#!/usr/bin/env python
# Copyright (c) Gribouillis at www.daniweb.com
 
from functools import update_wrapper 
 
def cachedProperty (func ,name =None ):
  """cachedProperty(func, name=None) -> a descriptor
This decorator implements an object's property which is computed
the first time it is accessed, and which value is then stored in
the object's __dict__ for later use. If the attribute is deleted,
the value will be recomputed the next time it is accessed.
  Usage:
    class X(object):
      @cachedProperty
      def foo(self):
        return computation()
"""
  if name is None :
    name =func .__name__ 
  def _get (self ):
    try :
      return self .__dict__ [name ]
    except KeyError :
      value =func (self )
      self .__dict__ [name ]=value 
      return value 
  update_wrapper (_get ,func )
  def _del (self ):
    self .__dict__ .pop (name ,None )
  return property (_get ,None ,_del )
 
if __name__ =="__main__":
# test code for cachedProperty
  import time 
 
  class X (object ):
    @cachedProperty 
    def foo (self ):
      print ("--> foo was called")
      return time .asctime ()
 
  def dictContent (instance ):
    return list (sorted (instance .__dict__ ))
 
  x =X ()
  print (dictContent (x ))# nothing in x.__dict__
  print (x .foo )# the attribute is computed and stored in x.__dict__
  time .sleep (2 )
  print (dictContent (x ))# x.__dict__ has a key 'foo'
  print (x .foo )# the cached value is used
  print (x .foo )# the cached value is used
  del x .foo # the cached value is deleted
  print (dictContent (x ))# x.__dict__ doesn't have a 'foo'
  print (x .foo )# the attribute is recomputed and stored
  print (dictContent (x ))# x.__dict__ has a 'foo'

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