def next_attempt(self):
        "Return sequence of chunks to retry"
        if self.size == self.UNIT: # a single sector failed
             yield self # try again in the next phase
        else:
            for ix in xrange(self.offset, self.offset+self.size, self.UNIT):
                yield self.__class__(ix, self.UNIT)
Return sequence of chunks to retry
recipes/Python/496839_Recover_files_from_damaged_media/recipe-496839.py