Improve missing import message (#7698)

It can be difficult to understand the missing module error message at times.

Changes

>  Cannot find module named '{}'

to

>  Cannot find implementation or library stub for module named '{}'

This improves #4542.

docs/source/error_code_list.rst

@@ -484,7 +484,7 @@ Example:
 
 .. code-block:: python
 
-    # Error: Cannot find module named 'acme'  [import]
+    # Error: Cannot find implementation or library stub for module named 'acme'  [import]
     import acme
 
 See :ref:`ignore-missing-imports` for how to work around these errors.

docs/source/existing_code.rst

@@ -42,8 +42,8 @@ find or that don't have stub files:
 
 .. code-block:: text
 
-    core/config.py:7: error: Cannot find module named 'frobnicate'
-    core/model.py:9: error: Cannot find module named 'acme'
+    core/config.py:7: error: Cannot find implementation or library stub for module named 'frobnicate'
+    core/model.py:9: error: Cannot find implementation or library stub for module named 'acme'
     ...
 
 This is normal, and you can easily ignore these errors. For example,

docs/source/running_mypy.rst

@@ -135,7 +135,7 @@ This can cause a lot of errors that look like the following::
 
     main.py:1: error: No library stub file for standard library module 'antigravity'
     main.py:2: error: No library stub file for module 'flask'
-    main.py:3: error: Cannot find module named 'this_module_does_not_exist'
+    main.py:3: error: Cannot find implementation or library stub for module named 'this_module_does_not_exist'
 
 There are several different things you can try doing, depending on the exact
 nature of the module.

mypy/build.py

@@ -2437,7 +2437,12 @@ def module_not_found(manager: BuildManager, line: int, caller_state: State,
         errors.report(line, 0, stub_msg, severity='note', only_once=True, code=codes.IMPORT)
     else:
         note = "See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports"
-        errors.report(line, 0, "Cannot find module named '{}'".format(target), code=codes.IMPORT)
+        errors.report(
+            line,
+            0,
+            "Cannot find implementation or library stub for module named '{}'".format(target),
+            code=codes.IMPORT
+        )
         errors.report(line, 0, note, severity='note', only_once=True, code=codes.IMPORT)
     errors.set_import_context(save_import_context)
 

mypy/test/testpep561.py

@@ -59,7 +59,7 @@ class SimpleMsg(Enum):
 
 
 class NamespaceMsg(Enum):
-    cfm_beta = ("{tempfile}:4: error: Cannot find module named "
+    cfm_beta = ("{tempfile}:4: error: Cannot find implementation or library stub for module named "
                 "'typedpkg_ns.ns.dne'")
     help_note = ('{tempfile}:4: note: See https://mypy.readthedocs.io/en/latest/'
                  'running_mypy.html#missing-imports')

test-data/unit/check-errorcodes.test

@@ -495,12 +495,12 @@ if int() is str():  # E: Non-overlapping identity check (left operand type: "int
 [case testErrorCodeMissingModule]
 from defusedxml import xyz  # E: No library stub file for module 'defusedxml'  [import] \
                             # N: (Stub files are from https://github.com/python/typeshed)
-from nonexistent import foobar  # E: Cannot find module named 'nonexistent'  [import] \
+from nonexistent import foobar  # E: Cannot find implementation or library stub for module named 'nonexistent'  [import] \
     # N: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
-import nonexistent2  # E: Cannot find module named 'nonexistent2'  [import]
-from nonexistent3 import *  # E: Cannot find module named 'nonexistent3'  [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 pkg import bad  # E: Module 'pkg' has no attribute 'bad'  [attr-defined]
-from pkg.bad2 import bad3  # E: Cannot find module named 'pkg.bad2'  [import]
+from pkg.bad2 import bad3  # E: Cannot find implementation or library stub for module named 'pkg.bad2'  [import]
 [file pkg/__init__.py]
 
