lazy.py

#coding: utf-8


# Property that is only computed once
def lazyproperty(f):
  @property
  def wrapper(self,*args,**kwargs):
    if not hasattr(self,'_'+f.__name__):
      setattr(self,'_'+f.__name__,f(self,*args,**kwargs))
    return getattr(self,'_'+f.__name__)
  return wrapper


if __name__ == '__main__':
  def mysum(l): # Example function to be called in my test getter
    print("Summing...")
    return sum(l)

  class Stuff():
    def __init__(self,data=[0,1,2]):
      self.data = data

    # s.sum will be computed on the first time we get it
    # but the same value will be returned on every other call
    # Simply delete self._sum to mark it as to be computed again
    @lazyproperty
    def sum(self):
      return mysum(self.data)

  s = Stuff()
  print("First call:")
  print("sum=",s.sum)
  print("second call:")
  print("sum=",s.sum)
  del s._sum
  print("Third call:")
  print("sum=",s.sum)