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
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