Use error subcodes to differentiate import errors (#14740)

Resolves #9789

Users could use `--disable-error-code=import-untyped` to only ignore
errors about libraries not having stubs, but continue to get errors
about e.g. typos in an import name.

The error subcode mechanism is new from #14570. Note that users will now
get a different error code depending on whether or not a package is
installed, and may not know that they can use the parent error code to
ignore the issue regardless. I think this is okay, in general type
checking results can change if you run them in two different
environments. Note also that with `--warn-unused-ignore` / `--strict`
mypy will complain about not having the most specific error code

mypy/build.py

@@ -2780,7 +2780,16 @@ def module_not_found(
     else:
         daemon = manager.options.fine_grained_incremental
         msg, notes = reason.error_message_templates(daemon)
-        errors.report(line, 0, msg.format(module=target), code=codes.IMPORT)
+        if reason == ModuleNotFoundReason.NOT_FOUND:
+            code = codes.IMPORT_NOT_FOUND
+        elif (
+            reason == ModuleNotFoundReason.FOUND_WITHOUT_TYPE_HINTS
+            or reason == ModuleNotFoundReason.APPROVED_STUBS_NOT_INSTALLED
+        ):
+            code = codes.IMPORT_UNTYPED
+        else:
+            code = codes.IMPORT
+        errors.report(line, 0, msg.format(module=target), code=code)
         top_level, second_level = get_top_two_prefixes(target)
         if second_level in legacy_bundled_packages or second_level in non_bundled_packages:
             top_level = second_level

mypy/errorcodes.py

@@ -107,6 +107,12 @@ def __hash__(self) -> int:
 IMPORT: Final = ErrorCode(
     "import", "Require that imported module can be found or has stubs", "General"
 )
+IMPORT_NOT_FOUND: Final = ErrorCode(
+    "import-not-found", "Require that imported module can be found", "General", sub_code_of=IMPORT
+)
+IMPORT_UNTYPED: Final = ErrorCode(
+    "import-untyped", "Require that imported module has stubs", "General", sub_code_of=IMPORT
+)
 NO_REDEF: Final = ErrorCode("no-redef", "Check that each name is defined once", "General")
 FUNC_RETURNS_VALUE: Final = ErrorCode(
     "func-returns-value", "Check that called function returns a value in value context", "General"

mypy/errors.py

@@ -8,7 +8,7 @@
 from typing_extensions import Literal, TypeAlias as _TypeAlias
 
 from mypy import errorcodes as codes
-from mypy.errorcodes import IMPORT, ErrorCode
+from mypy.errorcodes import IMPORT, IMPORT_NOT_FOUND, IMPORT_UNTYPED, ErrorCode
 from mypy.message_registry import ErrorMessage
 from mypy.options import Options
 from mypy.scope import Scope
@@ -510,7 +510,11 @@ def add_error_info(self, info: ErrorInfo) -> None:
             if info.message in self.only_once_messages:
                 return
             self.only_once_messages.add(info.message)
-        if self.seen_import_error and info.code is not IMPORT and self.has_many_errors():
+        if (
+            self.seen_import_error
+            and info.code not in (IMPORT, IMPORT_UNTYPED, IMPORT_NOT_FOUND)
+            and self.has_many_errors()
+        ):
             # Missing stubs can easily cause thousands of errors about
             # Any types, especially when upgrading to mypy 0.900,
             # which no longer bundles third-party library stubs. Avoid

test-data/unit/check-errorcodes.test

@@ -183,7 +183,7 @@ from defusedxml import xyz  # type: ignore[import]
 
 [case testErrorCodeBadIgnore]
 import nostub # type: ignore xyz  # E: Invalid "type: ignore" comment  [syntax] \
-                                  # E: Cannot find implementation or library stub for module named "nostub"  [import] \
+                                  # E: Cannot find implementation or library stub for module named "nostub"  [import-not-found] \
                                   # N: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
 import nostub # type: ignore[  # E: Invalid "type: ignore" comment  [syntax]
 import nostub # type: ignore[foo  # E: Invalid "type: ignore" comment  [syntax]
@@ -211,7 +211,7 @@ def f(x,  # type: int  # type: ignore[
     pass
 [out]
 main:2: error: Invalid "type: ignore" comment  [syntax]
-main:2: error: Cannot find implementation or library stub for module named "nostub"  [import]
+main:2: error: Cannot find implementation or library stub for module named "nostub"  [import-not-found]
 main:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
 main:3: error: Invalid "type: ignore" comment  [syntax]
 main:4: error: Invalid "type: ignore" comment  [syntax]
@@ -522,12 +522,12 @@ if int() is str():  # E: Non-overlapping identity check (left operand type: "int
 [builtins fixtures/primitives.pyi]
 
 [case testErrorCodeMissingModule]
-from defusedxml import xyz  # E: Cannot find implementation or library stub for module named "defusedxml"  [import]
-from nonexistent import foobar  # E: Cannot find implementation or library stub for module named "nonexistent"  [import]
-import nonexistent2  # E: Cannot find implementation or library stub for module named "nonexistent2"  [import]
-from nonexistent3 import *  # E: Cannot find implementation or library stub for module named "nonexistent3"  [import]
+from defusedxml import xyz  # E: Cannot find implementation or library stub for module named "defusedxml"  [import-not-found]
+from nonexistent import foobar  # E: Cannot find implementation or library stub for module named "nonexistent"  [import-not-found]
+import nonexistent2  # E: Cannot find implementation or library stub for module named "nonexistent2"  [import-not-found]
+from nonexistent3 import *  # E: Cannot find implementation or library stub for module named "nonexistent3"  [import-not-found]
 from pkg import bad  # E: Module "pkg" has no attribute "bad"  [attr-defined]
-from pkg.bad2 import bad3  # E: Cannot find implementation or library stub for module named "pkg.bad2"  [import] \
+from pkg.bad2 import bad3  # E: Cannot find implementation or library stub for module named "pkg.bad2"  [import-not-found] \
                            # N: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
 [file pkg/__init__.py]