python写入数据到csv中，头信息保证只写入一次

import csv
import codecs

with codecs.open(filename,'a+','utf-8') as csvfp:  
            # 指定csv文件的头部显示项
            global flag_only_one
            if(flag_only_one):
                file_headers = ["标题","网盘地址","提取码","解压密码"]
                writer = csv.DictWriter(csvfp, fieldnames= file_headers)
                writer.writeheader()
                flag_only_one = False
                
            writer = csv.writer(csvfp)
            # item list 这里的数据来源于其他代码 
            item = [
                    title[0],
                     bdwp_address[0],
                     bdwp_code[0][-4:],
                    unzip_psd[0]
                ]
            
            try:
                # 记录一下当前打印到哪了
                global cnt
                cnt = cnt + 1
                
                writer.writerow(item)
                print( str(cnt) + "." + str(title) + "--已录入-->" + filename)
            except UnicodeEncodeError:
                print("编码错误, 该数据无法写到文件中, 直接忽略该数据") 
                
if __name__ == '__main__':

    # flag标志位，只写入一次csv的headers
    flag_only_one = True
    cnt = 0