out]
+main.py:1: error: Unused "type: ignore" comment
+==
+main.py:1: error: Unused "type: ignore" comment
+
+[case testTypeIgnoreWithoutCodePreservedAfterChange]
+# flags: --enable-error-code ignore-without-code --no-error-summary
+[file main.py]
+a = 1  # type: ignore
+[file main.py.2]
+a = 1  # type: ignore
+# Comment to trigger reload.
+[out]
+main.py:1: error: "type: ignore" comment without error code
+==
+main.py:1: error: "type: ignore" comment without error code
+
 [case testFineGrainedFunctoolsPartial]
 import m