在上下文管理中，没有价值的结果是什么

Question

import contextlib
import time

@contextlib.contextmanager
def time_print(task_name):
    t = time.time()
    try:
        yield
    finally:
        print task_name, "took", time.time() - t, "seconds."


def doproc():
    x=1+1


with time_print("processes"):
    [doproc() for _ in range(500)]

# processes took 15.236166954 seconds.

使用这个装饰器时doproc何时执行？在

Answer 1

@Martijn Pieters的精彩解释。由于yield在您的案例中是多余的，您可以通过创建自己的上下文管理器（不使用yield和contextlib.contextmanager）。这更简单易读。所以在您的例子中，您可以实现如下内容。在

import time

class time_print(object):
    def __init__(self, task_name):
        self.task_name = task_name

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

    def __exit__(self):
        print self.task_name, "took", time.time() - self.t, "seconds."

def doproc():
    x=1+1


with time_print("processes"):
    # __enter__ is called
    [doproc() for _ in range(500)]
    # __exit__ is called

内部contextlib.contextmanager按照@Martijun Pieters的解释，调用\uuenter\和\uuu exit\uuu魔术函数。希望这有帮助！在