为什么不能调用 class 的属性？

Question

我正在修改openstack-nova的代码。

添加 RESTful api 后，我得到了这个错误：

eption during message handling: AttributeError: 'SchedulerReportClient' object has no attribute 'obj_to_primitive'
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server Traceback (most recent call last):
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/server.py", line 163, in _process_incoming
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     res = self.dispatcher.dispatch(message)
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 268, in dispatch
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     return self._do_dispatch(endpoint, method, ctxt, args)
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 195, in _do_dispatch
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     return self.serializer.serialize_entity(ctxt, result)
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/openstack/lib/python2.7/site-packages/nova/rpc.py", line 130, in serialize_entity
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     return self._base.serialize_entity(context, entity)
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/openstack/lib/python2.7/site-packages/nova/objects/base.py", line 237, in serialize_entity
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     entity)
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/openstack/lib/python2.7/site-packages/nova/objects/base.py", line 223, in _process_iterable
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     for k, v in values.items()})
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/openstack/lib/python2.7/site-packages/nova/objects/base.py", line 223, in <dictcomp>
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     for k, v in values.items()})
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/openstack/lib/python2.7/site-packages/nova/objects/base.py", line 245, in serialize_entity
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     entity = entity.obj_to_primitive()
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server   File "/openstack/lib/python2.7/site-packages/nova/scheduler/client/__init__.py", line 37, in __run_method
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server     return getattr(self.instance, __name)(*args, **kwargs)
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server AttributeError: 'SchedulerReportClient' object has no attribute 'obj_to_primitive'
2018-08-30 23:08:10.904 1637 ERROR oslo_messaging.rpc.server

关联代码为：

def serialize_entity(self, context, entity):
    if isinstance(entity, (tuple, list, set, dict)):
        entity = self._process_iterable(context, self.serialize_entity, entity)
    elif (hasattr(entity, 'obj_to_primitive') and callable(entity.obj_to_primitive)):
        entity = entity.obj_to_primitive()
    return entity

我的问题是，hasattr(entity, 'obj_to_primitive') 是正确的，那么为什么 entity.obj_to_primitive() 会引发异常？

Answer 1

查看堆栈跟踪的最后一行。您正在调用“方法”，但它只是 self.instance 上相应方法的包装器 that getattr 那是失败的。 outer attribute lookup成功创建wrapper，hasattr开心，wrapper确实是callable.

如果 getattr(self.instance, __name) 是在创建时计算的，hasattr 测试将起作用。如果包装器的唯一目的是调用（而不是，比如说，拥有自己的方法），它也可以执行自己的 callable 检查。但是 Python 将自己交给了 EAFP，所以我们到了。

为什么不能调用 class 的属性？

Why an attribute of the class cannot be called?

python

openstack

openstack-nova