mail_info = {
   "from": "xxxxx@qq.com",
   "to": "xxxxxxxxxxxxxxx@qq.com",
   "hostname": "smtp.qq.com",
   "username": "aaaaaaaaaa@qq.com",
   "password": "wqszrkejsssrqfnbaci",
   "mail_subject": "test",
   "mail_text": "hello, this is a test email, sended by py",
   "mail_encoding": "utf-8"
}

w = max(map(len,mail_info.keys()))
print(w)

for k in mail_info:
   # print(k.ljust(w),':',mail_info[k])
   print(format(k,'<%d'% w),":",mail_info[k])