深入浅析python with语句简介

# 自定义支持with语句的对象class DummyRes:  def __init__(self, tag):    self.tag = tag  def __enter__(self):    print("Enter >>> {}".format(self.tag))    return self  def __exit__(self, exc_type, exc_value, exc_tb):    print("Exit <<< {}".format(self.tag))    if exc_tb is None:      print("Exit without Exception {}".format(self.tag))      return False    else:      print("Exit with Exception {}".format(self.tag))      return True# 支持closing 上下文with语句对象class Closing:  def __init__(self, thing):    self.thing = thing  def __enter__(self):    return self  def __exit__(self, exc_type, exc_value, exc_tb):    self.thing.close()class ClosingDemo:  def __init__(self):    self.acquire()  def acquire(self):    print("Acquire RES")  def close(self):    print("Close RES")from contextlib import contextmanagerclass ContextDemo:  def __init__(self):    print("Context Demo init")    raise Exception    print("Context Demo init")  def print(self):    print("Context Demo print 1")    #raise Exception    print("Context Demo print 2")  def close(self):    print("Context Demo close")def context_demo():  print("context demo in")  raise Exception  print("context demo out")@contextmanagerdef demo():  print("Allocate Resoures")  try:    yield context_demo  finally:    print("raise exception")  #yield "*** contextmanager demo ***"  print("Free Resoures")if __name__ == "__main__":  # 1. 使用with语句 (自动关闭文件)  with open("test.txt", "w") as f:    f.write("write test")  # 2. 自动定义with语句  with DummyRes("test") as res:    print("With body 1")    raise Exception    print("With body 2")  # 3. 利用contextlib定义with语句  with demo():    print("exc demo")  # 4. closing 上下文 (适合有close操作的情况)  with Closing(ClosingDemo()):    print("Use Resoures")