copy first column to last colum of a csv file

treatcsv.py

def readCSV(filename):
    f = open(filename,"r")
    y = open('saida.csv',"w")
    for row in f:
        treat = row.strip("\n").strip("\r")+';\n'
        new_list = treat.split(';') #str to list
        new_list[-1] = new_list[0]
        new_string = ''
        for i in range(0,len(new_list)-1):
            new_string = new_string + new_list[i] + ';'
        new_string = new_string + new_list[-1] + '\n'
        y.write(new_string)
    f.close()
    y.close()