def combine_cols(cols):
    """Combine column names into one list of names.

    Args:
        cols (list{str | list{str}}):
            A list of names of columns or list of column names.

    Return:
        list{str}:
            Combined names of columns.
    """
    combined_cols = []
    for col in cols:
        if isinstance(col, str):
            combined_cols.append(col)
        else:
            combined_cols += col

    return combined_cols