How to implement stale-while-revalidate caching in Python

import time
from functools import lru_cache


class CachedService:
    def __init__(self, fetch_func, ttl=5):
        self.fetch_func = fetch_func
        self.ttl = ttl
        self._cache = {}
        self._timestamp = {}

    def get(self, key):
        now = time.time()
        if key in self._cache and now - self._timestamp[key] < self.ttl:
            return self._cache[key], False
        try:
            value = self.fetch_func(key)
            if value is None:
                raise ValueError("fetch returned None")
        except Exception:
            if key in self._cache:
                return self._cache[key], True
            raise
        self._cache[key] = value
        self._timestamp[key] = now
        return value, False


if __name__ == "__main__":
    calls = []

    def flaky_fetch(key):
        calls.append(key)
        if len(calls) < 3:
            raise ConnectionError("temporary failure")
        return f"fresh-{key}"

    svc = CachedService(flaky_fetch, ttl=10)
    print(svc.get("a"))   # raises
    print(svc.get("a"))   # raises
    print(svc.get("a"))   # fresh result, cached
    print(svc.get("a"))   # cache hit, not stale