Bug

format_sse_event() documents that:

The result always ends with \n\n (the event terminator).

It doesn't, when no field is set. The terminator is built by appending two empty entries to the list of field lines and then joining them with \n:

lines.append("")
lines.append("")
return "\n".join(lines).encode("utf-8")

When lines is empty the join produces a single "\n" instead of "\n\n":

>>> from fastapi.sse import format_sse_event
>>> format_sse_event()
b'\n'                # expected b'\n\n'
>>> format_sse_event(data_str='"x"')
b'data: "x"\n\n'     # correct

This is reachable from user code by yielding an empty ServerSentEvent() from an SSE path operation:

@app.get("/stream", response_class=EventSourceResponse)
async def stream():
    yield ServerSentEvent(data="first")
    yield ServerSentEvent()
    yield ServerSentEvent(data="second")

The empty event is emitted as a bare \n rather than a properly terminated (empty) event:

>>> # before
'data: "first"\n\n' '\n'   'data: "second"\n\n'
>>> # after
'data: "first"\n\n' '\n\n' 'data: "second"\n\n'

Fix

Append the terminator to the joined field lines instead of adding it as two list entries, so it is emitted unconditionally.

Output is byte-for-byte unchanged for every event that sets at least one field (data, event, id, retry or comment) — the only behavior change is the previously unterminated empty event.

Tests

Two tests added to tests/test_sse.py, both of which fail on master:

• test_format_sse_event_without_fields_is_terminated — unit level.
• test_empty_server_sent_event_is_terminated — end to end through an SSE path operation.