Cache Penetration Null Object Mock in Python

import time
from collections import defaultdict
from typing import Any, Optional


class Cache:
    def __init__(self):
        self.store: dict[str, Any] = {}
        self.ttl: dict[str, float] = {}
        self.null_marker = object()

    def get(self, key: str, ttl: int = 60, fallback:
            Any = None) -> Any:
        now = time.time()
        if key in self.ttl and now > self.ttl[key]:
            self.store.pop(key, None)
            self.ttl.pop(key, None)

        if key in self.store:
            return self.store[key]

        # Simulate cache penetration: miss returns None
        if fallback is not None:
            # Store null object to avoid repeated DB hits
            self.store[key] = self.null_marker
            self.ttl[key] = now + ttl
            return self.store[key]
        return None

    def set(self, key: str, value: Any, ttl: int = 60) -> None:
        self.store[key] = value
        self.ttl[key] = time.time() + ttl


if __name__ == "__main__":
    cache = Cache()
    # Miss for "user:1", use fallback -> store null object
    result1 = cache.get("user:1", fallback=None)
    # Subsequent hit: matches null object, but we treat as miss
    result2 = cache.get("user:1")
    # Real value set
    cache.set("user:1", {"name": "Alice"})
    result3 = cache.get("user:1")

    print(f"First call: {result1}")
    print(f"Second call (should be null object): {result2 is cache.null_marker}")
    print(f"After set: {result3}")