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"""Cache module, with a least recently used algorithm for the management of the
deletion of entries.
:copyright: 2002-2008 LOGILAB S.A. (Paris, FRANCE), all rights reserved.
:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""
__docformat__ = "restructuredtext en"
from threading import Lock
from logilab.common.decorators import locked
_marker = object()
class Cache(dict):
"""A dictionary like cache.
inv:
len(self._usage) <= self.size
len(self.data) <= self.size
"""
def __init__(self, size=100):
""" Warning : Cache.__init__() != dict.__init__().
Constructor does not take any arguments beside size.
"""
assert size >= 0, 'cache size must be >= 0 (0 meaning no caching)'
self.size = size
self._usage = []
self._lock = Lock()
super(Cache, self).__init__()
def _acquire(self):
self._lock.acquire()
def _release(self):
self._lock.release()
def _update_usage(self, key):
if not self._usage:
self._usage.append(key)
elif self._usage[-1] != key:
try:
self._usage.remove(key)
except ValueError:
# we are inserting a new key
# check the size of the dictionary
# and remove the oldest item in the cache
if self.size and len(self._usage) >= self.size:
super(Cache, self).__delitem__(self._usage[0])
del self._usage[0]
self._usage.append(key)
else:
pass # key is already the most recently used key
def __getitem__(self, key):
value = super(Cache, self).__getitem__(key)
self._update_usage(key)
return value
__getitem__ = locked(_acquire, _release)(__getitem__)
def __setitem__(self, key, item):
# Just make sure that size > 0 before inserting a new item in the cache
if self.size > 0:
super(Cache, self).__setitem__(key, item)
self._update_usage(key)
__setitem__ = locked(_acquire, _release)(__setitem__)
def __delitem__(self, key):
super(Cache, self).__delitem__(key)
self._usage.remove(key)
__delitem__ = locked(_acquire, _release)(__delitem__)
def clear(self):
super(Cache, self).clear()
self._usage = []
clear = locked(_acquire, _release)(clear)
def pop(self, key, default=_marker):
if key in self:
self._usage.remove(key)
#if default is _marker:
# return super(Cache, self).pop(key)
return super(Cache, self).pop(key, default)
pop = locked(_acquire, _release)(pop)
def popitem(self):
raise NotImplementedError()
def setdefault(self, key, default=None):
raise NotImplementedError()
def update(self, other):
raise NotImplementedError()
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