From d68c066246c9a70f179ca165614dac19b8c8beeb Mon Sep 17 00:00:00 2001
From: ad hoc <postma.marin@protonmail.com>
Date: Tue, 2 Dec 2025 05:39:55 +0100
Subject: [PATCH] =?utf8?q?=F0=9F=90=9B=20Fix=20support=20for=20form=20valu?=
 =?utf8?q?es=20with=20empty=20strings=20interpreted=20as=20missing=20(`Non?=
 =?utf8?q?e`=20if=20that's=20the=20default),=20for=20compatibility=20with?=
 =?utf8?q?=20HTML=20forms=20(#13537)?=
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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Motov Yurii <109919500+YuriiMotov@users.noreply.github.com>
Co-authored-by: Yurii Motov <yurii.motov.monte@gmail.com>
Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>
---
 fastapi/dependencies/utils.py |  3 ++-
 tests/test_form_default.py    | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 tests/test_form_default.py

diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index d43fa8a516..0f25a3c360 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -902,8 +902,9 @@ async def _extract_form_body(
             value = serialize_sequence_value(field=field, value=results)
         if value is not None:
             values[field.alias] = value
+    field_aliases = {field.alias for field in body_fields}
     for key, value in received_body.items():
-        if key not in values:
+        if key not in field_aliases:
             values[key] = value
     return values
 
diff --git a/tests/test_form_default.py b/tests/test_form_default.py
new file mode 100644
index 0000000000..2a12049d1a
--- /dev/null
+++ b/tests/test_form_default.py
@@ -0,0 +1,35 @@
+from typing import Optional
+
+from fastapi import FastAPI, File, Form
+from starlette.testclient import TestClient
+from typing_extensions import Annotated
+
+app = FastAPI()
+
+
+@app.post("/urlencoded")
+async def post_url_encoded(age: Annotated[Optional[int], Form()] = None):
+    return age
+
+
+@app.post("/multipart")
+async def post_multi_part(
+    age: Annotated[Optional[int], Form()] = None,
+    file: Annotated[Optional[bytes], File()] = None,
+):
+    return {"file": file, "age": age}
+
+
+client = TestClient(app)
+
+
+def test_form_default_url_encoded():
+    response = client.post("/urlencoded", data={"age": ""})
+    assert response.status_code == 200
+    assert response.text == "null"
+
+
+def test_form_default_multi_part():
+    response = client.post("/multipart", data={"age": ""})
+    assert response.status_code == 200
+    assert response.json() == {"file": None, "age": None}
-- 
2.47.3