On Sep 3, 7:48 am, [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, 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
Two thoughts: 1/ 'Reduce' has a 'default' argument-- they call it 'initial'. >>> reduce( max, [ 0, 1, 2, 3 ] ) 3 >>> reduce( max, [ 0, 1, 2, 'a' ] ) 'a' >>> reduce( max, [ 0, 1, 2, 'a', 'b' ] ) 'b' 2/ Introduce a 'max' class object that takes a default type or default argument. Query the default for an 'additive' identity, or query for a 'comparitive' identity, comparisons to which always return true; or call the constructor with no arguments to construct one. -- http://mail.python.org/mailman/listinfo/python-list
