Merge cef2d642de into 5ca11c59e3

fastapi/dependencies/utils.py

@@ -741,6 +741,11 @@ def _get_multidict_value(
         if field.required:
             return
         else:
+            # For FormData, return None to let Pydantic handle defaults
+            # This preserves model_fields_set metadata correctly
+            if isinstance(values, FormData):
+                return
+            # For non-FormData (Query, Header, Cookie), keep existing behavior
             return deepcopy(field.default)
     return value
 

tests/test_forms_fields_set.py

@@ -0,0 +1,139 @@
+"""
+Tests for Form fields preserving model_fields_set metadata.
+Related to issue #13399: https://github.com/fastapi/fastapi/issues/13399
+
+This test validates that Form models correctly track which fields were
+explicitly provided vs. which fields use defaults.
+"""
+
+from typing import Annotated
+
+from fastapi import FastAPI, Form
+from fastapi._compat import PYDANTIC_V2
+from fastapi.testclient import TestClient
+from pydantic import BaseModel
+
+
+class FormModelFieldsSet(BaseModel):
+    """Model for testing fields_set metadata preservation."""
+
+    field_1: bool = True
+    field_2: str = "default"
+    field_3: int = 42
+
+
+app = FastAPI()
+
+
+@app.post("/form-fields-set")
+async def form_fields_set_endpoint(model: Annotated[FormModelFieldsSet, Form()]):
+    # Use correct attribute name for each Pydantic version
+    if PYDANTIC_V2:
+        fields_set = list(model.model_fields_set)
+    else:
+        fields_set = list(model.__fields_set__)
+    return {
+        "fields_set": fields_set,
+        "data": model.dict() if not PYDANTIC_V2 else model.model_dump(),
+    }
+
+
+@app.post("/body-fields-set")
+async def body_fields_set_endpoint(model: FormModelFieldsSet):
+    # Use correct attribute name for each Pydantic version
+    if PYDANTIC_V2:
+        fields_set = list(model.model_fields_set)
+    else:
+        fields_set = list(model.__fields_set__)
+    return {"fields_set": fields_set}
+
+
+client = TestClient(app)
+
+
+class TestFormFieldsSetMetadata:
+    """Test that Form models correctly preserve fields_set metadata."""
+
+    def test_form_empty_data_has_empty_fields_set(self):
+        """Form with no data should have empty fields_set (matching JSON behavior)."""
+        resp = client.post("/form-fields-set", data={})
+        assert resp.status_code == 200, resp.text
+        fields_set = resp.json()["fields_set"]
+        assert fields_set == []
+
+    def test_body_empty_data_has_empty_fields_set(self):
+        """JSON body with no data should have empty fields_set."""
+        resp = client.post("/body-fields-set", json={})
+        assert resp.status_code == 200, resp.text
+        fields_set = resp.json()["fields_set"]
+        assert fields_set == []
+
+    def test_form_partial_data_tracks_provided_fields(self):
+        """Form with partial data should only show provided fields in fields_set."""
+        resp = client.post("/form-fields-set", data={"field_1": "False"})
+        assert resp.status_code == 200, resp.text
+        fields_set = resp.json()["fields_set"]
+        assert fields_set == ["field_1"]
+
+    def test_body_partial_data_tracks_provided_fields(self):
+        """JSON body with partial data should only show provided fields."""
+        resp = client.post("/body-fields-set", json={"field_1": False})
+        assert resp.status_code == 200, resp.text
+        fields_set = resp.json()["fields_set"]
+        assert fields_set == ["field_1"]
+
+    def test_form_all_fields_provided(self):
+        """Form with all fields should show all fields in fields_set."""
+        resp = client.post(
+            "/form-fields-set",
+            data={"field_1": "False", "field_2": "test", "field_3": "100"},
+        )
+        assert resp.status_code == 200, resp.text
+        fields_set = resp.json()["fields_set"]
+        assert set(fields_set) == {"field_1", "field_2", "field_3"}
+
+    def test_body_all_fields_provided(self):
+        """JSON body with all fields should show all fields in fields_set."""
+        resp = client.post(
+            "/body-fields-set",
+            json={"field_1": False, "field_2": "test", "field_3": 100},
+        )
+        assert resp.status_code == 200, resp.text
+        fields_set = resp.json()["fields_set"]
+        assert set(fields_set) == {"field_1", "field_2", "field_3"}
+
+    def test_form_field_set_to_default_value_is_tracked(self):
+        """Form field explicitly set to default value should appear in fields_set."""
+        # Same as default=True, but explicitly provided
+        resp = client.post("/form-fields-set", data={"field_1": "True"})
+        assert resp.status_code == 200, resp.text
+        fields_set = resp.json()["fields_set"]
+        assert fields_set == ["field_1"]
+
+    def test_body_field_set_to_default_value_is_tracked(self):
+        """JSON body field explicitly set to default value should appear in fields_set."""
+        resp = client.post("/body-fields-set", json={"field_1": True})
+        assert resp.status_code == 200, resp.text
+        fields_set = resp.json()["fields_set"]
+        assert fields_set == ["field_1"]
+
+    def test_form_body_consistency(self):
+        """
+        Verify that body and form behave consistently.
+        Form fields_set should match JSON body fields_set for equivalent data.
+        """
+        # Empty data - both should have empty fields_set
+        body_resp = client.post("/body-fields-set", json={})
+        form_resp = client.post("/form-fields-set", data={})
+        assert body_resp.json()["fields_set"] == []
+        assert form_resp.json()["fields_set"] == []
+
+        # Partial data - both should track same fields
+        body_resp = client.post(
+            "/body-fields-set", json={"field_1": False, "field_3": 99}
+        )
+        form_resp = client.post(
+            "/form-fields-set", data={"field_1": "False", "field_3": "99"}
+        )
+        assert set(body_resp.json()["fields_set"]) == {"field_1", "field_3"}
+        assert set(form_resp.json()["fields_set"]) == {"field_1", "field_3"}

tests/test_forms_single_model.py

@@ -99,13 +99,13 @@ def test_no_data():
                 "type": "missing",
                 "loc": ["body", "username"],
                 "msg": "Field required",
-                "input": {"tags": ["foo", "bar"], "with": "nothing"},
+                "input": {},  # Fixed: defaults no longer included (issue #13399)
             },
             {
                 "type": "missing",
                 "loc": ["body", "lastname"],
                 "msg": "Field required",
-                "input": {"tags": ["foo", "bar"], "with": "nothing"},
+                "input": {},  # Fixed: defaults no longer included (issue #13399)
             },
         ]
     }