axio-sse

Read text/event-stream: a decoder you feed, and a reader for what its payloads mean.

The package knows nothing about HTTP and imports no client. It has no dependencies, not even on axio.

Installation

pip install axio-sse

Usage

payloads(chunks, *, until="") — the JSON object of every event

All a stream with no discriminator needs. Comments, keep-alives and junk never arrive. until names the one data payload that closes the stream, so a sentinel that is not JSON never reaches you.

import asyncio
from axio_sse import payloads

async def chunks():
    yield b': keep-alive\n\n'
    yield b'data: {"choices":[{"delta":{"content":"Hel"}}]}\n\n'
    yield b'data: {"choices":[{"delta":{"content":"lo"}}]}\n\n'
    yield b"data: [DONE]"

async def main() -> None:
    got = [p["choices"][0]["delta"]["content"] async for p in payloads(chunks(), until="[DONE]")]
    assert got == ["Hel", "lo"]

asyncio.run(main())

Feed it whatever the transport hands you. A chunk may end mid-field, mid-terminator, or mid-UTF-8 sequence. The result does not depend on where it was cut. Chunks must still carry their line terminators, so aiter_lines() will not do. It strips them, and nothing ever dispatches.

A stream that stops without its final blank line still yields what it collected. The example above ends on data: [DONE] with no newline after it. That is how these streams really end.

events(chunks, *, until="") — the wire events themselves

import asyncio
from axio_sse import Event, events

async def chunks():
    yield b'data: {"first":\ndata: true}\n\n'
    yield b"event: named\r\ndata: sec"
    yield b"ond\r\n\r\n"

async def main() -> None:
    assert [e async for e in events(chunks())] == [
        Event(data='{"first":\ntrue}'),
        Event(data="second", event="named"),
    ]

asyncio.run(main())

Event carries the four fields the format defines — data, event, id, retry. An empty event means unnamed, which the format reads as "message". Event.payload() gives the JSON object, or None where the event carries none.

events() suspends nowhere of its own accord, so it needs no async framework: asyncio, trio and anyio all drive it. A yield in an async generator does not reach the event loop. A caller that must stay fair to other tasks — a TUI redrawing, a queue being served — therefore says so itself, with await asyncio.sleep(0) in its own loop where it knows what else is waiting.

Decoder — the format, with no loop

Decoder is the format and nothing else. It is synchronous and holds no connection. Every wire case is therefore testable without a loop. A thread or a non-asyncio caller can drive it too. Same shape as codecs.IncrementalDecoder, because the problem is the same: input cut at arbitrary points, output that only sometimes completes.

from axio_sse import Decoder, Event

decoder = Decoder()
assert decoder.decode(b"data: hel") == []
assert decoder.decode(b"lo\n\ndata: wor") == [Event(data="hello")]
assert decoder.decode(b"ld\n\n", final=True) == [Event(data="world")]

final=True closes the stream. What is still pending is discarded, which the format requires: an event that never reached its blank line is not dispatched. Dispatched anyway, a connection cut between a frame and the blank line after it makes a truncated turn read as a finished one.

The package takes chunks and never lines for a reason. aiohttp's readuntil raises LineTooLong past 131072 bytes. LineTooLong is not a ClientError. One large reasoning event kills a turn with no answer.

Wire — a payload shape

Declare the fields you read. Each is read by its declared name and type. A misspelled key is therefore a type error at the place that uses it, rather than a default quietly standing in for the value.

from dataclasses import dataclass, field
from axio_sse import Payload, Wire

@dataclass(frozen=True, slots=True)
class Usage(Wire):
    """Nested, and never dispatched to: it has no name of its own."""
    output_tokens: int = 0

@dataclass(frozen=True, slots=True)
class ResponseObject(Wire):
    usage: Usage = field(default_factory=Usage)

@dataclass(frozen=True, slots=True)
class OutputTextDelta(Wire, name="response.output_text.delta"):
    delta: str = ""

@dataclass(frozen=True, slots=True)
class Completed(Wire, name="response.completed"):
    response: ResponseObject = field(default_factory=ResponseObject)

A field the provider did not send, sent as null, or sent as the wrong type takes its default. That is what an optional provider field is. One bad field must not lose the whole event. A nested object is another Wire. A list of them is list[ThatWire]. Declare a field raw: Payload and it receives the whole payload, for a shape that varies too much to declare whole.

