def index(self, keyword, start=None, stop=None):
        """
        Returns the index if the first instance of the given keyword in the
        header, similar to `list.index` if the Header object is treated as a
        list of keywords.

        Parameters
        ----------
        keyword : str
            The keyword to look up in the list of all keywords in the header

        start : int, optional
            The lower bound for the index

        stop : int, optional
            The upper bound for the index

        """

        if start is None:
            start = 0

        if stop is None:
            stop = len(self._cards)

        if start < stop:
            step = -1
        else:
            step = 1

        norm_keyword = Card.normalize_keyword(keyword)

        for idx in range(start, stop, step):
            if self._cards[idx].keyword.upper() == norm_keyword:
                return idx
        else:
            raise ValueError('The keyword %r is not in the header.' % keyword)
dataset/ETHPy150Open spacetelescope/PyFITS/pyfits/header.py/Header.index/swapped operands of `<`