Add support for instance method reassignment when extra='allow'

pydantic/main.py

@@ -798,8 +798,14 @@ def __setattr__(self, name: str, value: Any) -> None:
             # TODO - matching error
             raise ValueError(f'"{self.__class__.__name__}" object has no field "{name}"')
         elif self.model_config.get('extra') == 'allow' and name not in self.model_fields:
-            # SAFETY: __pydantic_extra__ is not None when extra = 'allow'
-            self.__pydantic_extra__[name] = value  # type: ignore
+            if self.model_extra and name in self.model_extra:
+                self.__pydantic_extra__[name] = value  # type: ignore
+            else:
+                attr_exists_on_instance: Any = getattr(self, name, None)
+                if attr_exists_on_instance:
+                    _object_setattr(self, name, value)
+                else:
+                    self.__pydantic_extra__[name] = value  # type: ignore
         else:
             self.__dict__[name] = value
             self.__pydantic_fields_set__.add(name)

tests/test_main.py

@@ -342,6 +342,43 @@ class Model(BaseModel):
     assert model.c == 1
 
 
+def test_reassign_static_method_with_extra_allow():
+    class Model(BaseModel):
+        model_config = ConfigDict(extra='allow')
+
+        @staticmethod
+        def not_extra_func(name: str) -> str:
+            return f'hello {name}'
+
+    def not_extra_func_replacement(name: str) -> str:
+        return f'hi {name}'
+
+    m = Model()
+    assert m.not_extra_func(name='james') == 'hello james'
+
+    m.not_extra_func = not_extra_func_replacement
+    assert m.not_extra_func(name='james') == 'hi james'
+    assert 'not_extra_func' in m.__dict__
+
+
+def test_reassign_class_method_with_extra_allow():
+    class Model(BaseModel):
+        model_config = ConfigDict(extra='allow')
+        name: ClassVar[str] = 'julia'
+
+        @classmethod
+        def not_extra_func(cls) -> str:
+            return f'hello {cls.name}'
+
+    def not_extra_func_replacement(cls_sub: Type[Model] = Model) -> str:
+        return f'hi {cls_sub.name}'
+
+    assert Model.not_extra_func() == 'hello julia'
+
+    Model.not_extra_func = not_extra_func_replacement
+    assert Model.not_extra_func() == 'hi julia'
+
+
 def test_extra_ignored():
     class Model(BaseModel):
         model_config = ConfigDict(extra='ignore')