我的功能需要负时间才能完成.这世界到底发生了什么? [英] My function takes negative time to complete. What in the world happened?

问题描述

我提出这个问题主要是出于好奇.我写了一些代码，正在做一些非常耗时的工作.因此，在执行我的主力函数之前，我将它封装在对 time.clock() 的几次调用中.它看起来像这样:

I'm posing this question mostly out of curiosity. I've written some code that is doing some very time intensive work. So, before executing my workhorse function, I wrapped it up in a couple of calls to time.clock(). It looks something like this:

t1 = time.clock()
print this_function_takes_forever(how_long_parameter = 20)
t2 = time.clock()
print t2 - t1

这工作得很好.我的函数正确返回，t2 - t1 给了我 972.29 的结果，或者大约 16 分钟.

This worked fine. My function returned correctly and t2 - t1 gave me a result of 972.29, or about 16 minutes.

但是，当我将代码更改为此

However, when I changed my code to this

t1 = time.clock()
print this_function_takes_forever(how_long_parameter = 80)
t2 = time.clock()
print t2 - t1

我的函数仍然正常返回，但 t2 - t1 的结果是:

My function still returned fine, but the result of t2 - t1 was:

None
-1741

我很好奇是什么实现细节导致了这种情况.None 和负数都让我感到困惑.它与签名类型有关吗?这如何解释 None?

I'm curious as to what implementation detail causes this. Both the None, and the negative number are perplexing to me. Does it have something to do with a signed type? How does this explain the None?

推荐答案

Python文档说:

在 Unix 上，将当前处理器时间返回为以秒表示的浮点数.精度，实际上是处理器时间"含义的定义，取决于同名 C 函数的精度

On Unix, return the current processor time as a floating point number expressed in seconds. The precision, and in fact the very definition of the meaning of "processor time", depends on that of the C function of the same name

引用的 C 函数的 手册页然后解释了这个问题:

The manpage of the referenced C function then explains the issue:

请注意，时间可以回绕.在 CLOCKS_PER_SEC 等于 1000000 的 32 位系统上，此函数将大约每 72 分钟返回相同的值.

Note that the time can wrap around. On a 32-bit system where CLOCKS_PER_SEC equals 1000000 this function will return the same value approximately every 72 minutes.

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