上下文管理协议__enter__和__exit__

'''上下文管理协议__enter__和__exit__'''
with open('day62.py', 'r') as f:
   pass
# 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
# 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')
#     print(exc_type) # with代码块无异常，打印None
#     print(exc_val) # with代码块无异常，打印None
#     print(exc_tb) # with代码块无异常，打印None
#
# with Open('a.txt') as f1:
#  print('===>执行代码块') # 先触发执行__enter__，然后执行with中代码，最后执行__exit__
#  print(f1) # <__main__.Open object at 0x0000014AD1A58490> (Open类中__enter__返回的是实例本身，如果没有返回则打印None)
#  print(f1.name) # a.txt (如果Open类中__enter__没有返回实例本身，那么调用不到此实例属性)


# class Open_1:
#  def __init__(self, name):
#     self.name = name
#
#  def __enter__(self):
#     return self
#
#  def __exit__(self, exc_type, exc_val, exc_tb):
#     print(exc_type) # <class 'NameError'> 对应异常类
#     print(exc_val) # name 'ababab' is not defined 对应异常值
#     print(exc_tb) # <traceback object at 0x0000027C6F1A1D40> 对应异常追踪信息
#
# with Open_1('b.txt') as f2:
#  print(ababab) # 无此变量名
#  print(f2.name) # 遇到异常执行完__exit__就停止且没有再执行此步操作了
# print('******') # 此步操作也没有进行打印
# 且最后会报错
# Traceback (most recent call last):
#   File "C:/Users/XHP/PycharmProjects/python_quanzhan/s1/day63.py", line 43, in <module>
#     print(ababab) # 无此变量名
# NameError: name 'ababab' is not defined


# class Open_2:
#  def __init__(self, name):
#     self.name = name
#
#  def __enter__(self):
#     return self
#
#  def __exit__(self, exc_type, exc_val, exc_tb):
#     print(exc_type) # <class 'NameError'> 对应异常类
#     print(exc_val) # name 'ababab' is not defined 对应异常值
#     print(exc_tb) # <traceback object at 0x0000027C6F1A1D40> 对应异常追踪信息
#     return True
#
# with Open_2('c.txt') as f3:
#  print(ababab) # 无此变量名
#  print(f3.name) # 不会打印(__exit__的运行完毕就代表了整个with语句的执行完毕)
# print('******') # 会打印(__exit__加入了return True后，还会执行with代码块以外的操作)
# 无报错信息