def set_index(df, index, append=False, inplace=True):
    """
    Turns one, or multiple dataframe columns into an index
    Arguments are the dataframe to be modified, as well as a column name
    or multiple column names in a list

    Optional arguments are;
    - append (default False, can be True or False) which determines whether
    to append the index to the existing index
    - inplace (default True, can be True or False) which determines whether
    to change the existing dataframe, or return a new dataframe

    Note that if append is not true, the original index will be dropped
    """
    df = df.set_index(index, append=append, inplace=inplace)
    return df