On Tue, 22 Mar 2016 04:48 am, Ian Kelly wrote:
> You don't actually need a metaclass for this:
>
>>>> class Desc:
> ... def __get__(self, obj, type=None):
> ... if not type._cached_value:
> ... type._cached_value = compute_value(type)
> ... return type._cached_value
This won't quite work. What if the cached value happens to be falsey, yet
still expensive to compute? You should compare it to a known sentinel which
the expensive function will never return (possibly None). Or use a hasattr
test: if not hasattr(type, '_cached_value').
But my favourite is to combine them:
class Desc:
def __get__(self, obj, type):
sentinel = object() # guaranteed to be unique
value = getattr(type, _cached_value, sentinel)
if value is sentinel:
value = type._cached_value = compute_value(type)
return value
Also, you don't need the default type=None. The descriptor protocol should
never call __get__ without supplying the type. The obj may be None, but I
don't believe there are any circumstances where type will be None.
Which is good, because if it is:
AttributeError: 'NoneType' object has no attribute '_cached_value'
--
Steven
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