cached_property.py

class cached_property:
    '''
    Decorator that converts a method with a single self argument into a
    property cached on the instance.

    A cached property can be made out of an existing method:
    (e.g. ``url = cached_property(get_absolute_url)``).
    '''
    # ...  
    def __get__(self, instance, cls=None):
      """
      Call the function and put the return value in instance.__dict__ so that
      subsequent attribute access on the instance returns the cached value
      instead of calling cached_property.__get__().
      """
      if instance is None:
          return self
      res = instance.__dict__[self.name] = self.func(instance)
      return res