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@durden
Created May 8, 2013 19:24
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Decorator for read-only properties evaluated only once within TTL period
#
# © 2011 Christopher Arndt, MIT License
#
# Taken from: http://wiki.python.org/moin/PythonDecoratorLibrary
import time
class cached_property(object):
'''Decorator for read-only properties evaluated only once within TTL period.
It can be used to created a cached property like this::
import random
# the class containing the property must be a new-style class
class MyClass(object):
# create property whose value is cached for ten minutes
@cached_property(ttl=600)
def randint(self):
# will only be evaluated every 10 min. at maximum.
return random.randint(0, 100)
The value is cached in the '_cache' attribute of the object instance that
has the property getter method wrapped by this decorator. The '_cache'
attribute value is a dictionary which has a key for every property of the
object which is wrapped by this decorator. Each entry in the cache is
created only when the property is accessed for the first time and is a
two-element tuple with the last computed property value and the last time
it was updated in seconds since the epoch.
The default time-to-live (TTL) is 300 seconds (5 minutes). Set the TTL to
zero for the cached value to never expire.
To expire a cached property value manually just do::
del instance._cache[<property name>]
'''
def __init__(self, ttl=300):
self.ttl = ttl
def __call__(self, fget, doc=None):
self.fget = fget
self.__doc__ = doc or fget.__doc__
self.__name__ = fget.__name__
self.__module__ = fget.__module__
return self
def __get__(self, inst, owner):
now = time.time()
try:
value, last_update = inst._cache[self.__name__]
if self.ttl > 0 and now - last_update > self.ttl:
raise AttributeError
except (KeyError, AttributeError):
value = self.fget(inst)
try:
cache = inst._cache
except AttributeError:
cache = inst._cache = {}
cache[self.__name__] = (value, now)
return value
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