sse-starlette 3.4.8: EventSourceResponse server-sent events stream in Starlette

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: EventSourceResponse server-sent events stream in Starlette
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:pypi/sse-starlette@3.4.8
Demonstrate these symbols/APIs:
  - sse_starlette.sse.EventSourceResponse

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:cd0f9f883d039ac44ec50e5474f069db4ebd5ae4865a0734b8a2d9db6abd7bfa","contract":["assert EventSourceResponse sets media_type to text/event-stream with no-store and keep-alive headers","assert EventSourceResponse streams string and dictionary payloads formatted as SSE events","assert EventSourceResponse streams ServerSentEvent instances with custom event, id, and retry fields","assert ServerSentEvent encodes comment lines, multiline data, and custom separators into bytes","assert EventSourceResponse configures custom ping_interval and exposes default constants"],"goal":"EventSourceResponse server-sent events stream in Starlette","kind":"HOW","packages":["pkg:pypi/sse-starlette@3.4.8"],"schemaVersion":1,"symbols":["sse_starlette.sse.EventSourceResponse"]},"contractCommand":["python3","test/contract.py"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"pypi","language":"python","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"pip","packageManagerVersion":"24.0","runtime":"python","runtimeVersion":"3.12","schemaVersion":1},"license":"MIT-0","packages":["pkg:pypi/sse-starlette@3.4.8"],"schemaVersion":1,"subject":"pkg:pypi/sse-starlette@3.4.8","symbols":["sse_starlette.sse.EventSourceResponse"],"verifierAdapter":"python@1"}

[project]
name = "verify-sse-starlette"
version = "0.1.0"
dependencies = [
    "sse-starlette==3.4.8",
    "starlette>=0.37.0",
    "anyio>=4.0.0",
    "sniffio>=1.3.0",
    "typing-extensions>=4.8.0",
    "idna>=3.0",
    "httpx>=0.27.0",
    "httpcore>=1.0.0",
    "certifi",
    "h11>=0.14.0",
]

sse-starlette==3.4.8
starlette>=0.37.0
anyio>=4.0.0
sniffio>=1.3.0
typing-extensions>=4.8.0
idna>=3.0
httpx>=0.27.0
httpcore>=1.0.0
certifi
h11>=0.14.0

{
  "schemaVersion": 1,
  "goal": "EventSourceResponse server-sent events stream in Starlette",
  "kind": "HOW",
  "packages": [
    "pkg:pypi/sse-starlette@3.4.8"
  ],
  "symbols": [
    "sse_starlette.sse.EventSourceResponse"
  ]
}

import asyncio
from sse_starlette.sse import EventSourceResponse, ServerSentEvent
from starlette.applications import Starlette
from starlette.routing import Route
from starlette.testclient import TestClient


def test_event_source_response_headers():
    response = EventSourceResponse(None, status_code=200, headers={"X-Custom-Header": "SSE-Stream"})
    assert response.media_type == "text/event-stream"
    assert response.status_code == 200
    assert response.headers["content-type"] == "text/event-stream; charset=utf-8"
    assert response.headers["cache-control"] == "no-store"
    assert response.headers["connection"] == "keep-alive"
    assert response.headers["x-custom-header"] == "SSE-Stream"
    assert response.headers["x-accel-buffering"] == "no"


def test_event_source_response_stream_payloads():
    async def endpoint(request):
        async def event_generator():
            yield "simple-text"
            yield {"data": "json-val", "event": "update", "id": "msg-1"}
            yield ServerSentEvent(data="sse-data", event="custom", id="msg-2", retry=3000)
        return EventSourceResponse(event_generator(), ping=None)

    app = Starlette(routes=[Route("/events", endpoint)])
    client = TestClient(app)
    res = client.get("/events")

    assert res.status_code == 200
    assert res.headers["content-type"] == "text/event-stream; charset=utf-8"
    assert "data: simple-text" in res.text
    assert "event: update" in res.text
    assert "data: json-val" in res.text
    assert "id: msg-1" in res.text
    assert "event: custom" in res.text
    assert "data: sse-data" in res.text
    assert "id: msg-2" in res.text
    assert "retry: 3000" in res.text


def test_serversentevent_encoding():
    s1 = ServerSentEvent(data="hello world")
    assert s1.encode() == b"data: hello world\r\n\r\n"

    s2 = ServerSentEvent(data="line1\nline2", event="alert", id="42", retry=5000, comment="heartbeat")
    encoded = s2.encode().decode("utf-8")
    assert ": heartbeat\r\n" in encoded
    assert "id: 42\r\n" in encoded
    assert "event: alert\r\n" in encoded
    assert "data: line1\r\ndata: line2\r\n" in encoded
    assert "retry: 5000\r\n" in encoded

    s3 = ServerSentEvent(data="custom-sep", sep="\n")
    assert s3.encode() == b"data: custom-sep\n\n"


def test_event_source_response_ping_and_defaults():
    resp = EventSourceResponse(None, ping=30)
    assert resp.ping_interval == 30
    assert resp.DEFAULT_PING_INTERVAL == 15
    assert resp.DEFAULT_SEPARATOR == "\r\n"


def main():
    test_event_source_response_headers()
    test_event_source_response_stream_payloads()
    test_serversentevent_encoding()
    test_event_source_response_ping_and_defaults()
    print("All contract assertions passed.")


if __name__ == "__main__":
    main()