🐛 Fix response_model not invalidating None (#2725)

Co-authored-by: Taneli Hukkinen <hukkinj1@users.noreply.github.com>
Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

fastapi/utils.py

@@ -50,7 +50,7 @@ def create_response_field(
     type_: Type[Any],
     class_validators: Optional[Dict[str, Validator]] = None,
     default: Optional[Any] = None,
-    required: Union[bool, UndefinedType] = False,
+    required: Union[bool, UndefinedType] = True,
     model_config: Type[BaseConfig] = BaseConfig,
     field_info: Optional[FieldInfo] = None,
     alias: Optional[str] = None,

tests/test_validate_response.py

@@ -1,4 +1,4 @@
-from typing import List, Optional
+from typing import List, Optional, Union
 
 import pytest
 from fastapi import FastAPI
@@ -19,6 +19,19 @@ def get_invalid():
     return {"name": "invalid", "price": "foo"}
 
 
+@app.get("/items/invalidnone", response_model=Item)
+def get_invalid_none():
+    return None
+
+
+@app.get("/items/validnone", response_model=Union[Item, None])
+def get_valid_none(send_none: bool = False):
+    if send_none:
+        return None
+    else:
+        return {"name": "invalid", "price": 3.2}
+
+
 @app.get("/items/innerinvalid", response_model=Item)
 def get_innerinvalid():
     return {"name": "double invalid", "price": "foo", "owner_ids": ["foo", "bar"]}
@@ -41,6 +54,25 @@ def test_invalid():
         client.get("/items/invalid")
 
 
+def test_invalid_none():
+    with pytest.raises(ValidationError):
+        client.get("/items/invalidnone")
+
+
+def test_valid_none_data():
+    response = client.get("/items/validnone")
+    data = response.json()
+    assert response.status_code == 200
+    assert data == {"name": "invalid", "price": 3.2, "owner_ids": None}
+
+
+def test_valid_none_none():
+    response = client.get("/items/validnone", params={"send_none": "true"})
+    data = response.json()
+    assert response.status_code == 200
+    assert data is None
+
+
 def test_double_invalid():
     with pytest.raises(ValidationError):
         client.get("/items/innerinvalid")