Python实现上下文管理器的方法

import time
from contextlib import contextmanager

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

# Example use
with timethis('counting'):
  n = 10000000
  while n > 0:
    n -= 1

@contextmanager
def list_transaction(orig_list):
  working = list(orig_list)
  yield working
  orig_list[:] = working

> items = [1, 2, 3]
> with list_transaction(items) as working:
...   working.append(4)
...   working.append(5)
...
> items
[1, 2, 3, 4, 5]
> with list_transaction(items) as working:
...   working.append(6)
...   working.append(7)
...   raise RuntimeError('oops')
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: oops
> items
[1, 2, 3, 4, 5]
>

import time

class timethis:
  def __init__(self, label):
    self.label = label

  def __enter__(self):
    self.start = time.time()

  def __exit__(self, exc_ty, exc_val, exc_tb):
    end = time.time()
    print('{}: {}'.format(self.label, end - self.start))