Fix Optional Literal Form field handling when value is None

fastapi/dependencies/utils.py

@@ -347,6 +347,21 @@ def add_non_field_param_to_dependency(
     return None
 
 
+def _is_optional_type(annotation: Any) -> bool:
+    """
+    Check if a type annotation is Optional (Union with None).
+    
+    Optional[X] is equivalent to Union[X, None], so we check if:
+    1. The origin is Union
+    2. NoneType is one of the union args
+    """
+    origin = get_origin(annotation)
+    if origin is Union:
+        args = get_args(annotation)
+        return type(None) in args
+    return False
+
+
 @dataclass
 class ParamDetails:
     type_annotation: Any
@@ -411,6 +426,11 @@ def analyze_param(
             if value is not inspect.Signature.empty:
                 assert not is_path_param, "Path parameters cannot have default values"
                 field_info.default = value
+            else:
+                # Check if the type annotation is Optional (Union with None)
+                # If so, use None as default instead of RequiredParam
+                if _is_optional_type(type_annotation):
+                    field_info.default = None
                 else:
                     field_info.default = RequiredParam
         # Get Annotated Depends

tests/test_request_params/test_form/test_optional_literal_form.py

@@ -0,0 +1,84 @@
+from typing import Annotated, Literal, Optional
+from fastapi import FastAPI, Form
+from fastapi.testclient import TestClient
+
+
+def test_optional_literal_form_none():
+    """Test that omitting an optional form field returns None"""
+    app = FastAPI()
+
+    @app.post("/")
+    async def read_main(
+        attribute: Annotated[Optional[Literal["abc", "def"]], Form()]
+    ):
+        return {"attribute": attribute}
+
+    client = TestClient(app)
+
+    # FIXED: Use empty data instead of {"attribute": None}
+    response = client.post("/", data={})
+    assert response.status_code == 200
+    assert response.json() == {"attribute": None}
+
+
+def test_optional_literal_form_valid_values():
+    """Test that valid literal values work correctly"""
+    app = FastAPI()
+
+    @app.post("/")
+    async def read_main(
+        attribute: Annotated[Optional[Literal["abc", "def"]], Form()]
+    ):
+        return {"attribute": attribute}
+
+    client = TestClient(app)
+
+    # Test with "abc"
+    response = client.post("/", data={"attribute": "abc"})
+    assert response.status_code == 200
+    assert response.json() == {"attribute": "abc"}
+
+    # Test with "def"
+    response = client.post("/", data={"attribute": "def"})
+    assert response.status_code == 200
+    assert response.json() == {"attribute": "def"}
+
+
+def test_optional_literal_form_invalid_value():
+    """Test that invalid values return 422"""
+    app = FastAPI()
+
+    @app.post("/")
+    async def read_main(
+        attribute: Annotated[Optional[Literal["abc", "def"]], Form()]
+    ):
+        return {"attribute": attribute}
+
+    client = TestClient(app)
+
+    # Invalid value should fail
+    response = client.post("/", data={"attribute": "xyz"})
+    assert response.status_code == 422
+
+    # String "None" is also invalid
+    response = client.post("/", data={"attribute": "None"})
+    assert response.status_code == 422
+
+
+def test_optional_literal_form_empty_string():
+    """Test that empty string is treated as None for optional Form fields"""
+    app = FastAPI()
+
+    @app.post("/")
+    async def read_main(
+        attribute: Annotated[Optional[Literal["abc", "def"]], Form()]
+    ):
+        return {"attribute": attribute}
+
+    client = TestClient(app)
+
+    # Empty string is treated as "not provided" for optional Form fields
+    # This is consistent with FastAPI's behavior for Form data
+    response = client.post("/", data={"attribute": ""})
+    assert response.status_code == 200
+    assert response.json() == {"attribute": None}