@小鱼 好的，我去了解一下那个软件，抱歉现在还不太懂所以格式不对，给您添麻烦了。

@小鱼 好的，谢谢您。

@小鱼 好的，抱歉，我重新发一下。

我在跑小乌龟程序的时候，学习网上教程想看看节点，输入了rosrun rqt_graph rqt_graph，之后就报错了，具体代码如下，请大佬们给萌新指导一下。
rosrun rqt_graph rqt_graph
PluginManager._discover() force discovery of plugins
RospkgPluginProvider._find_plugins() crawling for plugins of type 'qt_gui'
Could not import "pyside" bindings of qt_gui_cpp library - so C++ plugins will not be available:
Traceback (most recent call last):
File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui_cpp/cpp_binding_helper.py", line 39, in <module>
from . import libqt_gui_cpp_shiboken
ImportError: cannot import name 'libqt_gui_cpp_shiboken'

RospkgPluginProvider._find_plugins() crawling for plugins of type 'rqt_gui'
QtBindingHelper using pyside
PerspectiveManager.switch_perspective() switching to perspective "@rqt_graph__RosGraph"
MainWindow.restore_settings()
PluginManager.restore_settings_without_plugins()
MainWindow.restore_state()
PluginManager.load_plugin(rqt_graph/RosGraph, 0)
MainWindow.save_setup()
RosPyPluginProvider._init_node() initialize ROS node "rqt_gui_py_node_2480"
RosPluginProvider.load(rqt_graph/RosGraph) exception raised in builtin.import(rqt_graph.ros_graph, [RosGraph]):
Traceback (most recent call last):
File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_gui/ros_plugin_provider.py", line 80, in load
attributes['module_name'], fromlist=[attributes['class_from_class_type']], level=0)
File "/home/fzz/.local/lib/python3.6/site-packages/shiboken2/files.dir/shibokensupport/feature.py", line 142, in _import
return original_import(name, *args, **kwargs)
File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_graph/ros_graph.py", line 45, in <module>
from qt_dotgraph.dot_to_qt import DotToQtGenerator
File "/home/fzz/.local/lib/python3.6/site-packages/shiboken2/files.dir/shibokensupport/feature.py", line 142, in _import
return original_import(name, *args, **kwargs)
File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_dotgraph/dot_to_qt.py", line 35, in <module>
import pydot
File "/home/fzz/.local/lib/python3.6/site-packages/shiboken2/files.dir/shibokensupport/feature.py", line 142, in _import
return original_import(name, *args, **kwargs)
ModuleNotFoundError: No module named 'pydot'

PluginManager._load_plugin_restore()
PluginManager._load_plugin() could not load plugin "rqt_graph/RosGraph":
Traceback (most recent call last):
File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/plugin_handler.py", line 102, in load
self._load()
File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/plugin_handler_direct.py", line 55, in _load
self._plugin = self._plugin_provider.load(self._instance_id.plugin_id, self._context)
File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/composite_plugin_provider.py", line 72, in load
instance = plugin_provider.load(plugin_id, plugin_context)
File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/composite_plugin_provider.py", line 72, in load
instance = plugin_provider.load(plugin_id, plugin_context)
File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_gui_py/ros_py_plugin_provider.py", line 61, in load
return super(RosPyPluginProvider, self).load(plugin_id, plugin_context)
File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/composite_plugin_provider.py", line 72, in load
instance = plugin_provider.load(plugin_id, plugin_context)
File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_gui/ros_plugin_provider.py", line 90, in load
raise e
File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_gui/ros_plugin_provider.py", line 80, in load
attributes['module_name'], fromlist=[attributes['class_from_class_type']], level=0)
File "/home/fzz/.local/lib/python3.6/site-packages/shiboken2/files.dir/shibokensupport/feature.py", line 142, in _import
return original_import(name, *args, **kwargs)
File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_graph/ros_graph.py", line 45, in <module>
from qt_dotgraph.dot_to_qt import DotToQtGenerator
File "/home/fzz/.local/lib/python3.6/site-packages/shiboken2/files.dir/shibokensupport/feature.py", line 142, in _import
return original_import(name, *args, **kwargs)
File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_dotgraph/dot_to_qt.py", line 35, in <module>
import pydot
File "/home/fzz/.local/lib/python3.6/site-packages/shiboken2/files.dir/shibokensupport/feature.py", line 142, in _import
return original_import(name, *args, **kwargs)
ModuleNotFoundError: No module named 'pydot'
谢谢！

@小鱼 已经可以成功运行小乌龟了

@小鱼 您好，我按照以下方法成功修改了默认python版本
Linux Python 版本切换方法
切换python版本

update-alternatives --display python
如果出现以下错误：
update-alternatives: 错误: 无 python 的候选项，那么按下面的流程操作。

#查看多个python版本

#ls /usr/bin/python
python2.7 python3.6
#添加Python2可选项，优先级为2

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 2
#添加Python3可选项，优先级为1

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 1
#配置

sudo update-alternatives --config python
#yongyu 删除配置

sudo update-alternatives --remove python /usr/bin/python2.7

来源：https://www.csdn.net/tags/NtzaAgysNDA5NjQtYmxvZwO0O0OO0O0O.html
感谢您的指导，谢谢！

@小鱼您好，谢谢您的意见，我按您说的尝试一下，感谢！

@小鱼 您好，可以向您请教一下要如何操作吗，我现在刚开始接触ros还不太懂，谢谢！

@小鱼 您好，我是u18.04，ros melodic，之前貌似是默认2，然后我根据网上教学一通瞎操作想搞成3，但是可能没有成功，所以之后roscore一直报错，我卸载后按照您的一键安装后依旧这样报错。

@首飞Kevin 谢谢您的回复，我明早尝试一下，感谢！