AttributeError: 'module' object has no attribute 'dumps'

Question

对于一个项目，我在 Python 中使用了 ROS-MQTT 网桥。这是从 ROS Wiki 获得的库。 该库可以在这里找到：https://github.com/groove-x/mqtt_bridge

其中一个 python 文件 (app.py) 使用以下代码行：

# load serializer and deserializer
  serializer = params.get ('serializer', 'json: dumps')
  deserializer = params.get ('deserializer', 'json: loads')

当运行roslaunch命令启动程序时，参考上面的代码出现如下错误：AttributeError: 'module' object has no attribute 'dumps'.

我查看了 this 问题，但答案对我的情况没有帮助。我在任何地方都找不到 json.py 文件，所以我也无法重命名它。如果我这样做 find sudo. -print | grep -i '. * [.] py' 我只会得到以下文件：

./app.py
./__init__.py
./bridge.py
./util.py
./mqtt_client.py

有兴趣者追溯：

Traceback (most recent call last):
  File "/opt/ros/melodic/lib/mqtt_bridge/mqtt_bridge_node.py", line 9, in <module>
    mqtt_bridge_node ()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/mqtt_bridge/app.py", line 50, in mqtt_bridge_node
    mqtt_client, serializer, deserializer, mqtt_private_path)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/mqtt_bridge/app.py", line 15, in create_config
    serializer = lookup_object (serializer)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/mqtt_bridge/util.py", line 12, in lookup_object
    obj = getattr (module, obj_name)
AttributeError: 'module' object has no attribute 'dumps'

对于完整的 app.py 文件感兴趣的人：

# -*- coding: utf-8 -*-
from __future__ import absolute_import

import inject
import paho.mqtt.client as mqtt
import rospy

from .bridge import create_bridge
from .mqtt_client import create_private_path_extractor
from .util import lookup_object

def create_config(mqtt_client, serializer, deserializer, mqtt_private_path):
    if isinstance(serializer, basestring):
        serializer = lookup_object(serializer)
    if isinstance(deserializer, basestring):
        deserializer = lookup_object(deserializer)
    private_path_extractor = create_private_path_extractor(mqtt_private_path)
    def config(binder):
        binder.bind('serializer', serializer)
        binder.bind('deserializer', deserializer)
        binder.bind(mqtt.Client, mqtt_client)
        binder.bind('mqtt_private_path_extractor', private_path_extractor)
    return config


def mqtt_bridge_node():
    # init node
    rospy.init_node('mqtt_bridge_node')

    # load parameters
    params = rospy.get_param("~", {})
    mqtt_params = params.pop("mqtt", {})
    conn_params = mqtt_params.pop("connection")
    mqtt_private_path = mqtt_params.pop("private_path", "")
    bridge_params = params.get("bridge", [])

    # create mqtt client
    mqtt_client_factory_name = rospy.get_param(
        "~mqtt_client_factory", ".mqtt_client:default_mqtt_client_factory")
    mqtt_client_factory = lookup_object(mqtt_client_factory_name)
    mqtt_client = mqtt_client_factory(mqtt_params)

    # load serializer and deserializer
    serializer = params.get('serializer', 'json:dumps')
    deserializer = params.get('deserializer', 'json:loads')

    # dependency injection
    config = create_config(
        mqtt_client, serializer, deserializer, mqtt_private_path)
    inject.configure(config)

    # configure and connect to MQTT broker
    mqtt_client.on_connect = _on_connect
    mqtt_client.on_disconnect = _on_disconnect
    mqtt_client.connect(**conn_params)

    # configure bridges
    bridges = []
    for bridge_args in bridge_params:
        bridges.append(create_bridge(**bridge_args))

    # start MQTT loop
    mqtt_client.loop_start()

    # register shutdown callback and spin
    rospy.on_shutdown(mqtt_client.disconnect)
    rospy.on_shutdown(mqtt_client.loop_stop)
    rospy.spin()


def _on_connect(client, userdata, flags, response_code):
    rospy.loginfo('MQTT connected')


def _on_disconnect(client, userdata, response_code):
    rospy.loginfo('MQTT disconnected')


__all__ = ['mqtt_bridge_node']

有人知道问题出在哪里吗？

Answer 1

经过 5 小时的尝试和研究后，我发现其中一个软件包依赖项丢失了。 该包正在检查缺少的依赖项，但那个除外。

通过在附加文件中添加一些印刷品，我发现它不是 json 丢失的。这是一个名为 msgpack 的依赖项。

执行 pip install msgpack-python==0.4.8 后我可以成功 运行 代码。