Redis GET SET EX TTL mock in Python

import time
import threading
from typing import Optional, Callable


class RedisTTLMock:
    def __init__(self):
        self._store: dict[str, tuple[str, float]] = {}
        self._lock = threading.Lock()

    def set(self, key: str, value: str, ex: Optional[int] = None) -> bool:
        expiry = time.time() + ex if ex is not None else float("inf")
        with self._lock:
            self._store[key] = (value, expiry)
        return True

    def get(self, key: str) -> Optional[str]:
        with self._lock:
            entry = self._store.get(key)
            if entry is None:
                return None
            value, expiry = entry
            if time.time() > expiry:
                del self._store[key]
                return None
            return value

    def ttl(self, key: str) -> int:
        with self._lock:
            entry = self._store.get(key)
            if entry is None:
                return -2
            _, expiry = entry
            remaining = expiry - time.time()
            if remaining <= 0:
                del self._store[key]
                return -2
            return int(remaining)


if __name__ == "__main__":
    r = RedisTTLMock()
    r.set("greeting", "hello", ex=2)
    print(r.get("greeting"))  # hello
    print(r.ttl("greeting"))  # 2
    time.sleep(3)
    print(r.get("greeting"))  # None
    print(r.ttl("greeting"))  # -2