nan.0 Format a SQL string with a parameter

start_date  = '2019-01-01'
end_date   = '2021-01-01'

SQL_string = '''
        SELECT *
        FROM DF
        WHERE DF.Date >= {St_dt}
        AND   DF.Date <= {ed_dt}
                 '''.format(start_date = start_date, end_date = end_date)