'Loads the credentials from the configured file.
@return: An iterable of C{username, password} couples.
@rtype: C{iterable}
@raise UnauthorizedLogin: when failing to read the credentials from the
file.'

def _loadCredentials(self):

    try:
        with open(self.filename, 'rb') as f:
            for line in f:
                line = line.rstrip()
                parts = line.split(self.delim)
                if ((self.ufield >= len(parts)) or (self.pfield >= len(parts))):
                    continue
                if self.caseSensitive:
                    (yield (parts[self.ufield], parts[self.pfield]))
                else:
                    (yield (parts[self.ufield].lower(), parts[self.pfield]))
    except IOError as e:
        self._log.error('Unable    to    load    credentials    db:    {e!r}', e=e)
        raise error.UnauthorizedLogin()
