Improve "Name is not defined" errors with fuzzy matching (#20693)

This PR collects all visible names in the current scope and uses fuzzy
matching to provide suggestions in the error messages

Author: KevinRK29

mypy/semanal.py

@@ -7529,6 +7529,18 @@ def name_not_defined(self, name: str, ctx: Context, namespace: str | None = None
             self.record_incomplete_ref()
             return
         message = f'Name "{name}" is not defined'
+        if (
+            not self.msg.prefer_simple_messages()
+            and "." not in name
+            and not (name.startswith("__") and name.endswith("__"))
+            and f"builtins.{name}" not in SUGGESTED_TEST_FIXTURES
+            and ctx.line not in self.errors.ignored_lines.get(self.errors.file, {})
+        ):
+            alternatives = self._get_names_in_scope()
+            alternatives.discard(name)
+            matches = best_matches(name, alternatives, n=3)
+            if matches:
+                message += f"; did you mean {pretty_seq(matches, 'or')}?"
         self.fail(message, ctx, code=codes.NAME_DEFINED)
 
         if f"builtins.{name}" in SUGGESTED_TEST_FIXTURES:
@@ -7553,6 +7565,39 @@ def name_not_defined(self, name: str, ctx: Context, namespace: str | None = None
             ).format(module=module, name=lowercased[fullname].rsplit(".", 1)[-1])
             self.note(hint, ctx, code=codes.NAME_DEFINED)
 
+    def _get_names_in_scope(self) -> set[str]:
+        """Collect all names visible in the current scope for fuzzy matching suggestions.
+
+        This includes:
+        - Local variables (from function scopes)
+        - Class attributes (only when directly in class body, not in methods)
+        - Global/module-level names
+        - Builtins
+        """
+        names: set[str] = set()
+
+        for table in self.locals:
+            if table is not None:
+                names.update(table.keys())
+
+        if self.is_class_scope():
+            assert self.type is not None
+            names.update(self.type.names.keys())
+
+        names.update(self.globals.keys())
+
+        b = self.globals.get("__builtins__", None)
+        if b:
+            assert isinstance(b.node, MypyFile)
+            for builtin_name in b.node.names.keys():
+                if not (
+                    len(builtin_name) > 1 and builtin_name[0] == "_" and builtin_name[1] != "_"
+                ):
+                    names.add(builtin_name)
+
+        # Filter out internal/dunder names that aren't useful as suggestions
+        return {n for n in names if not n.startswith("__")}
+
     def already_defined(
         self, name: str, ctx: Context, original_ctx: SymbolTableNode | SymbolNode | None, noun: str
     ) -> None:

test-data/unit/check-errorcodes.test

@@ -21,6 +21,77 @@ def f() -> None:
 [file m.py]
 [builtins fixtures/module.pyi]
 
+[case testErrorCodeUndefinedNameSuggestion]
+my_variable = 42
+my_constant = 100
+
+x = my_variabel  # E: Name "my_variabel" is not defined; did you mean "my_variable"?  [name-defined]
+
+def calculate_sum(items: int) -> int:
+    return items
+
+calculate_summ(1)  # E: Name "calculate_summ" is not defined; did you mean "calculate_sum"?  [name-defined]
+
+class MyClass:
+    pass
+
+y = MyClas()  # E: Name "MyClas" is not defined; did you mean "MyClass"?  [name-defined]
+
+unknown_xyz  # E: Name "unknown_xyz" is not defined  [name-defined]
+[builtins fixtures/module.pyi]
+
+[case testErrorCodeUndefinedNameSuggestionLocal]
+def foo() -> None:
+    local_value = 10
+    x = local_valeu  # E: Name "local_valeu" is not defined; did you mean "local_value"?  [name-defined]
+[builtins fixtures/module.pyi]
+
+[case testErrorCodeUndefinedNameSuggestionOuterScope]
+top_level_var = 42
+
+def foo() -> None:
+    x = top_level_vr  # E: Name "top_level_vr" is not defined; did you mean "top_level_var"?  [name-defined]
+[builtins fixtures/module.pyi]
+
+[case testErrorCodeUndefinedNameSuggestionClassBody]
+class Foo:
+    class_attr = 10
+    x = class_atr  # E: Name "class_atr" is not defined; did you mean "class_attr"?  [name-defined]
+[builtins fixtures/module.pyi]
+
+[case testErrorCodeUndefinedNameSuggestionFromImport]
+from m import some_function
+
+some_functon(1)  # E: Name "some_functon" is not defined; did you mean "some_function"?  [name-defined]
+
+[file m.py]
+def some_function(x: int) -> int:
+    return x
+[builtins fixtures/module.pyi]
+
+[case testErrorCodeUndefinedNameSuggestionBuiltin]
+x = lenn  # E: Name "lenn" is not defined; did you mean "len"?  [name-defined]
+[builtins fixtures/len.pyi]
+
+[case testErrorCodeUndefinedNameSuggestionClassMethod]
+class Foo:
+    class_attr = 10
+    def method(self) -> None:
+        x = class_atr  # E: Name "class_atr" is not defined  [name-defined]
+[builtins fixtures/module.pyi]
+
+[case testErrorCodeUndefinedNameSuggestionMultiple]
+total_count = 1
+total_counts = 2
+x = total_countt  # E: Name "total_countt" is not defined; did you mean "total_count" or "total_counts"?  [name-defined]
+[builtins fixtures/module.pyi]
+
+[case testErrorCodeUndefinedNameSuggestionIgnored]
+my_variable = 42
+x = my_variabel  # type: ignore[name-defined]
+y = my_variabel  # type: ignore
+[builtins fixtures/module.pyi]
+
 [case testErrorCodeUnclassifiedError]
 class A:
     def __init__(self) -> int: \

test-data/unit/pythoneval.test

@@ -608,7 +608,7 @@ def f(x: _T) -> None: pass
 s: FrozenSet
 [out]
 _program.py:2: error: Name "_T" is not defined
-_program.py:3: error: Name "FrozenSet" is not defined
+_program.py:3: error: Name "FrozenSet" is not defined; did you mean "frozenset"?
 
 [case testVarArgsFunctionSubtyping]
 import typing