Memoization and encapsulation

[Mike Meyer]

drewp at bigasterisk.com writes:
> class HashableList(list):
>     def __hash__(self):
>         return 0

This will suck for performance if you put a lot of lists in the same
dictionary. Faster is:

class FastHashableList(list):
    def __hash__(self):
        return id(self)

> I think python is broken here-- why aren't lists hashable, or why isn't
> there a straightforward way to make memoised() work?

There isn't a straightforward way to make memoised work because -
well, it's not a straightforward thing to make work.

For instance, consider the two hashable list implementations
above. Consider this:

@memoised
def broken(l):
    return len(l) + 1

x = HL([1, 2, 3])	# Or FHL
print broken(x)
x.append(4)
print broken(x)

Will your memoised handle this case correctly?

Which is also why lists aren't hashable - there's no good definition
of the hash of a list that isn't broken for some common use case.

   <mike
-- 
Mike Meyer <mwm at mired.org>			http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.