fix: preserve empty form values for required params

fastapi/dependencies/utils.py

@@ -762,22 +762,24 @@ def _get_multidict_value(
         value = values.getlist(alias)
     else:
         value = values.get(alias, None)
-    if (
+    if value is None or (
-        value is None
+        field_annotation_is_sequence(field.field_info.annotation)
-        or (
+        and len(value) == 0
-            isinstance(field.field_info, params.Form)
-            and isinstance(value, str)  # For type checks
-            and value == ""
-        )
-        or (
-            field_annotation_is_sequence(field.field_info.annotation)
-            and len(value) == 0
-        )
     ):
         if field.field_info.is_required():
             return
         else:
             return deepcopy(field.default)
+
+    if (
+        isinstance(field.field_info, params.Form)
+        and isinstance(value, str)  # For type checks
+        and value == ""
+    ):
+        if field.field_info.is_required():
+            return value
+        else:
+            return deepcopy(field.default)
     return value
 
 

tests/test_form_default.py

@@ -19,6 +19,11 @@ async def post_multi_part(
     return {"file": file, "age": age}
 
 
+@app.post("/urlencoded-required")
+async def post_url_encoded_required(name: Annotated[str, Form()]):
+    return name
+
+
 client = TestClient(app)
 
 
@@ -32,3 +37,9 @@ def test_form_default_multi_part():
     response = client.post("/multipart", data={"age": ""})
     assert response.status_code == 200
     assert response.json() == {"file": None, "age": None}
+
+
+def test_required_form_field_with_empty_string():
+    response = client.post("/urlencoded-required", data={"name": ""})
+    assert response.status_code == 200
+    assert response.text == ""