Python 时间变量没有改变

Question

我修改了一些在 Whosebug 上找到的 here 代码，但出现了一些奇怪的行为。谁能看到我的错误？

import atexit
from time import clock

line = "="*10 + ">>>"

def secondsToStr(s):
    return "{0}.{1}ms".format(round(s*1000), round(s*1000000))

##
# @brief Starts a timer. If the program crashes, the end function will be called anyway.
def start():
    atexit.register(stop)
    print(line,"Start time measurement")
    startTime = clock()
    print("new start time:", startTime)

##
# @brief Needs to be called after start(). Prints the time passed since start() was called.
def stop():
    end = clock()
    elapsed = end-startTime
    print(startTime, end)    # Inserted for debugging
    print(line, "Ellapsed time:", secondsToStr(elapsed))
    atexit.unregister(stop)

def now():
    return secondsToStr(clock())

startTime = clock()

想法是调用 start() 和 stop() 来测量程序在这两个调用之间花费的时间。奇怪的是，startTime 没有改变。因此，每次调用 stop() 都会给出自第一次调用 start() 以来的过去时间。这是一个示例输出：

==========>>> Start time measurement
new start time: 0.285078
Doing work
0.231932 1.766478
==========>>> Ellapsed time: 1535.1534546ms

==========>>> Start time measurement
new start time: 1.766624
More work
0.231932 1.975752
==========>>> Ellapsed time: 1744.1743820ms

==========>>> Start time measurement
new start time: 1.975821
More work
0.231932 1.976301
==========>>> Ellapsed time: 1744.1744369ms

为什么 startTime 永远不会改变？每次调用 start() 时它应该得到一个新值。

Answer 1

它不起作用的原因是因为在 start 中，startTime 是一个 local 变量。如果你想更新 global 变量，你需要明确地告诉 python：

def start():
    global startTime  # tell python to work with `startTime` in the global scope.
    atexit.register(stop)
    print(line,"Start time measurement")
    startTime = clock()
    print("new start time:", startTime)

但是请注意，99% 的情况下，当您使用 global 语句时，可能有更好的选择。在不知道您的确切用例的情况下，很难给出完美的建议。但是，我可能会在这里考虑上下文管理器：

import contextlib
from time import clock

line = "="*10 + ">>>"

def secondsToStr(s):
    return "{0}.{1}ms".format(round(s*1000), round(s*1000000))

@contextlib.contextmanager
def timer():
    start = clock()
    try:
        yield None
    finally:
        stop = clock()
        print(line, "Ellapsed time:", secondsToStr(elapsed))

您可以按如下方式使用它：

with timer():
    do_something()
    do_something_else()

在函数 return 之后，您将打印出这两个函数的组合运行时间。与您的原件一样，如果发生异常，它仍会打印。

Answer 2

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, but in any case, this is the function to use for benchmarking Python or timing algorithms.

这意味着 time.clock() 测量了从您的进程开始的时间。如果你想测量挂钟时间，你可以使用 time.time()

Python 时间变量没有改变

Python time variable is not changing

python

timing

clock

python-3.x