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 contextmanager
class 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")
@contextmanager
def 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")