I would like to propose adding a class to the stdlib to provide a more flexible
LRU caching mechanism. As things stand, the functools.lru_cache() decorator is
fine for memoization of pure functions, but does not accommodate situations
where cache entries become stale and must be invalidated/updated (e.g.
filecmp.cmp(); cf. issue #11802).
I was thinking it would be a good idea to factor out the the replacement code
into a separate class that could then be reused by code for which the
lru_cache() decorator is not applicable. The implementation of lru_cache()
itself would also become considerably simpler.
Implementation:
class LRUCache:
"""Least-recently-used cache class.
See: http://en.wikipedia.org/wiki/Cache_algorithms#Least_Recently_Used
"""
def __init__(self, maxsize):
"""Initialize an LRUCache.
If *maxsize* is None, the LRU replacement mechanism is disabled,
and the cache can grow without bound.
"""
if maxsize is None:
self.cache = dict()
else:
self.cache = OrderedDict()
self.lock = Lock()
self.hits = self.misses = 0
self.maxsize = maxsize
def __getitem__(self, key):
with self.lock:
try:
value = self.cache[key]
except KeyError:
self.misses += 1
raise
else:
self.hits += 1
if self.maxsize is not None:
self.cache.move_to_end(key)
return value
def __setitem__(self, key, value):
with self.lock:
self.cache[key] = value
if self.maxsize is not None and len(self.cache) > self.maxsize:
self.cache.popitem(0)
def info(self):
"""Report cache statistics"""
with self.lock:
return _CacheInfo(self.hits, self.misses,
self.maxsize, len(self.cache))
I'm not sure where this class should go; it would be convenient to just stick
it in functools along with lru_cache(), but perhaps the collections package
would be more appropriate?
Cheers,
Nadeem
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