python习题：写一个备份数据库的脚本

  备份数据库的命令 mysqldump（操作系统的命令）

import os,datetime

class BakDB(object):
    def __init__(self,ip,username,password,port=3306,path='/tmp/db_bak'):  # path传的是linux系统的路径
        self.ip=ip
        self.username=username
        self.password=password
        self.port=port
        self.path=path
        self.path_exist()

    def path_exist(self):
        if not os.path.isdir(self.path):
            os.mkdir(self.path)

    def bak_db(self):
        # mysqldump -uroot -p123456 -A > nhy123.sql
       # mysqldump -u%s -p%s -P%s -h%s -A > filename
        filename=str(datetime.date.today())+'.sql'
        abs_file = os.path.join(self.path,filename)
        command = '''mysqldump -u{username} -p{password} -P{port} -h{ip} -A > {filename}'''.format(
            username=self.username,
            password=self.password,
            port=self.port,
            filename=abs_file,
            ip=self.ip)
        os.system(command)
        print('数据库已备份完成！')
a=BakDB('127.0.0.1','root','123456')
a.bak_db()