lazy.py

class lazy(object):
    """
    Normal approach::

      >>> class Test(object):
      ...     @property
      ...     def attr(self):
      ...         print "attr property"
      ...         if not hasattr(self, '_attr'):
      ...             print "expensive load"
      ...             self._attr = 42
      ...         return self._attr
      >>> t = Test()
      >>> t.attr
      attr property
      expensive load
      42
      >>> t.attr
      attr property
      42
      >>> vars(t)
      {'_attr': 42}

    @lazy approach:

      >>> class Test(object):
              @lazy
              def attr(self):
                  print "expensive load"
                  return 42

      >>> t = Test()
      >>> vars(t)
      {}
      >>> t.attr
      expensive load
      42
      >>> vars(t)
      {'attr': 42}
      >>> t.attr
      42
    """

    __slots__ = ['setup']

    def __init__(self, setup):
        self.setup = setup

    def __get__(self, obj, type):
        if obj is None:
            return self
        s = self.setup(obj)
        setattr(obj, self.setup.__name__, s)
        return s