I thought this to be a great exercise so I went the extra length to turn it into a function for my little but growing library. I hope you enjoy :)
def indexer(string, target): '''indexer(string, target) -> [list of target indexes] enter in a string and a target and indexer will either return a list of all targeted indexes if at least one target is found or indexer will return None if the target is not found in sequence. >>> indexer('a long long day is long', 'long') [2, 7, 19] >>> indexer('a long long day is long', 'day') [12] >>> indexer('a long long day is long', 'short') None ''' res = [] if string.count(target) >= 1: res.append(string.find(target)) if string.count(target) >= 2: for item in xrange(string.count(target) - 1): res.append(string.find(target, res[-1] + 1)) return res if __name__ == '__main__': print indexer('a long long day is long', 'long') # -> [2, 7, 19] print indexer('a long long day is long', 'day') # -> [12] print indexer('a long long day is long', 'short') # -> None -- http://mail.python.org/mailman/listinfo/python-list