python上下文管理器使用场景及异常处理_python

# 创建一个文件写入字符串“Python”
f = open('123.txt', 'w')
f.write("python")
f.close()
# 使用with语句调用上下文实现文件写入操作
with open('123.txt', 'w') as f:
    f.write('python')

import pymysql
class DBConnection(object):
    def __init__(self,ip,user,passwd,db):
        self.ip = ip
        self.user = user
        self.passwd = passwd
        self.db = db
    def __enter__(self):
        self.conn = pymysql.connect(self.ip, user=self.user, passwd=self.passwd, db=self.db)
        self.cur = conn.cursor()
        return self.cur
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.cur.close()
        self.conn.close()
with DBConnection('192.168.121.xxx', user="xxx", passwd="123456", db="xxx") as cur:
    cur.execute("select * from studnet;")
    result = cur.fetchall()
    print(result)

class MyOpen(object):
    """自定义上下文管理类"""
    def __init__(self, file, mode):
        self._file = file
        self._mode = mode
    def __enter__(self):
        self._handle = open(self._file, self._mode)
        return self._handle
    def __exit__(self, exc_type, exc_val, exc_tb):
        # print('Type: ', exc_type)
        # print('Value:', exc_val)
        # print('TreacBack:', exc_tb)
        self._handle.close()
        print("异常已被处理")
        return True
# 读的模式打开文件,进行写操作，不支持
with MyOpen('123.txt', 'r') as f:
    f.write('python')
#输出：
异常已被处理