Reader — one method per event

A stream that says what each event is subclasses Reader and writes one @on(...) method per event. That class body is one endpoint's whole vocabulary. by on the class line names the payload key that holds the event's name. It defaults to "type".

Give @on a shape and the method is handed that shape. Give it names and the method is handed the Payload itself. That is what a method that only forwards an event wants. Declaring a shape for a payload nobody reads a field of would be a schema written for nothing.

One instance reads one stream. The turn's running totals and id maps live on self instead of travelling through a call. Construct one per response.

import asyncio
from collections.abc import Iterator
from axio_sse import Reader, on

class Responses(Reader[str]):
    """What the Responses API sends, and what each event becomes."""

    def __init__(self) -> None:
        self.output_tokens = 0

    @on(OutputTextDelta)
    def _text(self, wire: OutputTextDelta) -> Iterator[str]:
        yield wire.delta

    @on(Completed)
    def _completed(self, wire: Completed) -> None:
        self.output_tokens = wire.response.usage.output_tokens

    @on("response.created", "response.in_progress", "response.output_text.done")
    def _expected(self, payload: Payload) -> None:
        """The bookkeeping around the deltas. Named so strict fires only on something new."""

async def chunks():
    yield b'data: {"type":"response.created"}\n\n'
    yield b'data: {"type":"response.output_text.delta","delta":"Hi"}\n\n'
    yield b'data: {"type":"response.completed","response":{"usage":{"output_tokens":7}}}\n\n'

async def main() -> None:
    turn = Responses()
    assert [made async for made in turn.over(chunks())] == ["Hi"]
    assert turn.output_tokens == 7

asyncio.run(main())

A handler returns what the event became — an iterable, or None where the event only moved that state. Several names on one method is how a stream that sends one thing under two names is written. It is also how a group of events that means nothing here is written: a method with only a docstring. Both stay in the class body, so no second list exists to keep in step with the first.

strict — failing on the day the provider sends something new

An event no method claims is skipped and logged at DEBUG. Read with strict=True and it raises instead. That is what a test holds against the provider's own published list.

import pytest
from axio_sse import Event, UnknownEvent

assert Responses.names() == {
    "response.output_text.delta",
    "response.completed",
    "response.created",
    "response.in_progress",
    "response.output_text.done",
}

with pytest.raises(UnknownEvent, match="response.refusal.delta"):
    Responses().read(Event(data='{"type":"response.refusal.delta"}'), strict=True)

strict belongs to the call, not to the reader. A policy that outlived one call would leave a CI test's strictness set for the next caller.

EVENT_NAME — dispatching on the format's own field

Some streams name the event in the SSE event: field rather than in the payload.

import asyncio
from collections.abc import Iterator
from dataclasses import dataclass, field
from axio_sse import EVENT_NAME, Reader, Wire, on

@dataclass(frozen=True, slots=True)
class BlockDelta(Wire):
    text: str = ""

@dataclass(frozen=True, slots=True)
class ContentBlockDelta(Wire, name="content_block_delta"):
    delta: BlockDelta = field(default_factory=BlockDelta)

class Messages(Reader[str], by=EVENT_NAME):
    @on(ContentBlockDelta)
    def _delta(self, wire: ContentBlockDelta) -> Iterator[str]:
        yield wire.delta.text

async def chunks():
    yield b'event: content_block_delta\ndata: {"delta":{"text":"Hi"}}\n\n'
    yield b"event: ping\ndata: {}\n\n"

async def main() -> None:
    assert [made async for made in Messages().over(chunks())] == ["Hi"]

asyncio.run(main())

Payload — reading by path

Payload is a dict, so payload["x"], in and json.dumps all still work. The four readers exist so a handler carries no Any and no chain of .get({}). Each walks the path and gives the default wherever a step is missing, null, or the wrong type. That is what an optional provider field is.

from axio_sse import Payload

payload = Payload({"message": {"usage": {"input_tokens": 7}}, "output": [{"type": "function_call"}]})

assert payload.number("message", "usage", "input_tokens") == 7
assert payload.number("message", "usage", "output_tokens") == 0
assert payload.number("message", "usage", "output_tokens", default=3) == 3
assert payload.string("message", "role") == ""
assert payload.obj("message", "usage") == {"input_tokens": 7}
assert payload.objs("output") == [{"type": "function_call"}]
assert payload.objs("nothing") == []

number() never reads a true as 1. bool is an int in Python, so a flag would otherwise read as a count and stay unnoticed:

assert Payload({"flag": True}).number("flag") == 0

License

MIT