上下文管理器 with。。as

（1）文件读取--上下文管理器  with open(filename,'w') as f :

class MyOpen:
    def __init__(self, filename, mode, encoding='utf-8'):
        self.filename = filename
        self.mode = mode
        self.encoding = encoding

    def __enter__(self):
        self.f = open(self.filename, self.mode, encoding=self.encoding)
        return self.f

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.f.close()


with MyOpen('add.yml', 'r') as f:
    print(f.read())

(2) mysql数据表读取 --上下文管理器：

from mysql import connector


class MySql:
    def __init__(self, host, user, password, database, port):
        self.host = host
        self.user = user
        self.password = password
        self.database = database
        self.port = port

    def __enter__(self):
        self.conn = connector.connect(host=self.host, user=self.user, password=self.password,
                                      database=self.database, port=self.port)
        self.cur = self.conn.cursor()
        return self.cur

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.cur.close()
        self.conn.close()


with MySql('127.0.0.1', 'root', 'password', 'db', '3306') as cursor:
    cursor.execute('select * from projects')
    print(cursor.fetchall())