149 实现文件上下文管理(__enter__和__exit__)
- 我们知道在操作文件对象的时候可以这么写
with open('a.txt') as f:
'代码块'
- 上述叫做上下文管理协议,即with语句,为了让一个对象兼容with语句,必须在这个对象的类中声明__enter__和__exit__方法
一、上下文管理协议
class Open:
def __init__(self, name):
self.name = name
def __enter__(self):
print('出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量')
# return self
def __exit__(self, exc_type, exc_val, exc_tb):
print('with中代码块执行完毕时执行我啊')
with Open('a.txt') as f:
print('=====>执行代码块')
# print(f,f.name)
出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量
=====>执行代码块
with中代码块执行完毕时执行我啊
- __exit__()中的三个参数分别代表异常类型,异常值和追溯信息,with语句中代码块出现异常,则with后的代码都无法执行
class Open:
def __init__(self, name):
self.name = name
def __enter__(self):
print('出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量')
def __exit__(self, exc_type, exc_val, exc_tb):
print('with中代码块执行完毕时执行我啊')
print(exc_type)
print(exc_val)
print(exc_tb)
try:
with Open('a.txt') as f:
print('=====>执行代码块')
raise AttributeError('***着火啦,救火啊***')
except Exception as e:
print(e)
出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量
=====>执行代码块
with中代码块执行完毕时执行我啊
<class 'AttributeError'>
***着火啦,救火啊***
<traceback object at 0x1065f1f88>
***着火啦,救火啊***
- 如果__exit()返回值为True,那么异常会被清空,就好像啥都没发生一样,with后的语句正常执行
class Open:
def __init__(self, name):
self.name = name
def __enter__(self):
print('出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量')
def __exit__(self, exc_type, exc_val, exc_tb):
print('with中代码块执行完毕时执行我啊')
print(exc_type)
print(exc_val)
print(exc_tb)
return True
with Open('a.txt') as f:
print('=====>执行代码块')
raise AttributeError('***着火啦,救火啊***')
print('0' * 100) #------------------------------->会执行
出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量
=====>执行代码块
with中代码块执行完毕时执行我啊
<class 'AttributeError'>
***着火啦,救火啊***
<traceback object at 0x1062ab048>
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
二、模拟open
class Open:
def __init__(self, filepath, mode='r', encoding='utf-8'):
self.filepath = filepath
self.mode = mode
self.encoding = encoding
def __enter__(self):
# print('enter')
self.f = open(self.filepath, mode=self.mode, encoding=self.encoding)
return self.f
def __exit__(self, exc_type, exc_val, exc_tb):
# print('exit')
self.f.close()
return True
def __getattr__(self, item):
return getattr(self.f, item)
with Open('a.txt', 'w') as f:
print(f)
f.write('aaaaaa')
f.wasdf #抛出异常,交给__exit__处理
<_io.TextIOWrapper name='a.txt' mode='w' encoding='utf-8'>
三、优点
-
使用with语句的目的就是把代码块放入with中执行,with结束后,自动完成清理工作,无须手动干预
-
在需要管理一些资源比如文件,网络连接和锁的编程环境中,可以在__exit__中定制自动释放资源的机制,你无须再去关系这个问题,这将大有用处