recognizing empty iterators
Michele Simionato
mis6 at pitt.edu
Mon Jul 21 10:26:15 EDT 2003
After a recent thread on .endswith, I have been thinking about iterators.
I realized that I don't know a satisfactory way to check if an
iterator is empty. In other words I am looking for an
"isempty" function to use in "if" statements such as
if isempty(iterator):
do_something()
without side effects. Here are some unsatisfactory ways of implementing
"isempty":
#1: converting to a list or tuple
def isempty(iterator):
return not list(iterator)
Easy, but one has to create the entire list, thus defecting the basic
purpose of the iterator, i.e. lazy evaluation.
#2: checking for StopIteration
def isempty(iterator):
try:
iterator.next()
except StopIteration:
return True
else:
return False
This works, for instance
print isempty(iter([]))
gives True and
it=iter([1,2,3])
print isempty(it)
gives False. However, there is a side effect: after the check, the
iterator has advanced of one step and now "it.next()" gives 2, not 1.
In order this to work without side effects, I should be able to restart
the iterator from the beginning, but I don't know how to do that.
Is it possible?
#3: comparing with the empty iterator
emptyiterator=iter([])
it=iter([])
if it == emptyiterator: print 'Ok!'
This simply doesn't work.
I wonder if the itertools module should contain a function to check for
empty iterators, thus simplifying my life ;) Of course, I may well be
missing something obvious, if so, please enlighten me.
Michele
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