Fix explicit null data in server-sent events

fastapi/routing.py

@@ -514,7 +514,7 @@ def get_request_handler(
                         # validation (the user may mix types intentionally).
                         if item.raw_data is not None:
                             data_str: str | None = item.raw_data
-                        elif item.data is not None:
+                        elif item.data is not None or "data" in item.model_fields_set:
                             if hasattr(item.data, "model_dump_json"):
                                 data_str = item.data.model_dump_json()
                             else:

fastapi/sse.py

@@ -147,7 +147,8 @@ class ServerSentEvent(BaseModel):
 
     @model_validator(mode="after")
     def _check_data_exclusive(self) -> "ServerSentEvent":
-        if self.data is not None and self.raw_data is not None:
+        data_is_set = self.data is not None or "data" in self.model_fields_set
+        if data_is_set and self.raw_data is not None:
             raise ValueError(
                 "Cannot set both 'data' and 'raw_data' on the same "
                 "ServerSentEvent. Use 'data' for JSON-serialized payloads "

tests/test_sse.py

@@ -74,6 +74,11 @@ async def sse_items_string():
     yield ServerSentEvent(data="plain text data")
 
 
+@app.get("/items/stream-null", response_class=EventSourceResponse)
+async def sse_items_null():
+    yield ServerSentEvent(data=None)
+
+
 @app.post("/items/stream-post", response_class=EventSourceResponse)
 async def sse_items_post() -> AsyncIterable[Item]:
     for item in items:
@@ -216,6 +221,12 @@ def test_string_data_json_encoded(client: TestClient):
     assert 'data: "plain text data"\n' in response.text
 
 
+def test_null_data_json_encoded(client: TestClient):
+    response = client.get("/items/stream-null")
+    assert response.status_code == 200
+    assert response.text == "data: null\n\n"
+
+
 def test_server_sent_event_null_id_rejected():
     with pytest.raises(ValueError, match="null"):
         ServerSentEvent(data="test", id="has\0null")
@@ -263,6 +274,9 @@ def test_data_and_raw_data_mutually_exclusive():
     with pytest.raises(ValueError, match="Cannot set both"):
         ServerSentEvent(data="json", raw_data="raw")
 
+    with pytest.raises(ValueError, match="Cannot set both"):
+        ServerSentEvent(data=None, raw_data="raw")
+
 
 def test_sse_on_router_included_in_app(client: TestClient):
     response = client.get("/api/events")