One-Shot Property?

[Scott David Daniels]

Leif K-Brooks wrote:
> 
> class CachingProperty(object):
>     def __init__(self, attr_name, calculate_function):
>         self._name = attr_name
>         self._calculate = calculate_function
>     def __get__(self, obj, type=None):
>         if obj is None:
>             return self
>         else:
>             value = self._calculate(obj)
>         setattr(obj, self._name, value)
>         return value
> 
> And example code:
>  >>> class Foo(object):
> ...     def calculate_value(self):
> ...         print 'Calculating...'
> ...         return 42
> ...     foo = CachingProperty('foo', calculate_value)
> ...
>  >>> bar = Foo()
>  >>> bar.__dict__
> {}
>  >>> bar.foo
> Calculating...
> 42
>  >>> bar.foo # Notice that the print statement doesn't run this time
> 42
>  >>> bar.__dict__
> {'foo': 42}


To build on this for Python 2.4:

class Caches(object):
     def __init__(self, calculate_function):
         self._calculate = calculate_function

     def __get__(self, obj, _=None):
         if obj is None:
             return self
         value = self._calculate(obj)
         setattr(obj, self._calculate.func_name, value)
         return value


class Foo(object):
     @Caches
     def foo(self):
         print 'Calculating...'
         return 42

--Scott David Daniels
Scott.Daniels at Acm.Org