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fix(streaming): preserve first SSE event after BOM #3819

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fix(streaming): preserve first SSE event after BOM #3819

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6 changes: 6 additions & 0 deletions src/openai/_streaming.py
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Original file line number Diff line number Diff line change
Expand Up @@ -327,6 +327,7 @@ class SSEDecoder:
_event: str | None
_retry: int | None
_last_event_id: str | None
_first_line: bool

def __init__(self) -> None:
self._reset()
Expand All @@ -336,6 +337,7 @@ def _reset(self) -> None:
self._data = []
self._last_event_id = None
self._retry = None
self._first_line = True

def _decode_lines(self, lines: Iterator[bytes]) -> Iterator[ServerSentEvent]:
for raw_line in lines:
Expand Down Expand Up @@ -368,6 +370,10 @@ async def aiter_bytes(self, iterator: AsyncIterator[bytes]) -> AsyncIterator[Ser
def decode(self, line: str) -> ServerSentEvent | None:
# See: https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation # noqa: E501

if self._first_line:
self._first_line = False
line = line.removeprefix("\ufeff")

if not line:
if not self._event and not self._data and not self._last_event_id and self._retry is None:
return None
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8 changes: 8 additions & 0 deletions tests/test_sse_framing.py
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Expand Up @@ -45,6 +45,14 @@ async def test_every_chunk_boundary(sync: bool, ending: bytes) -> None:
assert [event.data for event in events] == ["ok"]


@pytest.mark.parametrize("sync", [True, False])
@pytest.mark.parametrize("fragment_size", [1, 2, 100000])
async def test_leading_utf8_bom(sync: bool, fragment_size: int) -> None:
frame = b"\xef\xbb\xbfdata:first\n\ndata:second\n\n"
events = await _decode(SSEDecoder(), _fragment(frame, fragment_size), sync)
assert [event.data for event in events] == ["first", "second"]


@pytest.mark.parametrize("sync", [True, False])
async def test_many_complete_events(sync: bool) -> None:
events = await _decode(SSEDecoder(), [b"data:ok\n\n" * 2000], sync)
Expand Down