Python 最近最少使用算法 LRUCache

# lrucache.py -- a simple LRU (Least-Recently-Used) cache class 

002

003	# Copyright 2004 Evan Prodromou <evan@bad.dynu.ca>

004	# Licensed under the Academic Free License 2.1

005

006	# Licensed for ftputil under the revised BSD license

007	# with permission by the author, Evan Prodromou. Many

008	# thanks, Evan! :-)

009

#

010	# The original file is available at

011	# http://pypi.python.org/pypi/lrucache/0.2 .

012

013	# arch-tag: LRU cache main module

014

015	"""a simple LRU (Least-Recently-Used) cache module

016

017	This module provides very simple LRU (Least-Recently-Used) cache

018	functionality.

019

020	An *in-memory cache* is useful for storing the results of an

021	'expe\nsive' process (one that takes a lot of time or resources) for

022	later re-use. Typical examples are accessing data from the filesystem,

023	a database, or a network location. If you know you'll need to re-read

024	the data again, it can help to keep it in a cache.

025

026	You *can* use a Python dictionary as a cache for some purposes.

027	However, if the results you're caching are large, or you have a lot of

028	possible results, this can be impractical memory-wise.

029

030	An *LRU cache*, on the other hand, only keeps _some_ of the results in

031	memory, which keeps you from overusing resources. The cache is bounded

032	by a maximum size; if you try to add more values to the cache, it will

033	automatically discard the values that you haven't read or written to

034	in the longest time. In other words, the least-recently-used items are

035	discarded. [1]_

036

037	.. [1]: 'Discarded' here means 'removed from the cache'.

038

039

"""

040

041	from __future__ import generators

042	import time

043	from heapq import heappush, heappop, heapify

044

045	# the suffix after the hyphen denotes modifications by the

046	#  ftputil project with respect to the original version

047	__version__ = "0.2-1"

048	__all__ = ['CacheKeyError', 'LRUCache', 'DEFAULT_SIZE']

049	__docformat__ = 'reStructuredText en'

050

051	DEFAULT_SIZE = 16

052	"""Default size of a new LRUCache object, if no 'size' argument is given."""

053

054	class CacheKeyError(KeyError):

055	"""Error raised when cache requests fail

056

057	When a cache record is accessed which no longer exists (or never did),

058	this error is raised. To avoid it, you may want to check for the existence

059	of a cache record before reading or deleting it."""

060

pass

061

062	class LRUCache(object):

063	"""Least-Recently-Used (LRU) cache.

064

065	Instances of this class provide a least-recently-used (LRU) cache. They

066	emulate a Python mapping type. You can use an LRU cache more or less like

067	a Python dictionary, with the exception that objects you put into the

068	cache may be discarded before you take them out.

069

070	Some example usage::

071

072	cache = LRUCache(32) # new cache

073	cache['foo'] = get_file_contents('foo') # or whatever

074

075	if 'foo' in cache: # if it's still in cache...

076	# use cached version

077	contents = cache['foo']

078

else:

079	# recalculate

080	contents = get_file_contents('foo')

081	# store in cache for next time

082	cache['foo'] = contents

083

084	print cache.size # Maximum size

085

086	print len(cache) # 0 <= len(cache) <= cache.size

087

088	cache.size = 10 # Auto-shrink on size assignment

089

090	for i in range(50): # note: larger than cache size

091	cache[i] = i

092

093	if 0 not in cache: print 'Zero was discarded.'

094

095	if 42 in cache:

096	del cache[42] # Manual deletion

097

098	for j in cache:   # iterate (in LRU order)

099	print j, cache[j] # iterator produces keys, not values

100

"""

101

102	class __Node(object):

103	"""Record of a cached value. Not for public consumption."""

104

105	def __init__(self, key, obj, timestamp, sort_key):

106	object.__init__(self)

107	self.key = key

108	self.obj = obj

109	self.atime = timestamp

110	self.mtime = self.atime

111	self._sort_key = sort_key

112

113	def __cmp__(self, other):

114	return cmp(self._sort_key, other._sort_key)

115

116	def __repr__(self):

117	return "<%s %s => %s (%s)>" % \

118	(self.__class__, self.key, self.obj, \

119	time.asctime(time.localtime(self.atime)))

120

121	def __init__(self, size=DEFAULT_SIZE):

122	# Check arguments

123	if size <= 0:

124	raise ValueError, size

125	elif type(size) is not type(0):

126	raise TypeError, size

127	object.__init__(self)

128	self.__heap = []

129	self.__dict = {}

130	"""Maximum size of the cache.

131	If more than 'size' elements are added to the cache,

132	the least-recently-used ones will be discarded."""

133	self.size = size

134	self.__counter = 0

135

136	def _sort_key(self):

137	"""Return a new integer value upon every call.

138

139	Cache nodes need a monotonically increasing time indicator.

140	time.time() and time.clock() don't guarantee this in a

141	platform-independent way.

142

"""

143	self.__counter += 1

144	return self.__counter

145

146	def __len__(self):

147	return len(self.__heap)

148

149	def __contains__(self, key):

150	return self.__dict.has_key(key)

151

152	def __setitem__(self, key, obj):

153	if self.__dict.has_key(key):

154	node = self.__dict[key]

155	# update node object in-place

156	node.obj = obj

157	node.atime = time.time()

158	node.mtime = node.atime

159	node._sort_key = self._sort_key()

160	heapify(self.__heap)

161

else:

162	# size may have been reset, so we loop

163	while len(self.__heap) >= self.size:

164	lru = heappop(self.__heap)

165	del self.__dict[lru.key]

166	node = self.__Node(key, obj, time.time(), self._sort_key())

167	self.__dict[key] = node

168	heappush(self.__heap, node)

169

170	def __getitem__(self, key):

171	if not self.__dict.has_key(key):

172	raise CacheKeyError(key)

173

else:

174	node = self.__dict[key]

175	# update node object in-place

176	node.atime = time.time()

177	node._sort_key = self._sort_key()

178	heapify(self.__heap)

179	return node.obj

180

181	def __delitem__(self, key):

182	if not self.__dict.has_key(key):

183	raise CacheKeyError(key)

184

else:

185	node = self.__dict[key]

186	del self.__dict[key]

187	self.__heap.remove(node)

188	heapify(self.__heap)

189	return node.obj

190

191	def __iter__(self):

192	copy = self.__heap[:]

193	while len(copy) > 0:

194	node = heappop(copy)

195	yield node.key

196	raise StopIteration

197

198	def __setattr__(self, name, value):

199	object.__setattr__(self, name, value)

200	# automagically shrink heap on resize

201	if name == 'size':

202	while len(self.__heap) > value:

203	lru = heappop(self.__heap)

204	del self.__dict[lru.key]

205

206	def __repr__(self):

207	return "<%s (%d elements)>" % (str(self.__class__), len(self.__heap))

208

209	def mtime(self, key):

210	"""Return the last modification time for the cache record with key.

211	May be useful for cache instances where the stored values can get

212	'stale', such as caching file or network resource contents."""

213	if not self.__dict.has_key(key):

214	raise CacheKeyError(key)

215

else:

216	node = self.__dict[key]

217	return node.mtime

218

219	if __name__ == "__main__":

220	cache = LRUCache(25)

221	print cache

222	for i in range(50):

223	cache[i] = str(i)

224	print cache

225	if 46 in cache:

226	print "46 in cache"

227	del cache[46]

228	print cache

229	cache.size = 10

230	print cache

231	cache[46] = '46'

232	print cache

233	print len(cache)

234	for c in cache:

235

print c

236	print cache

237	print cache.mtime(46)

238	for c in cache:

239

print c