Prevent final reassignment in match case (#19496)

omerhadari · web-flow · commit c6b40df63ce0 · 2025-07-26T21:15:42.000-07:00
Fixes #19507 This PR adds a check to prevent reassignment of final variables in a match statement. Currently, the following passes without an error: ```Python from typing import Final FOO: Final = 8 a = 10 match a: case FOO: pass print(FOO) # FOO is reassigned, prints 10 ``` MyPy already checks that the type of FOO isn't changed if used like this. I added a check in the same place that makes sure it's not `Final` either. Since this tests the match syntax, I put the test where I did. If it's not the appropriate place for it I will move it as instructed :)
diff --git a/mypy/checker.py b/mypy/checker.py
@@ -5577,6 +5577,8 @@ def infer_variable_types_from_type_maps(
                 previous_type, _, _ = self.check_lvalue(expr)
                 if previous_type is not None:
                     already_exists = True
+                    if isinstance(expr.node, Var) and expr.node.is_final:
+                        self.msg.cant_assign_to_final(expr.name, False, expr)
                     if self.check_subtype(
                         typ,
                         previous_type,
diff --git a/test-data/unit/check-python310.test b/test-data/unit/check-python310.test
@@ -2839,3 +2839,13 @@ match value_type:
     case _:
         assert_never(value_type)
 [builtins fixtures/tuple.pyi]
+
+[case testAssignmentToFinalInMatchCaseNotAllowed]
+from typing import Final
+
+FOO: Final[int] = 10
+
+val: int = 8
+match val:
+    case FOO:  # E: Cannot assign to final name "FOO"
+        pass
