python之在 python 中测量时间的异常结果

这是我的秒表课

import time 
 
class StopWatch: 
 
    def __init__(self, tag="", startTime=time.time(), elapsedTime=-1): 
        self._tag=tag 
        self._elapsedTime=elapsedTime 
        self._startTime=startTime 
 
    # other codes here............# 
 
    def stop(self, tag=None): 
        if tag is not None: 
            self._tag = tag 
        self._elapsedTime = time.time() - self._startTime 
        return self 
 
    def getStartTime(self): 
        return self._startTime 

秒表测试文件为:

import unittest 
import time 
from metric.StopWatch import StopWatch 
 
class StopWatchTest(unittest.TestCase): 
 
    def test_stopwatch(self): 
        now=time.time() 
        print "now:%f" % now 
 
        stopwatch=StopWatch("firstHook") 
        print "start time:%r" % stopwatch.getStartTime() 
        self.assertTrue(stopwatch.getStartTime()>now, "start time is wrong") 

我得到的错误是:

now:1364791630.047630 
start time:1364791629.158797 

..

AssertionError: start time is wrong 

稍后执行的代码显示较早的时间。这背后的原因是什么？

请您参考如下方法:

默认参数只计算一次。

def __init__(self, tag="", startTime=time.time(), elapsedTime=-1): 
    self._tag=tag 
    self._elapsedTime = elapsedTime 
    self._startTime = startTime 

要修复，我们需要显式检查默认值并在函数体中评估它们:

def __init__(self, tag="", startTime=None, elapsedTime=-1): 
    self._tag=tag 
    self._elapsedTime = elapsedTime 
    if startTime is None: 
        startTime = time.time() 
    self._startTime = startTime