Browse Source
When a Pydantic BaseModel is used as a Form() parameter, fields that
were absent from the submission or submitted as blank strings ("") were
being pre-filled with their default values before model construction.
Pydantic then treated those pre-filled defaults as explicitly-set values,
making model_fields_set incorrect and preventing model validators from
distinguishing unset fields from fields set to their default.
Two cases needed fixing:
1. Absent fields - the field key is not in the form data at all
(e.g. an unchecked HTML checkbox, which browsers omit entirely).
2. Blank string fields - the field key is present but with value ""
(e.g. a cleared text input, which browsers send as key=).
Fix: add form_input=True parameter to _get_multidict_value. When set,
both absent and blank-string values return None rather than the field
default. _extract_form_body now calls _get_multidict_value(form_input=True)
so neither case gets pre-populated into the values dict. Pydantic then
applies defaults through its own mechanism, which correctly leaves those
fields out of model_fields_set.
Required fields are unaffected: Pydantic still raises a validation error
when a required field is absent from the submitted data.
Closes #13399
Co-authored-by: sneakers-the-rat <[email protected]>
pull/15258/head
Sai Tarrun Pitta
4 months ago
3 changed files with 232 additions and 7 deletions
@ -0,0 +1,222 @@ |
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""" |
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Tests for correct model_fields_set and default handling in Form endpoints. |
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Covers: |
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- Absent fields (never sent by the client) |
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- Blank strings (HTML forms send "" for empty text inputs) |
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- Required fields still return 422 when absent |
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- Behaviour matches JSON body endpoints |
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- Aliases work correctly |
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Related: https://github.com/fastapi/fastapi/issues/13399 |
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""" |
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from typing import Annotated |
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import pytest |
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from fastapi import FastAPI, Form |
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from fastapi.testclient import TestClient |
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from pydantic import BaseModel, Field |
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app = FastAPI() |
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# --------------------------------------------------------------------------- |
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# Models |
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# --------------------------------------------------------------------------- |
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class BoolModel(BaseModel): |
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"""Checkbox-style: bool with non-falsy default.""" |
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checkbox: bool = True |
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class StringModel(BaseModel): |
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"""Text input with a non-empty default.""" |
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text: str = "default" |
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alias_field: str = Field(alias="with", default="nothing") |
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class MixedModel(BaseModel): |
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field_bool: bool = True |
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field_str: str = "hello" |
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field_nullable: str | None = None |
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class RequiredModel(BaseModel): |
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required_field: str |
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optional_field: str = "default" |
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# --------------------------------------------------------------------------- |
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# Endpoints |
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# --------------------------------------------------------------------------- |
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@app.post("/bool") |
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async def bool_endpoint(model: Annotated[BoolModel, Form()]): |
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return {"fields_set": sorted(model.model_fields_set), "values": model.model_dump()} |
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@app.post("/string") |
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async def string_endpoint(model: Annotated[StringModel, Form()]): |
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return {"fields_set": sorted(model.model_fields_set), "values": model.model_dump(by_alias=True)} |
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@app.post("/mixed") |
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async def mixed_endpoint(model: Annotated[MixedModel, Form()]): |
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return {"fields_set": sorted(model.model_fields_set), "values": model.model_dump()} |
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@app.post("/required") |
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async def required_endpoint(model: Annotated[RequiredModel, Form()]): |
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return {"fields_set": sorted(model.model_fields_set), "values": model.model_dump()} |
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# JSON body counterpart for parity checks |
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@app.post("/mixed-json") |
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async def mixed_json_endpoint(model: MixedModel): |
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return {"fields_set": sorted(model.model_fields_set), "values": model.model_dump()} |
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client = TestClient(app) |
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# --------------------------------------------------------------------------- |
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# Absent fields — never sent by the client |
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# --------------------------------------------------------------------------- |
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def test_absent_bool_field_not_in_fields_set(): |
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"""Unchecked checkbox (absent from form) must NOT appear in model_fields_set.""" |
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resp = client.post("/bool", data={}) |
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assert resp.status_code == 200, resp.text |
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assert resp.json()["fields_set"] == [] |
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def test_absent_bool_field_default_still_applied(): |
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"""Default value is still present in the response even when field not submitted.""" |
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resp = client.post("/bool", data={}) |
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assert resp.status_code == 200, resp.text |
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assert resp.json()["values"]["checkbox"] is True |
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def test_submitted_bool_field_in_fields_set(): |
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"""Submitted checkbox appears in model_fields_set.""" |
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resp = client.post("/bool", data={"checkbox": "false"}) |
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assert resp.status_code == 200, resp.text |
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assert resp.json()["fields_set"] == ["checkbox"] |
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assert resp.json()["values"]["checkbox"] is False |
