openstack with和yield结合方法

源码是这样的：

with self.virtapi.wait_for_instance_event( #错误位置
                    instance, events, deadline=timeout,
                    error_callback=self._neutron_failed_callback):
                self.plug_vifs(instance, network_info) #添加虚拟网卡，后面是添加防火墙
                self.firewall_driver.setup_basic_filtering(instance,
                                                           network_info)
                self.firewall_driver.prepare_instance_filter(instance,
                                                             network_info)
                with self._lxc_disk_handler(instance, instance.image_meta,
                                            block_device_info, disk_info):
                    guest = self._create_domain(
                        xml, pause=pause, power_on=power_on,
                        post_xml_callback=post_xml_callback)

                self.firewall_driver.apply_instance_filter(instance,
                                                           network_info)

with后面类必须要有一个__enter__()函数和一个__exit__()函数，然后看代码wait_for_instance_event，发现里面有yield，很奇怪。

@contextlib.contextmanager
    def wait_for_instance_event(self, instance, event_names, deadline=300,
                                error_callback=None):
        a
        yield
        b

这里解决 with和yield问题的是contextlib.contextmanager这个装饰器

@contextlib.contextmanager
    def wait_for_instance_event(self, instance, event_names, deadline=300,
                                error_callback=None):
        a
        yield
        b

contextmanager代码里有较详细的注释：

def contextmanager(func):
    """@contextmanager decorator.

    Typical usage:

        @contextmanager
        def some_generator():

            try:
                yield
            finally:


    This makes this:

        with some_generator() as :


    equivalent to this:              下面是实际的代码执行流程


        try:
             =
                            with的body部分在这里执行
        finally:


    """
    @wraps(func)
    def helper(*args, **kwds):
        return GeneratorContextManager(func(*args, **kwds))
    return helper

contextmanager是通过GeneratorContextManager实现的，GeneratorContextManager实现了__enter__和__exit__，这两个函数都调了next()函数来结合yield。

class GeneratorContextManager(object):
    """Helper for @contextmanager decorator."""

    def __init__(self, gen):
        self.gen = gen

    def __enter__(self):
        try:
            return self.gen.next()
        except StopIteration:
            raise RuntimeError("generator didn't yield")

    def __exit__(self, type, value, traceback):
        if type is None:
            try:
                self.gen.next()
            except StopIteration:
                return
            else:
                raise RuntimeError("generator didn't stop")
        else:
            if value is None:
                # Need to force instantiation so we can reliably
                # tell if we get the same exception back
                value = type()
            try:
                self.gen.throw(type, value, traceback)
                raise RuntimeError("generator didn't stop after throw()")
            except StopIteration, exc:
                # Suppress the exception *unless* it's the same exception that
                # was passed to throw().  This prevents a StopIteration
                # raised inside the "with" statement from being suppressed
                return exc is not value
            except:
                # only re-raise if it's *not* the exception that was
                # passed to throw(), because __exit__() must not raise
                # an exception unless __exit__() itself failed. But throw()
                # has to raise the exception to signal propagation, so this
                # fixes the impedance mismatch between the throw() protocol
                # and the __exit__() protocol.
                #
                if sys.exc_info()[1] is not value:
                    raise

灵活的python。