In article <[EMAIL PROTECTED]>, [EMAIL PROTECTED] wrote: > Empty Python lists [] don't know the type of the items it will > contain, so this sounds strange: > > >>> sum([]) > 0 > > Because that [] may be an empty sequence of someobject: > > >>> sum(s for s in ["a", "b"] if len(s) > 2) > 0 > > In a statically typed language in that situation you may answer the > initializer value of the type of the items of the list, as I do in the > sum() in D. > > This sounds like a more correct/clean thing to do: > > >>> max([]) > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > ValueError: max() arg is an empty sequence > > So it may be better to make the sum([]) too raise a ValueError,
I don't see why you feel the two should act the same. At least in mathematics, the sum of the elements of the empty set _is_ 0, while the maximum element of the empty set is undefined. And both for good reason: (i) If A and B are disjoint sets we certainly want to have sum(A union B) = sum(A) + sum(B). This requires sum(empty set) = 0. (ii) If A is a subset of B then we should have max(A) <= max(B). This requires that max(empty set) be something that's smaller than everything else. So we give up on that. > in > Python 3/3.1 (if this isn't already true). On the other hand often > enough I have code like this: > > >>> max(fun(x) for x in iterable if predicate(x)) > > This may raise the ValueError both if iterable is empty of if the > predicate on its items is always false, so instead of catching > exceptions, that I try to avoid, I usually end with a normal loop, > that's readable and fast: > > max_value = smallvalue > for x in iterable: > if predicate(x): > max_value = max(max_value, fun(x)) > > Where running speed matters, I may even replace that max(max_value, > fun(x)) with a more normal if/else. > > A possible alternative is to add a default to max(), like the next() > built-in of Python 2.6: > > >>> max((fun(x) for x in iterable if predicate(x)), default=smallvalue) > > This returns smallvalue if there are no items to compute the max of. > > Bye, > bearophile -- David C. Ullrich -- http://mail.python.org/mailman/listinfo/python-list