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# --------------------------------------------------------------------------- |
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# Blank string fields — HTML forms send "" for empty text inputs |
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# --------------------------------------------------------------------------- |
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def test_blank_string_treated_as_unset(): |
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"""Empty text input ('') must NOT appear in model_fields_set; default applied.""" |
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resp = client.post("/string", data={"text": "", "with": ""}) |
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assert resp.status_code == 200, resp.text |
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body = resp.json() |
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assert body["fields_set"] == [] |
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assert body["values"]["text"] == "default" |
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assert body["values"]["with"] == "nothing" |
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def test_non_empty_string_in_fields_set(): |
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"""Non-empty submission must appear in model_fields_set.""" |
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resp = client.post("/string", data={"text": "hello"}) |
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assert resp.status_code == 200, resp.text |
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body = resp.json() |
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assert "text" in body["fields_set"] |
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assert body["values"]["text"] == "hello" |
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# --------------------------------------------------------------------------- |
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# Mixed model — no data submitted at all |
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# --------------------------------------------------------------------------- |
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def test_no_data_fields_set_empty(): |
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"""Submitting no form data produces an empty model_fields_set.""" |
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resp = client.post("/mixed", data={}) |
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assert resp.status_code == 200, resp.text |
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assert resp.json()["fields_set"] == [] |
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def test_partial_data_only_submitted_fields_in_set(): |
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"""Only submitted fields appear in model_fields_set.""" |
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resp = client.post("/mixed", data={"field_bool": "false"}) |
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assert resp.status_code == 200, resp.text |
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assert resp.json()["fields_set"] == ["field_bool"] |
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def test_all_data_all_fields_in_set(): |
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"""All submitted fields appear in model_fields_set.""" |
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resp = client.post( |
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"/mixed", |
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data={"field_bool": "true", "field_str": "world", "field_nullable": "x"}, |
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) |
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assert resp.status_code == 200, resp.text |
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assert resp.json()["fields_set"] == ["field_bool", "field_nullable", "field_str"] |
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# --------------------------------------------------------------------------- |
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# Required fields — must still return 422 when absent |
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# --------------------------------------------------------------------------- |
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def test_required_field_absent_returns_422(): |
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"""Required field absent from form must still return a 422 validation error.""" |
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resp = client.post("/required", data={}) |
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assert resp.status_code == 422, resp.text |
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detail = resp.json()["detail"] |
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locs = [e["loc"] for e in detail] |
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assert ["body", "required_field"] in locs |
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def test_required_field_present_optional_absent(): |
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"""Required field present, optional absent → only required field in fields_set.""" |
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resp = client.post("/required", data={"required_field": "value"}) |
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assert resp.status_code == 200, resp.text |
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body = resp.json() |
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assert body["fields_set"] == ["required_field"] |
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assert body["values"]["optional_field"] == "default" |
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# --------------------------------------------------------------------------- |
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# Parity with JSON body |
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# --------------------------------------------------------------------------- |
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@pytest.mark.parametrize( |
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"submitted, expected_set", |
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[ |
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({}, []), |
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({"field_bool": "true"}, ["field_bool"]), |
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({"field_str": "world"}, ["field_str"]), |
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( |
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{"field_bool": "false", "field_str": "hi", "field_nullable": "x"}, |
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["field_bool", "field_nullable", "field_str"], |
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), |
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], |
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) |
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def test_form_fields_set_matches_json(submitted, expected_set): |
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"""Form and JSON endpoints produce the same model_fields_set for equivalent input.""" |
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form_resp = client.post("/mixed", data=submitted) |
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json_payload = {} |
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if "field_bool" in submitted: |
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json_payload["field_bool"] = submitted["field_bool"].lower() == "true" |
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if "field_str" in submitted: |
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json_payload["field_str"] = submitted["field_str"] |
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if "field_nullable" in submitted: |
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json_payload["field_nullable"] = submitted["field_nullable"] |
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json_resp = client.post("/mixed-json", json=json_payload) |
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assert form_resp.status_code == 200, form_resp.text |
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assert json_resp.status_code == 200, json_resp.text |
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assert form_resp.json()["fields_set"] == json_resp.json()["fields_set"] == expected_set |
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