一、上下文管理协议其实就是 with obje as f
在文件操作时用过，with open("a.txt","r") as f:
当时的好处就是不用手动关闭文件了
学到现在我们知道了，其实open也是一个类，之所以能用 with as 是类里做了“小手脚”
class Foo:
def __init__(self,name):
self.name = name
def __enter__(self):
print("执行enter")
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print("执行exit")
with Foo('a.txt') as f:
print(f)
print(f.name)
print("======>")
print("======>")
print("000000000000000")
上述过程：
1. with obj ------->触发 obj.__enter__(), 拿到的返回值赋值给 f (with obj as f 等同于 f= obj.__enter__())
2. with obj as f 内的代码执行完毕后触发 __exit__()
二、__exit__中的异常处理
先了解下异常
分为：异常类，异常值，追踪信息
①没有异常的情况下，整个代码块运行完毕后去触发 __exit__，它的三个参数都为None
class Foo:
def __init__(self,name):
self.name = name
def __enter__(self):
print("执行enter")
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print("执行exit")
print(exc_type) #异常类
print(exc_val) #异常值
print(exc_tb) #追踪信息
with Foo('a.txt') as f:
print(f)
print(f.name)
# print(jinling) #触发 __exit__()
print("======>")
print("======>")
print("000000000000000")
②在有异常的情况下，从异常出现的位置直接触发 __exit__
a:如果__exit__的返回值为True 代表吞掉了异常
b:如果__exit__的返回值不为True,代表突出了异常
c: __exit__运行完毕代表了整个 with 语句的执行完毕
class Foo:
def __init__(self,name):
self.name = name
def __enter__(self):
print("执行enter")
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print("执行exit")
print(exc_type) #异常类
print(exc_val) #异常值
print(exc_tb) #追踪信息
# return True
with Foo('a.txt') as f:
print(f)
print(f.name)
print(jinling) #触发 __exit__()
print("======>")
print("======>")
print("000000000000000")
class Foo:
def __init__(self,name):
self.name = name
def __enter__(self):
print("执行enter")
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print("执行exit")
print(exc_type) #异常类
print(exc_val) #异常值
print(exc_tb) #追踪信息
return True
with Foo('a.txt') as f:
print(f)
print(f.name)
print(jinling) #触发 __exit__()
print("======>")
print("======>")
print("000000000000000") #因为__exit__ 吞掉了异常，所以最后一句00000000000正常输出，但是with块内异常下面的代码不能正常输出
