oslo_utils 中的 timeutils

在E:\nova\nova\compute\manager.py 中有如下语句：
from oslo_utils import timeutils
在manager.py中使用timeutils的例子如下：
        with timeutils.StopWatch() as timer:
            self.network_api.deallocate_for_instance(
                context, instance, requested_networks=requested_networks)
        # nova-network does an rpc call so we're OK tracking time spent here
        LOG.info('Took %0.2f seconds to deallocate network for instance.',
                 timer.elapsed(), instance=instance)
这里就可以通过timer.elapsed() 来显示从timeutils.StopWatch() 到LOG.info 之间也就是deallocate_for_instance 执行的时间.
在oslo_utils 中StopWatch 的实现如下；
https://github.com/openstack/oslo.utils/blob/master/oslo_utils/timeutils.py

class StopWatch(object):

#可以看到StopWatch 有两种状态，分别表示start和stop
    _STARTED = 'STARTED'
    _STOPPED = 'STOPPED'
#init函数就是基本的赋值，本例中的duration 用的是默认值None
    def __init__(self, duration=None):
        if duration is not None and duration < 0:
            raise ValueError("Duration must be greater or equal to"
                             " zero and not %s" % duration)
        self._duration = duration
        self._started_at = None
        self._stopped_at = None
        self._state = None
        self._splits = ()
#开始计数
    def start(self):
        """Starts the watch (if not already started).
        NOTE(harlowja): resets any splits previously captured (if any).
        """
#如果当前的状态已经是start了，则返回。
        if self._state == self._STARTED:
            return self
#记录当前的时间
        self._started_at = now()
        self._stopped_at = None
#设置当前的状态为_STARTED
        self._state = self._STARTED
        self._splits = ()
        return self
    @staticmethod
    def _delta_seconds(earlier, later):
        # Uses max to avoid the delta/time going backwards (and thus negative).
        return max(0.0, later - earlier)
#显示调用elapsed显示从with 调用StopWatch 开始到现在的时间
    def elapsed(self, maximum=None):
        """Returns how many seconds have elapsed."""
        if self._state not in (self._STARTED, self._STOPPED):
            raise RuntimeError("Can not get the elapsed time of a stopwatch"
                               " if it has not been started/stopped")
        if self._state == self._STOPPED:
            elapsed = self._delta_seconds(self._started_at, self._stopped_at)
        else:
            elapsed = self._delta_seconds(self._started_at, now())
        if maximum is not None and elapsed > maximum:
            elapsed = max(0.0, maximum)
        return elapsed

#通过with 调用StopWatch的时候，会自动调用__enter__，在__enter__ 中有会调用start 来开始计数
    def __enter__(self):
        """Starts the watch."""
        self.start()
        return self

StopWatch 这个class 有实现__enter__，因此当在with 中调用StopWatch 是会自动调用__enter__，在__enter__ 中有会调用自身的start函数
来记录下当前的时间，当前时间是通过now函数获得，并把时间记录到内部变量_started_at中这样当用户下次在主动调用elapsed的时候就会主动
通过now函数得到当前的时间减去之前记录在_started_at的时间.