This is intended to be a drop-in replacement for np.argsort which
    handles NaNs. It adds ascending and na_position parameters.
    GH #6399, #5231

def nargsort(items, kind='quicksort', ascending=True, na_position='last'):
    """
    This is intended to be a drop-in replacement for np.argsort which
    handles NaNs. It adds ascending and na_position parameters.
    GH #6399, #5231
    """

    # specially handle Categorical
    if is_categorical_dtype(items):
        if na_position not in {'first', 'last'}:
            raise ValueError('invalid na_position: {!r}'.format(na_position))

        mask = isna(items)
        cnt_null = mask.sum()
        sorted_idx = items.argsort(ascending=ascending, kind=kind)
        if ascending and na_position == 'last':
            # NaN is coded as -1 and is listed in front after sorting
            sorted_idx = np.roll(sorted_idx, -cnt_null)
        elif not ascending and na_position == 'first':
            # NaN is coded as -1 and is listed in the end after sorting
            sorted_idx = np.roll(sorted_idx, cnt_null)
        return sorted_idx

    with warnings.catch_warnings():
        # https://github.com/pandas-dev/pandas/issues/25439
        # can be removed once ExtensionArrays are properly handled by nargsort
        warnings.filterwarnings(
            "ignore", category=FutureWarning,
            message="Converting timezone-aware DatetimeArray to")
        items = np.asanyarray(items)
    idx = np.arange(len(items))
    mask = isna(items)
    non_nans = items[~mask]
    non_nan_idx = idx[~mask]
    nan_idx = np.nonzero(mask)[0]
    if not ascending:
        non_nans = non_nans[::-1]
        non_nan_idx = non_nan_idx[::-1]
    indexer = non_nan_idx[non_nans.argsort(kind=kind)]
    if not ascending:
        indexer = indexer[::-1]
    # Finally, place the NaNs at the end or the beginning according to
    # na_position
    if na_position == 'last':
        indexer = np.concatenate([indexer, nan_idx])
    elif na_position == 'first':
        indexer = np.concatenate([nan_idx, indexer])
    else:
        raise ValueError('invalid na_position: {!r}'.format(na_position))
    return indexer