don't prefill defaults in form input

fastapi/dependencies/utils.py

@@ -714,7 +714,10 @@ def _validate_value_with_model_field(
 
 
 def _get_multidict_value(
-    field: ModelField, values: Mapping[str, Any], alias: Union[str, None] = None
+    field: ModelField,
+    values: Mapping[str, Any],
+    alias: Union[str, None] = None,
+    return_default=True,
 ) -> Any:
     alias = alias or field.alias
     if is_sequence_field(field) and isinstance(values, (ImmutableMultiDict, Headers)):
@@ -730,10 +733,10 @@ def _get_multidict_value(
         )
         or (is_sequence_field(field) and len(value) == 0)
     ):
-        if field.required:
-            return
-        else:
+        if return_default and not field.required:
             return deepcopy(field.default)
+        else:
+            return None
     return value
 
 
@@ -839,7 +842,7 @@ async def _extract_form_body(
     first_field_info = first_field.field_info
 
     for field in body_fields:
-        value = _get_multidict_value(field, received_body)
+        value = _get_multidict_value(field, received_body, return_default=False)
         if (
             isinstance(first_field_info, params.File)
             and is_bytes_field(field)

tests/test_forms_defaults.py

@@ -0,0 +1,192 @@
+from typing import Annotated, Optional
+
+import pytest
+from fastapi import FastAPI, Form
+from pydantic import BaseModel, Field, model_validator
+from starlette.testclient import TestClient
+
+
+class Parent(BaseModel):
+    init_input: dict
+    # importantly, no default here
+
+    @model_validator(mode="before")
+    def validate_inputs(cls, value: dict) -> dict:
+        """
+        model validators in before mode should receive values passed
+        to model instantiation before any further validation
+        """
+        # we should not be double-instantiating the models
+        assert isinstance(value, dict)
+        value["init_input"] = value.copy()
+
+        # differentiate between explicit Nones and unpassed values
+        if "true_if_unset" not in value:
+            value["true_if_unset"] = True
+        return value
+
+
+class StandardModel(Parent):
+    default_true: bool = True
+    default_false: bool = False
+    default_none: Optional[bool] = None
+    default_zero: int = 0
+    true_if_unset: Optional[bool] = None
+
+
+class FieldModel(Parent):
+    default_true: bool = Field(default=True)
+    default_false: bool = Field(default=False)
+    default_none: Optional[bool] = Field(default=None)
+    default_zero: int = Field(default=0)
+    true_if_unset: Optional[bool] = Field(default=None)
+
+
+class AnnotatedFieldModel(Parent):
+    default_true: Annotated[bool, Field(default=True)]
+    default_false: Annotated[bool, Field(default=False)]
+    default_none: Annotated[Optional[bool], Field(default=None)]
+    default_zero: Annotated[int, Field(default=0)]
+    true_if_unset: Annotated[Optional[bool], Field(default=None)]
+
+
+class AnnotatedFormModel(Parent):
+    default_true: Annotated[bool, Form(default=True)]
+    default_false: Annotated[bool, Form(default=False)]
+    default_none: Annotated[Optional[bool], Form(default=None)]
+    default_zero: Annotated[int, Form(default=0)]
+    true_if_unset: Annotated[Optional[bool], Form(default=None)]
+
+
+class ResponseModel(BaseModel):
+    fields_set: list = Field(default_factory=list)
+    dumped_fields_no_exclude: dict = Field(default_factory=dict)
+    dumped_fields_exclude_default: dict = Field(default_factory=dict)
+    dumped_fields_exclude_unset: dict = Field(default_factory=dict)
+    init_input: dict
+
+    @classmethod
+    def from_value(cls, value: Parent) -> "ResponseModel":
+        return ResponseModel(
+            init_input=value.init_input,
+            fields_set=list(value.model_fields_set),
+            dumped_fields_no_exclude=value.model_dump(),
+            dumped_fields_exclude_default=value.model_dump(exclude_defaults=True),
+            dumped_fields_exclude_unset=value.model_dump(exclude_unset=True),
+        )
+
+
+app = FastAPI()
+
+
+@app.post("/form/standard")
+async def form_standard(value: Annotated[StandardModel, Form()]) -> ResponseModel:
+    return ResponseModel.from_value(value)
+
+
+@app.post("/form/field")
+async def form_field(value: Annotated[FieldModel, Form()]) -> ResponseModel:
