from contextlib import contextmanager
import time
@contextmanager
def demo(label):
  start = time.time()
  try:
    yield
  finally:
    end = time.time()
    print('{}: {}'.format(label, end - start))

with demo('counting'):
  n = 10000000
  while n > 0:
    n -= 1
# counting: 1.32399988174

from contextlib import contextmanager
class File():
  def query(self):
    print('查询文件')

@contextmanager
def open():
  print('打开文件')
  yield File()
  print('关闭文件')

with open() as f:
  f.query()
  
# 结果 打开文件 查询文件 关闭文件

from contextlib import contextmanager
class DataBase():
  def query(self):
    print('写入操作')
  @contextmanager
  def open(self):
    try:
      yield
      self.commit()
    except Exception as e:
      self.rollback()
      raise e

db = DataBase()
with db.open():
  db.query()