且將生活一飲而盡之上下文管理與裝飾器

上下文管理協議

• 定義：上下文管理協議，即with語句，爲了讓一個對象兼容with語句，必須在這個對象的類中聲明__enter__和__exit__方法

• 示例一：

  class MyResource:
      def __enter__(self):
          print("connected")
          return self
  
      def __exit__(self, *args, **kwargs):
          print("close")
          # 遇到異常須要關閉的時候 裏面返回True
          # 外層就再也不拋異常  否者拋異常
          return True
  
      def query(self):
          print("query data")
  
  
  try:
      with MyResource() as obj_A:  # 這裏面的obj_A 就是__enter__方法return self 實例化的對象
          obj_A.query()
  except Exception as e:
      print(e)
  """
  打印結果：
  connected
  query data
  close
  """

• 示例二：

  class MyResource:
      def __enter__(self):
          print("connected")
          return self
  
      def __exit__(self, *args, **kwargs):
          print("close")
          # 遇到異常須要關閉的時候 裏面返回True
          # 外層就再也不拋異常  否者拋異常
          return True
  
      def query(self):
          print("query data")
  
  
  try:
      with MyResource() as obj_A:
          1 / 0  # 這個位置就直接退出
          obj_A.query()
  except Exception as e:
      print(e)
  """
  打印結果：
  connected
  close
  """

• contextmanager 實現上下文管理協議

  class MyResource:
      def query(self):
          print("query data")
  
  from contextlib import contextmanager
  
  @contextmanager
  def make_myresource():
      print("connected")
      yield MyResource()   # 這裏面將實例化的MyResource()給r
      print("close")
  
  with make_myresource() as r:
      r.query()
  """
  result:
  connected
  query data
  close
  """

• 且將生活一飲而盡之上下文管理

  from contextlib import contextmanager
  
  
  @contextmanager
  def book_mark():
      print("<<", end='')
      yield
      print('>>', end='')
  
  
  with book_mark():
      print("且將生活一飲而盡", end='')
  """
  <<且將生活一飲而盡>>
  """

• 且將生活一飲而盡之裝飾器

  def outer(f):
      def inner(*args, **kwargs):
          print("<<", end="")
          f(*args, **kwargs)
          print(">>", end="")
      return inner
  
  @outer
  def println(str):
      print(str, end="")
  
  println("且將生活一飲而盡")
  """
  <<且將生活一飲而盡>>
  """