13.13小节完成
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XiongNeng
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----------
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问题
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You want to be able to record the time it takes to perform various tasks.
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你想记录程序执行多个任务所花费的时间
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解决方案
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The time module contains various functions for performing timing-related functions.
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However, it’s often useful to put a higher-level interface on them that mimics a stop
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watch. For example:
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``time`` 模块包含很多函数来执行跟时间有关的函数。
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尽管如此，通常我们会在此基础之上构造一个更高级的接口来模拟一个计时器。例如：
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import time
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.. code-block:: python
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class Timer:
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def __init__(self, func=time.perf_counter):
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self.elapsed = 0.0
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self._func = func
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self._start = None
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import time
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def start(self):
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if self._start is not None:
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raise RuntimeError('Already started')
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self._start = self._func()
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class Timer:
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def __init__(self, func=time.perf_counter):
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self.elapsed = 0.0
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self._func = func
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self._start = None
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def stop(self):
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if self._start is None:
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raise RuntimeError('Not started')
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end = self._func()
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self.elapsed += end - self._start
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self._start = None
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def start(self):
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if self._start is not None:
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raise RuntimeError('Already started')
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self._start = self._func()
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def reset(self):
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self.elapsed = 0.0
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def stop(self):
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if self._start is None:
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raise RuntimeError('Not started')
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end = self._func()
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self.elapsed += end - self._start
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self._start = None
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@property
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def running(self):
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return self._start is not None
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def reset(self):
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self.elapsed = 0.0
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def __enter__(self):
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self.start()
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return self
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@property
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def running(self):
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return self._start is not None
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def __exit__(self, *args):
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self.stop()
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def __enter__(self):
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self.start()
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return self
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This class defines a timer that can be started, stopped, and reset as needed by the user.
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It keeps track of the total elapsed time in the elapsed attribute. Here is an example that
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shows how it can be used:
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def __exit__(self, *args):
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self.stop()
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def countdown(n):
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while n > 0:
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n -= 1
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这个类定义了一个可以被用户根据需要启动、停止和重置的计时器。
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它会在 ``elapsed`` 属性中记录整个消耗时间。
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下面是一个例子来演示怎样使用它：
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# Use 1: Explicit start/stop
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t = Timer()
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t.start()
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countdown(1000000)
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t.stop()
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print(t.elapsed)
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.. code-block:: python
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# Use 2: As a context manager
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with t:
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def countdown(n):
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while n > 0:
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n -= 1
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# Use 1: Explicit start/stop
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t = Timer()
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t.start()
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countdown(1000000)
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t.stop()
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print(t.elapsed)
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print(t.elapsed)
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# Use 2: As a context manager
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with t:
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countdown(1000000)
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with Timer() as t2:
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countdown(1000000)
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print(t2.elapsed)
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print(t.elapsed)
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with Timer() as t2:
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countdown(1000000)
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print(t2.elapsed)
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----------
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讨论
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----------
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This recipe provides a simple yet very useful class for making timing measurements and
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tracking elapsed time. It’s also a nice illustration of how to support the context-
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management protocol and the with statement.
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One issue in making timing measurements concerns the underlying time function used
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to do it. As a general rule, the accuracy of timing measurements made with functions
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such as time.time() or time.clock() varies according to the operating system. In
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contrast, the time.perf_counter() function always uses the highest-resolution timer
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available on the system.
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As shown, the time recorded by the Timer class is made according to wall-clock time,
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and includes all time spent sleeping. If you only want the amount of CPU time used by
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the process, use time.process_time() instead. For example:
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本节提供了一个简单而实用的类来实现时间记录以及耗时计算。
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同时也是对使用with语句以及上下文管理器协议的一个很好的演示。
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t = Timer(time.process_time)
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with t:
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countdown(1000000)
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print(t.elapsed)
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在计时中要考虑一个底层的时间函数问题。一般来说，
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使用 ``time.time()`` 或 ``time.clock()`` 计算的时间精度因操作系统的不同会有所不同。
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而使用 ``time.perf_counter()`` 函数可以确保使用系统上面最精确的计时器。
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Both the time.perf_counter() and time.process_time() return a “time” in fractional
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seconds. However, the actual value of the time doesn’t have any particular meaning. To
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make sense of the results, you have to call the functions twice and compute a time
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difference.
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More examples of timing and profiling are given in Recipe 14.13.
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上述代码中由 ``Timer`` 类记录的时间是钟表时间，并包含了所有休眠时间。
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如果你只想计算该进程所花费的CPU时间，应该使用 ``time.process_time()`` 来代替：
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.. code-block:: python
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t = Timer(time.process_time)
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with t:
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countdown(1000000)
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print(t.elapsed)
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``time.perf_counter()`` 和 ``time.process_time()`` 都会返回小数形式的秒数时间。
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实际的时间值没有任何意义，为了得到有意义的结果，你得执行两次函数然后计算它们的差值。
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更多关于计时和性能分析的例子请参考14.13小节。
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@@ -56,28 +56,28 @@ Roadmap
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| 13章翻译完成
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2015/09/01 - 2015/09/30:
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2015/09/01 - 2015/11/30:
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::
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| 14章翻译完成
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2015/10/01 - 2015/10/31:
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2015/12/01 - 2015/12/20:
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::
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| 15章翻译完成
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2015/11/01 - 2015/11/15:
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2015/12/21 - 2015/12/31:
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::
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| 对全部翻译进行校对一次
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2015/11/16 - 2015/11/20:
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2016/01/01 - 2016/01/10:
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::
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