python上下文管理器的基本介绍

with EXPR as VAR:

    BLOCK

import time

class demo:

    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))

with demo('counting'):

    n = 10000000

    while n > 0:

        n -= 1

# counting: 1.36000013351