 [case testErrorCodeAlreadyDefined]

test-data/unit/check-flags.test

@@ -453,7 +453,7 @@ main:2: note: (Using --follow-imports=error, module not passed on command line)
 [case testIgnoreMissingImportsFalse]
 from mod import x
 [out]
-main:1: error: Cannot find module named 'mod'
+main:1: error: Cannot find implementation or library stub for module named 'mod'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testIgnoreMissingImportsTrue]
@@ -610,7 +610,7 @@ from missing import MyType
 def f(x: MyType) -> None:
     pass
 [out]
-main:2: error: Cannot find module named 'missing'
+main:2: error: Cannot find implementation or library stub for module named 'missing'
 main:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:4: error: Argument 1 to "f" becomes "Any" due to an unfollowed import
 

test-data/unit/check-incremental.test

@@ -2085,7 +2085,7 @@ x = 1
 [rechecked n]
 [stale]
 [out2]
-tmp/n.py:1: error: Cannot find module named 'm'
+tmp/n.py:1: error: Cannot find implementation or library stub for module named 'm'
 tmp/n.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testDeleteFileWithinCycle]
@@ -3382,7 +3382,7 @@ import a
 [out1]
 
 [out2]
-main:2: error: Cannot find module named 'a'
+main:2: error: Cannot find implementation or library stub for module named 'a'
 main:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testIncrementalInheritanceAddAnnotation]
@@ -3525,7 +3525,7 @@ def f() -> None: pass
 def f(x: int) -> None: pass
 [out]
 [out2]
-main:1: error: Cannot find module named 'p.q'
+main:1: error: Cannot find implementation or library stub for module named 'p.q'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 [out3]
 main:2: error: Too few arguments for "f"
@@ -4140,10 +4140,10 @@ def __getattr__(attr: str) -> Any: ...
 # empty
 [builtins fixtures/module.pyi]
 [out]
-tmp/c.py:1: error: Cannot find module named 'a.b.c'
+tmp/c.py:1: error: Cannot find implementation or library stub for module named 'a.b.c'
 tmp/c.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 [out2]
-tmp/c.py:1: error: Cannot find module named 'a.b.c'
+tmp/c.py:1: error: Cannot find implementation or library stub for module named 'a.b.c'
 tmp/c.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testAddedMissingStubs]

test-data/unit/check-literal.test

@@ -851,7 +851,7 @@ func(f)  # E: Argument 1 to "func" has incompatible type "Union[Literal['foo'],
 [case testLiteralDisallowAny]
 from typing import Any
 from typing_extensions import Literal
-from missing_module import BadAlias     # E: Cannot find module named 'missing_module' \
+from missing_module import BadAlias     # E: Cannot find implementation or library stub for module named 'missing_module' \
                                         # N: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 a: Literal[Any]                         # E: Parameter 1 of Literal[...] cannot be of type "Any"

test-data/unit/check-modules.test

@@ -208,7 +208,7 @@ else:
 import nonexistent
 None + ''
 [out]
-main:1: error: Cannot find module named 'nonexistent'
+main:1: error: Cannot find implementation or library stub for module named 'nonexistent'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Unsupported left operand type for + ("None")
 
@@ -220,7 +220,7 @@ m.x = ''
 [file m.py]
 x = 1
 [out]
-main:1: error: Cannot find module named 'nonexistent'
+main:1: error: Cannot find implementation or library stub for module named 'nonexistent'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Unsupported left operand type for + ("None")
 main:4: error: Incompatible types in assignment (expression has type "str", variable has type "int")
@@ -233,7 +233,7 @@ m.x = ''
 [file m.py]
 x = 1
 [out]
-main:1: error: Cannot find module named 'nonexistent'
+main:1: error: Cannot find implementation or library stub for module named 'nonexistent'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Unsupported left operand type for + ("None")
 main:4: error: Incompatible types in assignment (expression has type "str", variable has type "int")
@@ -242,32 +242,32 @@ main:4: error: Incompatible types in assignment (expression has type "str", vari
 import nonexistent, another
 None + ''
 [out]
-main:1: error: Cannot find module named 'nonexistent'
+main:1: error: Cannot find implementation or library stub for module named 'nonexistent'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
-main:1: error: Cannot find module named 'another'
+main:1: error: Cannot find implementation or library stub for module named 'another'
 main:2: error: Unsupported left operand type for + ("None")
 
