Build a Caching ProxyStep 2 of 5

Add Caching

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Step 2: Add Caching

A pass-through proxy is boring. The point of a proxy is to add behavior without changing the interface. The classic addition is a cache: remember each key's result so a repeat request skips the expensive service entirely.

Give the proxy a dictionary. On fetch, if the key is already cached, return the stored value and do not touch the real service. Otherwise call the service once, store the result, and return it.

  • Proxy.fetch(key). Return the cached value if present, else fetch once, cache, and return

The tell-tale sign it works: fetching the same key twice leaves the service's call counter at 1.

Hints
Starter code
class RealService:
    def __init__(self, backend):
        self.backend = dict(backend)
        self.calls = 0

    def fetch(self, key):
        self.calls += 1
        return self.backend.get(key)


class Proxy:
    def __init__(self, real):
        self.real = real
        self._cache = {}

    def fetch(self, key):
        """Return the cached value if present, else fetch once and cache it."""
        # Step 1 behavior: delegate to the real service.
        # TODO: check self._cache first; only call the service on a miss, then store the result.
        return self.real.fetch(key)
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