有时候需要等待一个时间不确定的事件的发生。如果直接sleep最大时长，然后判断预期，则严重影响效率。可以改用轮询机制，一旦条件满足，立即返回；反之等到最后超时。
def waitfor(getter, timeout=3, interval=0.5, *args):
starttime = datetime.datetime.now()
while True:
if getter(args):
return
else:
runtime = datetime.datetime.now() - starttime
print runtime
if runtime.seconds >= timeout:
raise Exception
time.sleep(interval)
current_value = 1
def testgetval(args):
wanted_value = args[0]
global current_value
current_value += 1
print '%d, %d' % (wanted_value, current_value)
return current_value > wanted_value
if __name__ == '__main__':
waitfor(testgetval, 1, 0.3, 2)
print '======================='
waitfor(testgetval, 1, 0.3, 8)2, 2
0:00:00.001000
2, 3
====================
8, 4
0:00:00.002000
8, 5
0:00:00.303000
8, 6
0:00:00.605000
8, 7
0:00:00.907000
8, 8
0:00:01.209000
Traceback (most rece
File "multiver.py"
waitfor(testgetv
File "multiver.py"
raise Exception
Exception
第一轮测试，在1秒中内成功返回
第二轮测试，在预定的时间内未得到预期结果，抛出超时异常