 [case testTypeCheckWithUnknownModule5]
 import nonexistent as x
 None + ''
 [out]
-main:1: error: Cannot find module named 'nonexistent'
+main:1: error: Cannot find implementation or library stub for module named 'nonexistent'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Unsupported left operand type for + ("None")
 
 [case testTypeCheckWithUnknownModuleUsingFromImport]
 from nonexistent import x
 None + ''
 [out]
-main:1: error: Cannot find module named 'nonexistent'
+main:1: error: Cannot find implementation or library stub for module named 'nonexistent'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Unsupported left operand type for + ("None")
 
 [case testTypeCheckWithUnknownModuleUsingImportStar]
 from nonexistent import *
 None + ''
 [out]
-main:1: error: Cannot find module named 'nonexistent'
+main:1: error: Cannot find implementation or library stub for module named 'nonexistent'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Unsupported left operand type for + ("None")
 
@@ -276,24 +276,24 @@ import xyz
 xyz.foo()
 xyz()
 [out]
-main:1: error: Cannot find module named 'xyz'
+main:1: error: Cannot find implementation or library stub for module named 'xyz'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testAccessingUnknownModule2]
 import xyz, bar
 xyz.foo()
 bar()
 [out]
-main:1: error: Cannot find module named 'xyz'
+main:1: error: Cannot find implementation or library stub for module named 'xyz'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
-main:1: error: Cannot find module named 'bar'
+main:1: error: Cannot find implementation or library stub for module named 'bar'
 
 [case testAccessingUnknownModule3]
 import xyz as z
 xyz.foo()
 z()
 [out]
-main:1: error: Cannot find module named 'xyz'
+main:1: error: Cannot find implementation or library stub for module named 'xyz'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Name 'xyz' is not defined
 
@@ -302,14 +302,14 @@ from xyz import y, z
 y.foo()
 z()
 [out]
-main:1: error: Cannot find module named 'xyz'
+main:1: error: Cannot find implementation or library stub for module named 'xyz'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testAccessingNameImportedFromUnknownModule2]
 from xyz import *
 y
 [out]
-main:1: error: Cannot find module named 'xyz'
+main:1: error: Cannot find implementation or library stub for module named 'xyz'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Name 'y' is not defined
 
@@ -318,14 +318,14 @@ from xyz import y as z
 y
 z
 [out]
-main:1: error: Cannot find module named 'xyz'
+main:1: error: Cannot find implementation or library stub for module named 'xyz'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Name 'y' is not defined
 
 [case testUnknownModuleRedefinition]
 # Error messages differ with the new analyzer
 
-import xab  # E: Cannot find module named 'xab'  # N: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
+import xab  # E: Cannot find implementation or library stub for module named 'xab'  # N: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 def xab(): pass  # E: Name 'xab' already defined (possibly by an import)
 
 [case testAccessingUnknownModuleFromOtherModule]
@@ -336,7 +336,7 @@ x.z
 import nonexistent
 [builtins fixtures/module.pyi]
 [out]
-tmp/x.py:1: error: Cannot find module named 'nonexistent'
+tmp/x.py:1: error: Cannot find implementation or library stub for module named 'nonexistent'
 tmp/x.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:3: error: Module has no attribute "z"
 
@@ -346,7 +346,7 @@ def f():
 def foobar(): pass
 foobar('')
 [out]
-main:2: error: Cannot find module named 'foobar'
+main:2: error: Cannot find implementation or library stub for module named 'foobar'
 main:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:4: error: Too many arguments for "foobar"
 
