Proxy Pattern Exercise

You are asked to implement a Proxy for a slow video service. The proxy must:

• Lazy-load the real service (do not create it until actually needed).
• Cache compressed downloads per (video_id, quality) so repeated requests are instant and do not hit the real service again.

Scenario

RealVideoService is slow to start up and slow to compress videos for download. We provide a proxy interface placeholder in proxy.py that you must implement.

The tests in tests/test_proxy.py verify:

1. The proxy does not construct the real service until a method is actually called.
2. The proxy caches compressed results and avoids calling the real service again for the same (video_id, quality).
3. Different qualities are not mixed up in the cache.
4. The proxy can still delegate to the real service when the cache misses.

The real service uses time.sleep to simulate slowness. Tests monkeypatch it so the suite runs fast.

Files

• video_service.py — contains the RealVideoService used by the proxy and a small interface note.
• proxy.py — YOU IMPLEMENT THIS. Follow the TODOs.
• tests/test_proxy.py — pytest tests that validate your solution.

How to run

python -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt
pytest -q

What to implement

In proxy.py, implement ProxyVideoService such that:

• It accepts a factory callable service_factory that constructs the real service only when needed.
• download_compressed(video_id: str, quality: str) -> bytes:
  • On first request for a given (video_id, quality), construct the real service (if not already created), delegate to it, store the result in an internal cache, and return it.
  • On subsequent requests for the same pair, return the cached bytes without calling the real service again.

Extra goals

• Add a maxsize to the cache (LRU).
• Add simple metrics: cache hits/misses.