An obscure python loop feature

I just stumbled over one of python’s dirty little secrets while going through the PySNMP examples. SNMP is a very cool protocol because it’s blazing fast, but boy is it ugly to use. PySNMP stays true by also exposing a difficult to use API. Say hello to

from pysnmp.entity.rfc3413.oneliner import cmdgen

This somehow reminds me of the guidelines on how to write unmaintaineable code (laughter guaranteed). One of the examples featured a for loop followed up by an else clause:

for oid, val in varBindRow:
            if not val.isSameTypeWith(rfc1905.endOfMibView):
                break
else:
    reactor.stop()
    return

I first thought it was an indentation bug in the code, but it is actually valid python: > Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the list (with for) or when the condition becomes false (with while), but not when the loop is terminated by a break statement. This is exemplified by the following loop, which searches for prime numbers:

for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
           print n, 'equals', x, '*', n/x
           break
    else:
         print n, 'is a prime number'

You can avoid using state variables with it but I think it costs too much readability in algorithms where you should be extracting functions as much as possible.
In this case the code fails to reveal what x means. IMHO it would be better to make explicit that you are searching for the first nontrivial divisor, and a number is prime when it has no nontrivial divisors. This allows you to extract a function first_nontrivial_divisor(number) and replace dubious break-based flow control with a return statement.