A small memoizing cache for arbitrary callables has a broken key function that mishandles positional versus keyword arguments and defaults. Fix the keying first, then extend the cache so entries survive a process restart and recover correctly. Level 1: fix the keying so every spelling of the same call shares one entry, different calls never do, and falsy results are cache hits. Level 2: make the cache survive a restart, and recover cleanly if the process died in the middle of a write.

Fix a function-cache key function, then make it durable

A small memoizing cache for arbitrary callables has a broken key function that mishandles positional versus keyword arguments and defaults. Fix the keying first, then extend the cache so entries survive a process restart and recover correctly.

Level 1: fix the keying so every spelling of the same call shares one entry, different calls never do, and falsy results are cache hits.
Level 2: make the cache survive a restart, and recover cleanly if the process died in the middle of a write.

import functools


def make_key(fn, args, kwargs, version="1", signature=None):
    # Buggy: find every way this gets "the same call" wrong.
    return hash((args, tuple(kwargs.items())))


class Cache:
    def __init__(self, path=None, fsync=True):
        self.data = {}

    def get(self, key):
        return self.data.get(key)

    def put(self, key, value):
        self.data[key] = value


def cached(cache=None, version="1"):
    def decorate(fn):
        store = cache if cache is not None else Cache()

        @functools.wraps(fn)
        def wrapper(*args, **kwargs):
            key = make_key(fn, args, kwargs, version)
            if store.get(key):
                return store.get(key)
            value = fn(*args, **kwargs)
            store.put(key, value)
            return value

        wrapper.cache = store
        return wrapper
    return decorate

Fix a function-cache key function, then make it durable

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