🐛 Don't pre-fill form parameters with default values in `_get_multidict_value` #14283

YuriiMotov · 2025-11-02T09:43:19Z

Credits

The author of solution is @sneakers-the-rat. Initial PR: #13464.
I just separated these changes from (IMO) unnecessary\unrelated changes, refactored the code a bit and simplified tests to make it easier to review.

Problem description

Current implementation of _get_multidict_value puts the default value if the parameter value is empty string or is missing.
This makes it impossible to determine if this value was passed explicitly or not.

See code example in the details

from typing import Annotated

from fastapi import FastAPI, Form
from fastapi.testclient import TestClient
from pydantic import BaseModel


class ExampleModel(BaseModel):
    field_1: bool = True

app = FastAPI()

@app.post("/form")
async def form_endpoint(model: Annotated[ExampleModel, Form()]):
    return {"fields_set": list(model.model_fields_set)}

client = TestClient(app)


def test_form():
    resp = client.post("/form", data={})
    assert resp.status_code == 200, resp.text
    fields_set = resp.json()["fields_set"]
    assert fields_set == []  # AssertionError: assert ['field_1'] == []

Also, there is a bug that treating empty strings in input as None doesn't work if the default value is None:

@app.post("/")
def read_root(user_id: Annotated[int | None, Form()] = None):
    return user_id

See test in the details

from typing import Annotated

from fastapi import FastAPI, Form
from fastapi.testclient import TestClient

app = FastAPI()

@app.post("/")
def read_root(
    user_id: Annotated[int | None, Form()] = None,
):
    return user_id

def test_pass_empty_str():
    client = TestClient(app)
    response = client.post("/", data={"user_id": ""})
    assert response.status_code == 200, response.text
    # ... unable to parse string as an integer","input":""

    assert response.json() is None

Current ststus

Waiting for the #13537 to be merged. It will simplify this PR a bit

Related discussions and PRs

Dependency Models created from Form input data are loosing metadata(field set) and are enforcing validation on default values. #13399 (original issue)
🐛 Don't prefill default values on form input #13464 (original PR)
Form() does not treat emptry string as None anymore #12377 (empty str as input for form fields)
Using `None` in form field declarations with Pydantic #12150 (empty str as input for form fields declared as model)

Some clarifications

Why `if key not in values:` changed to `if key not in field_aliases:`?

In _extract_form_body we now pass form_input=True to _get_multidict_value that disables the pre-filling with default values the form parameters with missing or empty str values.

Later in _extract_form_body there were lines that added extra parameters to values.

    for key, value in received_body.items():
        if key not in values:
            values[key] = value
    return values

It worked previously because we had values for all explicitly declared parameters (we used defaults if parameter was not passed or passed as "").
But now it's not true, so we need to update this condition to if key not in field_aliases:. Otherwise it would break the logic of treating empty str as absence of the value for Form parameters.

Why `isinstance(field.field_info, (params.Form, temp_pydantic_v1_params.Form))` was removed?

This was the way to distinguish Form parameters from others. But now we have form_input parameter that effectively does it.

Also, as we previously only checked Form class, the logic of treating empty str as None didn't work for File parameters. So, "Send empty value" option in Swagger didn't work for File parameters.
Now it will work.

We can consider moving this to separate PR which should be merged before this one.

Tests

Part of tests are added just for completeness to cover different ways of Form field declaration.

On master the following tests fail:

test_defaults_form_param[empty_strings_sent] - empty string to File parameter is not treated as None
test_defaults_form_model[empty_strings_sent] - same as previous but with parameter declared as form model
test_form_model_fields_set - currently FastAPI pre-fills values with defaults, so model_fields_set doesn't work as expected

commit 1b8d0d7 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Wed Aug 20 03:24:25 2025 -0700 ok but seriously commit d3ccab4 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Tue Aug 19 23:38:25 2025 -0700 rm being able to determine the input format of a model commit fec0a06 Merge: 3f2e0f5 cad08bb Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Mon Apr 14 20:03:14 2025 -0700 Merge branch 'form-defaults' of https://github.com/sneakers-the-rat/fastapi into form-defaults commit 3f2e0f5 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Mon Apr 14 20:01:50 2025 -0700 lint commit cad08bb Author: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com> Date: Tue Apr 15 02:47:42 2025 +0000 🎨 [pre-commit.ci] Auto format from pre-commit.com hooks commit f63e983 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Mon Apr 14 19:46:34 2025 -0700 docs for handling default values, pass field to validation context commit 529d486 Merge: a9acab8 159824e Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Sat Mar 8 17:45:40 2025 -0800 Merge branch 'form-defaults' of https://github.com/sneakers-the-rat/fastapi into form-defaults commit a9acab8 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Sat Mar 8 17:42:38 2025 -0800 lint commit 159824e Author: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com> Date: Sun Mar 9 01:42:31 2025 +0000 🎨 [pre-commit.ci] Auto format from pre-commit.com hooks commit e761843 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Sat Mar 8 17:42:21 2025 -0800 pydantic v1 compat commit 64f2528 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Sat Mar 8 17:38:03 2025 -0800 fix handling form data with fields that are not annotated as Form() commit 7fade13 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Sat Mar 8 17:37:32 2025 -0800 fix just the extra values problem (again, purposefully with failing tests to demonstrate the problem, fixing in next commit) commit 49f6b83 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Sat Mar 8 17:36:03 2025 -0800 add failing tests for empty input values to get a CI run baseline for them commit 15eb678 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Thu Mar 6 19:35:53 2025 -0800 mypy lint commit 1a58af4 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Thu Mar 6 19:31:28 2025 -0800 finish pydantic 1 compat commit a2ad8b1 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Thu Mar 6 19:22:11 2025 -0800 python 3.8 and pydantic 1 compat commit 76c4d31 Author: sneakers-the-rat <sneakers-the-rat@protonmail.com> Date: Thu Mar 6 19:06:44 2025 -0800 don't prefill defaults in form input

…prefill-form

sneakers-the-rat and others added 7 commits October 30, 2025 17:34

Reverted the changes in docs unrelated to the main idea of PR

2ceaa96

Merge remote-tracking branch 'upstream/master' into issue-13399_dont-…

2f4404d

…prefill-form

Refactor _get_multidict_value for readability

218d06c

Refactor and clarify comment in _extract_form_body

265a0fe

Format

8033b02

Update tests

4c68df1

YuriiMotov added the bug Something isn't working label Nov 2, 2025

YuriiMotov force-pushed the issue-13399_dont-prefill-form branch from 5a0ee8d to 4c68df1 Compare November 2, 2025 09:52

Fix test - use default value instead of default_factory

fb17bbd

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🐛 Don't pre-fill form parameters with default values in `_get_multidict_value` #14283

🐛 Don't pre-fill form parameters with default values in `_get_multidict_value` #14283

YuriiMotov commented Nov 2, 2025 •

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🐛 Don't pre-fill form parameters with default values in _get_multidict_value #14283

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YuriiMotov commented Nov 2, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Credits

Problem description

Current ststus

Related discussions and PRs

Some clarifications

Why if key not in values: changed to if key not in field_aliases:?

Why isinstance(field.field_info, (params.Form, temp_pydantic_v1_params.Form)) was removed?

Tests

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🐛 Don't pre-fill form parameters with default values in `_get_multidict_value` #14283

🐛 Don't pre-fill form parameters with default values in `_get_multidict_value` #14283

YuriiMotov commented Nov 2, 2025 •

edited

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Why `if key not in values:` changed to `if key not in field_aliases:`?

Why `isinstance(field.field_info, (params.Form, temp_pydantic_v1_params.Form))` was removed?