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

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🐛 Don't pre-fill form parameters with default values in _get_multidict_value#14283
YuriiMotov wants to merge 10 commits into
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YuriiMotov:issue-13399_dont-prefill-form

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@YuriiMotov YuriiMotov commented Nov 2, 2025 •
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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'] == []

Related discussions and PRs

Some clarifications

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 every field to be annotated with Form, the logic of treating empty str as None didn't work for form fields declared as model.
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.

sneakers-the-rat and others added 7 commits October 30, 2025 17:34
@sneakers-the-rat @YuriiMotov
Squashed commit of the following:
978351d 
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
@YuriiMotov
Reverted the changes in docs unrelated to the main idea of PR
2ceaa96
@YuriiMotov
Merge remote-tracking branch 'upstream/master' into issue-13399_dont-…
2f4404d 
…prefill-form
@YuriiMotov
Refactor _get_multidict_value for readability
218d06c
@YuriiMotov
Refactor and clarify comment in _extract_form_body
265a0fe
@YuriiMotov
Format
8033b02
@YuriiMotov
Update tests
4c68df1
@YuriiMotov YuriiMotov added the bug Something isn't working label Nov 2, 2025
@YuriiMotov YuriiMotov force-pushed the issue-13399_dont-prefill-form branch from 5a0ee8d to 4c68df1 Compare November 2, 2025 09:52
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@github-actions github-actions Bot added the conflicts Automatically generated when a PR has a merge conflict label Dec 2, 2025
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@github-actions github-actions Bot removed the conflicts Automatically generated when a PR has a merge conflict label Dec 2, 2025
@YuriiMotov YuriiMotov marked this pull request as ready for review December 4, 2025 07:20
@github-actions github-actions Bot added the conflicts Automatically generated when a PR has a merge conflict label Dec 17, 2025
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This pull request has a merge conflict that needs to be resolved.

@YuriiMotov YuriiMotov mentioned this pull request Dec 20, 2025
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@YuriiMotov YuriiMotov marked this pull request as draft April 28, 2026 10:35
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@YuriiMotov @sneakers-the-rat @svlandeg