SSE Client with Reconnect Parser

import json
import time
import urllib.error
import urllib.request
from collections.abc import Iterable, Iterator


def parse_sse(lines: Iterable[str]) -> Iterator[dict[str, str]]:
    event: dict[str, str] = {}
    data: list[str] = []
    for raw in lines:
        line = raw.rstrip("\r\n")
        if not line:
            if data:
                event["data"] = "\n".join(data)
                yield event
            event, data = {}, []
            continue
        if line.startswith(":"):
            continue
        field, _, value = line.partition(":")
        value = value.lstrip(" ")
        if field == "data":
            data.append(value)
        elif field:
            event[field] = value
    if data:
        event["data"] = "\n".join(data)
        yield event


def iter_sse(
    url: str, retries: int = 3, backoff: float = 0.5
) -> Iterator[dict[str, str]]:
    attempt = 0
    while True:
        try:
            request = urllib.request.Request(
                url, headers={"Accept": "text/event-stream"}
            )
            with urllib.request.urlopen(request, timeout=30) as response:
                yield from parse_sse(line.decode("utf-8") for line in response)
            return
        except (OSError, urllib.error.URLError):
            attempt += 1
            if attempt > retries:
                raise
            time.sleep(backoff * attempt)


FIXTURE = """event: build
id: 7
data: {"status": "ok"}
data: second line
"""


if __name__ == "__main__":
    events = list(parse_sse(FIXTURE.splitlines()))
    print(events)
    for event in events:
        first_line = event["data"].splitlines()[0]
        print(event.get("event", "message"), json.loads(first_line))