for: else: - any practical uses for the else clause?

[Johan Steyn]

On 29 Sep 2006 11:26:10 -0700, Klaas <mike.klaas at gmail.com> wrote:

> else: does not trigger when there is no data on which to iterate, but
> when the loop terminated normally (ie., wasn't break-ed out).  It is
> meaningless without break.

  The else clause *is* executed when there is no data on which to iterate.
Your example even demonstrates that clearly:

 >>> for x in []:
> ...     print 'nothing'
> ... else:
> ...     print 'done'
> ...
> done


The else clause is executed because the loop still terminates normally with
an empty list - albeit without having looped at all.

I agree that it is meaningless without a break statement, but I still find
it useful when I want to determine whether I looped over the whole list or
not. For example, if I want to see whether or not a list contains an odd
number:

for i in list:
    if i % 2 == 1:
        print "Found an odd number."
        break
else:
    print "No odd number found."

Without the else clause I would need to use an extra variable as a "flag"
and check its value outside the loop:

found = False
for i in list:
    if i % 2 == 1:
        print "Found an odd number."
        found = True
        break
if not found:
    print "No odd number found."

OK, so using "else" only saves me 2 lines and a variable - not much to write
home about, but I still like it.

Johan.
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