+    return ResponseModel.from_value(value)
+
+
+@app.post("/form/annotated-field")
+async def form_annotated_field(
+    value: Annotated[AnnotatedFieldModel, Form()],
+) -> ResponseModel:
+    return ResponseModel.from_value(value)
+
+
+@app.post("/form/annotated-form")
+async def form_annotated_form(
+    value: Annotated[AnnotatedFormModel, Form()],
+) -> ResponseModel:
+    return ResponseModel.from_value(value)
+
+
+@app.post("/json/standard")
+async def json_standard(value: StandardModel) -> ResponseModel:
+    return ResponseModel.from_value(value)
+
+
+@app.post("/json/field")
+async def json_field(value: FieldModel) -> ResponseModel:
+    return ResponseModel.from_value(value)
+
+
+@app.post("/json/annotated-field")
+async def json_annotated_field(value: AnnotatedFieldModel) -> ResponseModel:
+    return ResponseModel.from_value(value)
+
+
+@app.post("/json/annotated-form")
+async def json_annotated_form(value: AnnotatedFormModel) -> ResponseModel:
+    return ResponseModel.from_value(value)
+
+
+MODEL_TYPES = {
+    "standard": StandardModel,
+    "field": FieldModel,
+    "annotated-field": AnnotatedFieldModel,
+    "annotated-form": AnnotatedFormModel,
+}
+ENCODINGS = ("form", "json")
+
+
+@pytest.fixture(scope="module")
+def client() -> TestClient:
+    with TestClient(app) as test_client:
+        yield test_client
+
+
+@pytest.mark.parametrize("encoding", ENCODINGS)
+@pytest.mark.parametrize("model_type", MODEL_TYPES.keys())
+def test_no_prefill_defaults_all_unset(encoding, model_type, client, monkeypatch):
+    """
+    When the model is instantiated by the server, it should not have its defaults prefilled
+    """
+
+    endpoint = f"/{encoding}/{model_type}"
+    if encoding == "form":
+        res = client.post(endpoint, data={})
+    else:
+        res = client.post(endpoint, json={})
+
+    assert res.status_code == 200
+    response_model = ResponseModel(**res.json())
+    assert response_model.init_input == {}
+    assert len(response_model.fields_set) == 2
+    assert response_model.dumped_fields_no_exclude["true_if_unset"] is True
+
+
+@pytest.mark.parametrize("encoding", ENCODINGS)
+@pytest.mark.parametrize("model_type", MODEL_TYPES.keys())
+def test_no_prefill_defaults_partially_set(encoding, model_type, client, monkeypatch):
+    """
+    When the model is instantiated by the server, it should not have its defaults prefilled,
+    and pydantic should be able to differentiate between unset and default values when some are passed
+    """
+    endpoint = f"/{encoding}/{model_type}"
+    if encoding == "form":
+        data = {"true_if_unset": "False", "default_false": "True", "default_zero": "0"}
+        res = client.post(endpoint, data=data)
+    else:
+        data = {"true_if_unset": False, "default_false": True, "default_zero": 0}
+        res = client.post(endpoint, json=data)
+
+    data_with_init_input = data.copy()
+    data_with_init_input["init_input"] = data.copy()
+
+    assert res.status_code == 200
+    response_model = ResponseModel(**res.json())
+    assert response_model.init_input == data
+    assert len(response_model.fields_set) == 4
+    dumped_exclude_unset = MODEL_TYPES[model_type](**data).model_dump(
+        exclude_unset=True
+    )
+    assert response_model.dumped_fields_exclude_unset == dumped_exclude_unset
+    assert response_model.dumped_fields_no_exclude["true_if_unset"] is False
+    dumped_exclude_default = MODEL_TYPES[model_type](**data).model_dump(
+        exclude_defaults=True
+    )
+    assert "default_zero" not in dumped_exclude_default
+    assert "default_zero" not in response_model.dumped_fields_exclude_default

tests/test_forms_single_model.py

@@ -104,13 +104,13 @@ def test_no_data():
                     "type": "missing",
                     "loc": ["body", "username"],
                     "msg": "Field required",
-                    "input": {"tags": ["foo", "bar"], "with": "nothing"},
+                    "input": {},
                 },
                 {
                     "type": "missing",
                     "loc": ["body", "lastname"],
                     "msg": "Field required",
-                    "input": {"tags": ["foo", "bar"], "with": "nothing"},
+                    "input": {},
                 },
             ]
         }