fix ISSUE 13533

fastapi/dependencies/utils.py

@@ -899,6 +899,8 @@ async def _extract_form_body(
     received_body: FormData,
 ) -> Dict[str, Any]:
     values = {}
+    # Track which keys are defined as body_fields to avoid re-adding empty strings
+    body_field_aliases = {field.alias for field in body_fields}
 
     for field in body_fields:
         value = _get_multidict_value(field, received_body)
@@ -928,10 +930,17 @@ async def _extract_form_body(
                 for sub_value in value:
                     tg.start_soon(process_fn, sub_value.read)
             value = serialize_sequence_value(field=field, value=results)
-        if value is not None:
+        # Don't add empty strings for Form fields - _get_multidict_value already handled them
+        if value is not None and not (
+            isinstance(field_info, (params.Form, temp_pydantic_v1_params.Form))
+            and isinstance(value, str)
+            and value == ""
+        ):
             values[field.alias] = value
+    # Only add extra fields that are not already defined as body_fields
+    # This prevents overwriting correctly processed default values
     for key, value in received_body.items():
-        if key not in values:
+        if key not in body_field_aliases and key not in values:
             values[key] = value
     return values
 

tests/test_forms_empty_string_default_13533.py

@@ -0,0 +1,103 @@
+"""
+Test for issue #13533: Multiple regressions in the handling of forms & form validation
+
+This test verifies that empty strings in form fields are correctly handled:
+- For optional fields with None default, empty string should result in None
+- For optional fields with int type, empty string should result in None (not parsing error)
+- This applies to both x-www-form-urlencoded and multipart/form-data
+"""
+
+from typing import Annotated, Optional
+
+from fastapi import FastAPI, File, Form
+from fastapi.testclient import TestClient
+
+# Test app for URL-encoded forms with optional string
+app_urlencoded_str = FastAPI()
+
+
+@app_urlencoded_str.post("/test")
+def endpoint_urlencoded_str(
+    name: Annotated[Optional[str], Form(embed=True)] = None,
+):
+    return {"name": name}
+
+
+# Test app for URL-encoded forms with optional int
+app_urlencoded_int = FastAPI()
+
+
+@app_urlencoded_int.post("/test")
+def endpoint_urlencoded_int(
+    age: Annotated[Optional[int], Form()] = None,
+):
+    return {"age": age}
+
+
+# Test app for multipart forms
+app_multipart = FastAPI()
+
+
+@app_multipart.post("/test")
+def endpoint_multipart(
+    file: Annotated[Optional[bytes], File()] = None,
+    name: Annotated[Optional[str], Form(embed=True)] = None,
+):
+    return {"file_size": len(file) if file else None, "name": name}
+
+
+def test_urlencoded_empty_string_optional_str():
+    """
+    Regression test for #13533: Empty string in URL-encoded form
+    with optional str field should use default value (None)
+    """
+    client = TestClient(app_urlencoded_str)
+    response = client.post("/test", data={"name": ""})
+    assert response.status_code == 200
+    assert response.json() == {"name": None}
+
+
+def test_urlencoded_empty_string_optional_int():
+    """
+    Regression test for #13533: Empty string in URL-encoded form
+    with optional int field should use default value (None), not cause parsing error
+    """
+    client = TestClient(app_urlencoded_int)
+    response = client.post("/test", data={"age": ""})
+    assert response.status_code == 200
+    assert response.json() == {"age": None}
+
+
+def test_multipart_empty_string_optional_str():
+    """
+    Regression test for #13533: Empty string in multipart form
+    with optional str field should use default value (None)
+    """
+    client = TestClient(app_multipart)
+    response = client.post("/test", data={"name": ""})
+    assert response.status_code == 200
+    assert response.json() == {"file_size": None, "name": None}
+
+
+def test_urlencoded_with_actual_value():
+    """Verify that actual values still work correctly"""
+    client = TestClient(app_urlencoded_str)
+    response = client.post("/test", data={"name": "John"})
+    assert response.status_code == 200
+    assert response.json() == {"name": "John"}
+
+
+def test_urlencoded_int_with_actual_value():
+    """Verify that actual int values still work correctly"""
+    client = TestClient(app_urlencoded_int)
+    response = client.post("/test", data={"age": "25"})
+    assert response.status_code == 200
+    assert response.json() == {"age": 25}
+
+
+def test_multipart_with_actual_value():
+    """Verify that actual values in multipart forms still work correctly"""
+    client = TestClient(app_multipart)
+    response = client.post("/test", data={"name": "John"})
+    assert response.status_code == 200
+    assert response.json() == {"file_size": None, "name": "John"}