Track extra fields set after init in model_fields_set (pydantic#12817)

Author: navalprakhar

pydantic/main.py

@@ -1096,6 +1096,7 @@ def _setattr_handler(self, name: str, value: Any) -> Callable[[BaseModel, str, A
                 elif attr is None:
                     # attribute does not exist, so put it in extra
                     self.__pydantic_extra__[name] = value
+                    self.__pydantic_fields_set__.add(name)
                     return None  # Can not return memoized handler with possibly freeform attr names
                 else:
                     # attribute _does_ exist, and was not in extra, so update it

tests/test_edge_cases.py

@@ -3135,3 +3135,22 @@ def my_model_validator(self):
         @classmethod
         def __get_validators__(cls):
             raise AssertionError('This should not be called')
+
+
+def test_model_fields_set_includes_extra_after_assignment():
+    """Regression test for https://github.com/pydantic/pydantic/issues/11134.
+
+    Extra fields set after initialization should be included in model_fields_set.
+    """
+    from pydantic import BaseModel, ConfigDict
+
+    class Model(BaseModel):
+        model_config = ConfigDict(extra='allow')
+        field: int
+
+    m = Model.model_validate({'field': 1, 'extra_at_init': 2})
+    assert m.model_fields_set == {'field', 'extra_at_init'}
+
+    m.extra_after_init = 3
+    assert m.model_fields_set == {'field', 'extra_at_init', 'extra_after_init'}
+    assert m.model_extra == {'extra_at_init': 2, 'extra_after_init': 3}