On Jul 29, 9:52 pm, Carl Banks <[EMAIL PROTECTED]> wrote: > On Jul 29, 11:17 pm, Terry Reedy <[EMAIL PROTECTED]> wrote: > > > > > Carl Banks wrote: > > >> As I wrote in the second reply email I sent, check out my integer set > > >> recipe on ASPN (and to save you the search: > > >> http://code.activestate.com/recipes/466286/). > > > > Couple points: > > > > 1. Any container type that returns a length that isn't exactly the > > > number of elements in it is broken. > > > 2. The need for __nonzero__ in this case depends on a limitation in > > > the language. > > > 3. On the other hand, I will concede that sometimes calculating len is > > > a lot more expensive than determining emptiness, and at a basic level > > > it's important to avoid these costs. You have found a practical use > > > case for __nonzero__. > > > I thought of another one: testing whether an iterator is 'empty' (will > > raise StopIteration on the next next() (3.0) call) or not. As virtual > > collections, iterators generally have neither __len__ or __bool__. But > > __bool__ (but only __bool__) can be added to any iterator by wrapping it > > with something like the following 3.0 code (not tested): > > > class look_ahead_it(): > > def __init__(self, iterable): > > self.it = iter(iterable) > > self.fill_next() > > > def __iter__(self): > > return self > > def __next__(self): > > tem = self.next > > if tem is self.empty: > > raise StopIteration > > else: > > self.fill_next() > > return tem > > > empty = object() > > def fill_next(self) > > try: > > self.next = next(self.it) > > except StopIteration: > > self.next = self.empty > > > def __bool__(self): > > return self.next is not self.empty > > Iterators are funny: if there's any reason you should not use "if x" > it's because of them. Built-in iterators are always true, so if > you're writing a function that accepts an iterable you should never > use the "if x" to test whether it's empty, because it fails for a > whole class of iterables. > > However, given that that wart exists, your example does work for "if > x" and not with "if len(x)!=0". > > Then again, it really only works to accommodate faulty code, because > no code that expects an iterable should be using that test in the > first place. (Unless you wrap every iterable as soon as you get it, > but then you're not bound to use __nonzero__.) > > Carl Banks
All this discussion about "if x" has me wondering. I use it all the time because a fully explicit test just seems redundant. But maybe it does have some value in terms of readability and type checking. After all, it is possible to forget whether something is a number or a list. Having said that, it would sure be nice to be able to write if myList is not empty: instead of if len(myList) != 0: -- http://mail.python.org/mailman/listinfo/python-list
