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

pydantic/main.py

@@ -798,8 +798,17 @@ 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:
+                try:
+                    getattr(self, name)
+                except AttributeError:
+                    # attribute does not already exist on instance, so put it in extra
+                    self.__pydantic_extra__[name] = value  # type: ignore
+                else:
+                    # attribute _does_ already exist on instance, and was not in extra, so update it
+                    _object_setattr(self, name, value)
         else:
             self.__dict__[name] = value
             self.__pydantic_fields_set__.add(name)

tests/test_main.py

@@ -7,6 +7,7 @@
 from dataclasses import dataclass
 from datetime import date, datetime
 from enum import Enum
+from functools import partial
 from typing import (
     Any,
     Callable,
@@ -342,6 +343,25 @@ class Model(BaseModel):
     assert model.c == 1
 
 
+def test_reassign_instance_method_with_extra_allow():
+    class Model(BaseModel):
+        model_config = ConfigDict(extra='allow')
+        name: str
+
+        def not_extra_func(self) -> str:
+            return f'hello {self.name}'
+
+    def not_extra_func_replacement(self_sub: Model) -> str:
+        return f'hi {self_sub.name}'
+
+    m = Model(name='james')
+    assert m.not_extra_func() == 'hello james'
+
+    m.not_extra_func = partial(not_extra_func_replacement, m)
+    assert m.not_extra_func() == 'hi james'
+    assert 'not_extra_func' in m.__dict__
+
+
 def test_extra_ignored():
     class Model(BaseModel):
         model_config = ConfigDict(extra='ignore')