How to Mock a Redis Session Store in Python

import time
import json
from collections import defaultdict


class RedisSessionStore:
    """In-memory mock of a Redis-backed session store."""

    def __init__(self, ttl=3600):
        self._data = defaultdict(dict)
        self._expires = {}
        self._ttl = ttl

    def set(self, session_id, field, value):
        if session_id not in self._data:
            self._expires[session_id] = time.time() + self._ttl
        self._data[session_id][field] = value

    def get(self, session_id, field):
        self._evict_expired()
        if session_id in self._data:
            return self._data[session_id].get(field)
        return None

    def delete(self, session_id):
        self._data.pop(session_id, None)
        self._expires.pop(session_id, None)

    def _evict_expired(self):
        now = time.time()
        for sid, expiry in list(self._expires.items()):
            if expiry <= now:
                self.delete(sid)

    def exists(self, session_id):
        self._evict_expired()
        return session_id in self._data


if __name__ == "__main__":
    store = RedisSessionStore(ttl=2)

    store.set("user:1", "name", "Alice")
    store.set("user:1", "cart", json.dumps(["item1", "item2"]))
    store.set("user:2", "name", "Bob")

    print("Before expiry:", store.get("user:1", "name"))
    print("Exists user:1:", store.exists("user:1"))
    print("All fields user:1:", store._data["user:1"])

    time.sleep(3)
    print("After 3s exist user:1:", store.exists("user:1"))
    print("After expiry get:", store.get("user:1", "name"))

    store.delete("user:2")
    print("After delete exists user:2:", store.exists("user:2"))