1 class Open: 2 def __init__(self,name): 3 self.name = name 4 5 def __enter__(self): 6 print("触发enter") 7 8 def __exit__(self, exc_type, exc_val, exc_tb): 9 print(exc_tb,' ',exc_val,' ',exc_type) #接收异常的三个参数 10 print("退出exit") 11 12 with Open('a.txt') as f: 13 print("===========") 14 print("===========") 15 # print(dfasdklf) 16 print("===========") 17 ''' 18 输出: 19 触发enter 20 =========== 21 =========== 22 =========== 23 None 24 None 25 None 26 退出exit 27 '''
下面，让程序产生异常：
1 class Open: 2 def __init__(self,name): 3 self.name = name 4 5 def __enter__(self): 6 print("触发enter") 7 8 def __exit__(self, exc_type, exc_val, exc_tb): 9 print(exc_tb,' ',exc_val,' ',exc_type) #接收异常的三个参数 10 print("退出exit") 11 12 with Open('a.txt') as f: 13 print("===========") 14 print("===========") 15 print(dfasdklf) 16 print("===========") 17 ''' 18 输出: 19 触发enter 20 =========== 21 =========== #总共两个，说明出现异常立即退出 22 <traceback object at 0x0000014E7651D0C8> 23 name 'dfasdklf' is not defined 24 <class 'NameError'> 25 退出exit 26 '''
自定义用于打开文件的上下文管理器：
1 class MyOpenClass: 2 def __init__(self,name,mode): 3 self.name= name 4 self.mode= mode 5 6 def __enter__(self): 7 print("进入上下文管理器") 8 if self.mode == 'r': 9 self.f = open(self.name,self.mode) 10 else: 11 self.f= open(self.name,self.mode) 12 return self #重点，要将self， 它返回的就是as 后面的那个 13 14 def __exit__(self, exc_type, exc_val, exc_tb): 15 self.f.close() 16 print("退出上下文管理器") 17 def read(self): 18 return self.f.read() 19 def write(self,s): 20 self.f.write(s) 21 if __name__ =="__main__": 22 with MyOpenClass("d:/test.txt",'r') as f1,MyOpenClass("d:/test02.txt",'w') as f2: 23 txt = f1.read() #从f1中读 24 print(txt) 25 f2.write(txt) # 写到f2中去