@@ -356,7 +356,7 @@ def f():
 def x(): pass
 x('')
 [out]
-main:2: error: Cannot find module named 'foobar'
+main:2: error: Cannot find implementation or library stub for module named 'foobar'
 main:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:4: error: Too many arguments for "x"
 
@@ -2177,8 +2177,8 @@ import c
 
 [out]
 -- TODO: it would be better for this to be in the other order
-tmp/b.py:1: error: Cannot find module named 'c'
-main:1: error: Cannot find module named 'c'
+tmp/b.py:1: error: Cannot find implementation or library stub for module named 'c'
+main:1: error: Cannot find implementation or library stub for module named 'c'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testIndirectFromImportWithinCycle1]
@@ -2312,7 +2312,7 @@ from typing import Any
 def __getattr__(attr: str) -> Any: ...
 [builtins fixtures/module.pyi]
 [out]
-main:1: error: Cannot find module named 'a.b'
+main:1: error: Cannot find implementation or library stub for module named 'a.b'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testModuleGetattrInit4]
@@ -2357,12 +2357,12 @@ def __getattr__(attr: str) -> Any: ...
 # empty (i.e. complete subpackage)
 [builtins fixtures/module.pyi]
 [out]
-main:1: error: Cannot find module named 'a.b.c.d'
+main:1: error: Cannot find implementation or library stub for module named 'a.b.c.d'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
-main:1: error: Cannot find module named 'a.b.c'
+main:1: error: Cannot find implementation or library stub for module named 'a.b.c'
 
 [case testModuleGetattrInit8a]
-import a.b.c  # E: Cannot find module named 'a.b.c'  # N: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
+import a.b.c  # E: Cannot find implementation or library stub for module named 'a.b.c'  # N: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 import a.d  # OK
 [file a/__init__.pyi]
 from typing import Any
@@ -2388,7 +2388,7 @@ def __getattr__(attr: str) -> Any: ...
 ignore_missing_imports = True
 [builtins fixtures/module.pyi]
 [out]
-main:3: error: Cannot find module named 'a.b.d'
+main:3: error: Cannot find implementation or library stub for module named 'a.b.d'
 main:3: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testIndirectFromImportWithinCycleUsedAsBaseClass-skip]
@@ -2619,7 +2619,7 @@ from foo.bar import x
 [file foo/bar.py]
 x = 0
 [out]
-main:1: error: Cannot find module named 'foo.bar'
+main:1: error: Cannot find implementation or library stub for module named 'foo.bar'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testNamespacePackage]

test-data/unit/check-python2.test

@@ -167,7 +167,7 @@ main:8: error: Argument 2 to "f" has incompatible type "int"; expected "Tuple[in
 import asyncio
 import Bastion
 [out]
-main:1: error: Cannot find module named 'asyncio'
+main:1: error: Cannot find implementation or library stub for module named 'asyncio'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: No library stub file for standard library module 'Bastion'
 main:2: note: (Stub files are from https://github.com/python/typeshed)

test-data/unit/check-semanal-error.test

@@ -18,7 +18,7 @@ m.foo()
 m.x = m.y
 1() # E
 [out]
-main:1: error: Cannot find module named 'm'
+main:1: error: Cannot find implementation or library stub for module named 'm'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:4: error: "int" not callable
 
@@ -28,7 +28,7 @@ x.foo()
 x.a = x.b
 1() # E
 [out]
-main:1: error: Cannot find module named 'm'
+main:1: error: Cannot find implementation or library stub for module named 'm'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:4: error: "int" not callable
 
@@ -37,7 +37,7 @@ from m import * # E
 x # E
 1() # E
 [out]
-main:1: error: Cannot find module named 'm'
+main:1: error: Cannot find implementation or library stub for module named 'm'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 main:2: error: Name 'x' is not defined
 main:3: error: "int" not callable