python：waitfor轮询

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