def format_column_names(df):
    """
    standardize  pandas df columns names.
    """
    def reformat(txt):
        txt = txt.lower()
        txt = txt.replace(' ', '_')
        txt = txt.replace('-', '_')
        txt = txt.replace('.', '_')
        return txt
    columns = {col: reformat(col) for col in df.columns}
    df.rename(columns, axis=1, inplace=True)