🐛 Use the negative of allow_mutation on frozen Field

bump_pydantic/codemods/field.py

@@ -100,7 +100,15 @@ def leave_field_call(self, original_node: cst.Call, updated_node: cst.Call) -> c
         for arg in updated_node.args:
             if m.matches(arg, m.Arg(keyword=m.Name())):
                 keyword = RENAMED_KEYWORDS.get(arg.keyword.value, arg.keyword.value)  # type: ignore
-                new_args.append(arg.with_changes(keyword=arg.keyword.with_changes(value=keyword)))  # type: ignore
+                value = arg.value
+                # The `allow_mutation` keyword argument is a special case. It's the negative of `frozen`.
+                if arg.keyword and arg.keyword.value == "allow_mutation":
+                    if m.matches(arg.value, m.Name(value="False")):
+                        value = cst.Name("True")
+                    elif m.matches(arg.value, m.Name(value="True")):
+                        value = cst.Name("False")
+                new_arg = arg.with_changes(keyword=arg.keyword.with_changes(value=keyword), value=value)  # type: ignore
+                new_args.append(new_arg)  # type: ignore
             else:
                 new_args.append(arg)
 

tests/unit/test_field.py

@@ -102,3 +102,20 @@ class Potato(BaseModel):
             potato: Literal[MyEnum.POTATO] = MyEnum.POTATO
         """
         self.assertCodemod(before, after)
+
+    def test_flip_frozen_boolean(self) -> None:
+        before = """
+        from pydantic import BaseSettings, Field
+
+        class Settings(BaseSettings):
+            potato: int = Field(..., allow_mutation=False)
+            strawberry: int = Field(..., allow_mutation=True)
+        """
+        after = """
+        from pydantic import BaseSettings, Field
+
+        class Settings(BaseSettings):
+            potato: int = Field(..., frozen=True)
+            strawberry: int = Field(..., frozen=False)
+        """
+        self.assertCodemod(before, after)