且将生活一饮而尽之上下文管理与装饰器

上下文管理协议

• 定义：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

• 示例一：

  class MyResource:
      def __enter__(self):
          print("connected")
          return self
  
      def __exit__(self, *args, **kwargs):
          print("close")
          # 遇到异常需要关闭的时候 里面返回True
          # 外层就不再抛异常  否者抛异常
          return True
  
      def query(self):
          print("query data")
  
  
  try:
      with MyResource() as obj_A:  # 这里面的obj_A 就是__enter__方法return self 实例化的对象
          obj_A.query()
  except Exception as e:
      print(e)
  """
  打印结果：
  connected
  query data
  close
  """
  

• 示例二：

  class MyResource:
      def __enter__(self):
          print("connected")
          return self
  
      def __exit__(self, *args, **kwargs):
          print("close")
          # 遇到异常需要关闭的时候 里面返回True
          # 外层就不再抛异常  否者抛异常
          return True
  
      def query(self):
          print("query data")
  
  
  try:
      with MyResource() as obj_A:
          1 / 0  # 这个位置就直接退出
          obj_A.query()
  except Exception as e:
      print(e)
  """
  打印结果：
  connected
  close
  """
  
  

• contextmanager 实现上下文管理协议

  class MyResource:
      def query(self):
          print("query data")
          
  from contextlib import contextmanager
  
  @contextmanager
  def make_myresource():
      print("connected")
      yield MyResource()   # 这里面将实例化的MyResource()给r
      print("close")
      
  with make_myresource() as r:
      r.query()
  """
  result:
  connected
  query data
  close
  """
  

• 且将生活一饮而尽之上下文管理

  from contextlib import contextmanager
  
  
  @contextmanager
  def book_mark():
      print("<<", end='')
      yield
      print('>>', end='')
  
  
  with book_mark():
      print("且将生活一饮而尽", end='')
  """
  <<且将生活一饮而尽>>
  """
  

• 且将生活一饮而尽之装饰器

  def outer(f):
      def inner(*args, **kwargs):
          print("<<", end="")
          f(*args, **kwargs)
          print(">>", end="")
      return inner
  
  @outer
  def println(str):
      print(str, end="")
  
  println("且将生活一饮而尽")
  """
  <<且将生活一饮而尽>>
  """