Boolean tests [was Re: Attack a sacred Python Cow]
tjreedy at udel.edu
Wed Jul 30 05:17:49 CEST 2008
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):
def __init__(self, iterable):
self.it = iter(iterable)
tem = self.next
if tem is self.empty:
empty = object()
self.next = next(self.it)
self.next = self.empty
return self.next is not self.empty
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