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There is a difference between Python and C implementations of functools.lru_cache(). Python implementation caches the hash of the key, C implementation doesn't. May be this is not too important (at least I have no an example that shows the benefit of caching the hash), but there is a place for possible optimization. Proposed patch uses private functions _PyDict_GetItem_KnownHash() and _PyDict_SetItem_KnownHash() to avoid the second calculating of the hash for new keys.
The hash is still calculated second time when the key is deleted from full cache. To avoid this _PyDict_DelItem_KnownHash() is needed.