cached_property.py

   1 # -*- coding: utf-8 -*-
   2
   3 __author__ = 'Daniel Greenfeld'
   4 __email__ = 'pydanny@gmail.com'
   5 __version__ = '0.1.4'
   6 __license__ = 'BSD'
   7
   8 import time
   9
  10
  11 class cached_property(object):
  12     """ A property that is only computed once per instance and then replaces
  13         itself with an ordinary attribute. Deleting the attribute resets the
  14         property.
  15
  16         Source: https://github.com/bottlepy/bottle/commit/fa7733e075da0d790d809aa3d2f53071897e6f76
  17         """
  18
  19     def __init__(self, func):
  20         self.__doc__ = getattr(func, '__doc__')
  21         self.func = func
  22
  23     def __get__(self, obj, cls):
  24         if obj is None:
  25             return self
  26         value = obj.__dict__[self.func.__name__] = self.func(obj)
  27         return value
  28
  29
  30 class timed_cached_property(object):
  31     '''Decorator for read-only properties evaluated only once within TTL period.
  32
  33     It can be used to created a cached property like this::
  34
  35         import random
  36
  37         # the class containing the property must be a new-style class
  38         class MyClass(object):
  39             # create property whose value is cached for ten minutes
  40             @cached_property(ttl=600)
  41             def randint(self):
  42                 # will only be evaluated every 10 min. at maximum.
  43                 return random.randint(0, 100)
  44
  45     The value is cached  in the '_cache' attribute of the object instance that
  46     has the property getter method wrapped by this decorator. The '_cache'
  47     attribute value is a dictionary which has a key for every property of the
  48     object which is wrapped by this decorator. Each entry in the cache is
  49     created only when the property is accessed for the first time and is a
  50     two-element tuple with the last computed property value and the last time
  51     it was updated in seconds since the epoch.
  52
  53     The default time-to-live (TTL) is 0, which also means the cache never expires.
  54
  55     To expire a cached property value manually just do::
  56
  57         del instance._cache[<property name>]
  58
  59     © 2011 Christopher Arndt, MIT License
  60     source: https://wiki.python.org/moin/PythonDecoratorLibrary#Cached_Properties
  61
  62     '''
  63     def __init__(self, ttl=0):
  64         self.ttl = ttl
  65
  66     def __call__(self, fget, doc=None):
  67         self.fget = fget
  68         self.__doc__ = doc or fget.__doc__
  69         self.__name__ = fget.__name__
  70         self.__module__ = fget.__module__
  71         return self
  72
  73     def __get__(self, inst, owner):
  74         now = time.time()
  75         try:
  76             value, last_update = inst._cache[self.__name__]
  77             if self.ttl > 0 and now - last_update > self.ttl:
  78                 raise AttributeError
  79         except (KeyError, AttributeError):
  80             value = self.fget(inst)
  81             try:
  82                 cache = inst._cache
  83             except AttributeError:
  84                 cache = inst._cache = {}
  85             cache[self.__name__] = (value, now)
  86         return value