lazy properties?

[Andrea Crotti]

Seeing the wonderful "lazy val" in Scala I thought that I should try to 
get the following also in Python.
The problem is that I often have this pattern in my code:

class Sample:
     def __init__(self):
         self._var = None

     @property
     def var(self):
         if self._var is None:
             self._var = long_computation()
         else:
             return self._var


which is quite useful when you have some expensive attribute to compute 
that is not going to change.
I was trying to generalize it in a @lazy_property but my attempts so far 
failed, any help on how I could do that?

What I would like to write is
     @lazy_property
     def var_lazy(self):
         return long_computation()

and this should imply that the long_computation is called only once..