用于FTP发送/接收软件的while循环中的Try/except块

import ftplib

def send_file_ftp(filename, server, username, password):
    retry_count = 3
    success = False

    while retry_count > 0 and not success:
        try:
            ftp = ftplib.FTP(server)
            ftp.login(username, password)
            with open(filename, 'rb') as file:
                ftp.storbinary('STOR ' + filename, file)
            ftp.quit()
            success = True
        except ftplib.all_errors as e:
            print('An error occurred:', str(e))
            retry_count -= 1

    if success:
        print('File', filename, 'sent successfully.')
    else:
        print('Failed to send file', filename)

# 调用示例
send_file_ftp('example.txt', 'ftp.example.com', 'username', 'password')