After learning the fourth lesson of Mr. Gu Yue’s ROS, I started the keyboard control of the robot and reported this error in the launch file. The following is the error content:

process[mbot_teleop-1]: started with pid [4942]
  File "/home/zxf/catkin_ws/src/mbot_teleop/scripts/mbot_teleop.py", line 78
    print msg
          ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(msg)?
[mbot_teleop-1] process has died [pid 4942, exit code 1, cmd /home/zxf/catkin_ws/src/mbot_teleop/scripts/mbot_teleop.py __name:=mbot_teleop __log:=/home/zxf/.ros/log/4e9341ca-0155-11ec-8ec5-cf95496c0738/mbot_teleop-1.log].
log file: /home/zxf/.ros/log/4e9341ca-0155-11ec-8ec5-cf95496c0738/mbot_teleop-1*.log

I searched the Internet and found that it was because of the version of python2. X was incompatible with the version of python3. X. The reason for the error lies in the print output syntax. A simple example is hello ROS

python2.x
print "hello ROS"     
Output: hello ROS
print 'hello ROS'
Output: hello ROS
print ('hello ROS')
Output: hello ROS
print ("hello ROS")
Output: hello ROS

The above four methods can be applied in Python 2. X, but not necessarily in Python 3. X

python3.x
print("hello ROS")
Output: hello ROS
print(hello ROS)
Output: hello ROS

I use Ubuntu 20.04, so it is the Python 3 version, which is different from that used by Gu Yue, so I need to modify ~/catkin_ws/src/mbot_teleop/scripts/mbot_ Teleop.py file

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import rospy
from geometry_msgs.msg import Twist
import sys, select, termios, tty

msg = """
Control mbot!
---------------------------
Moving around:
   u    i    o
   j    k    l
   m    ,    .

q/z : increase/decrease max speeds by 10%
w/x : increase/decrease only linear speed by 10%
e/c : increase/decrease only angular speed by 10%
space key, k : force stop
anything else : stop smoothly

CTRL-C to quit
"""

moveBindings = {
        'i':(1,0),
        'o':(1,-1),
        'j':(0,1),
        'l':(0,-1),
        'u':(1,1),
        ',':(-1,0),
        '.':(-1,1),
        'm':(-1,-1),
           }

speedBindings={
        'q':(1.1,1.1),
        'z':(.9,.9),
        'w':(1.1,1),
        'x':(.9,1),
        'e':(1,1.1),
        'c':(1,.9),
          }

def getKey():
    tty.setraw(sys.stdin.fileno())
    rlist, _, _ = select.select([sys.stdin], [], [], 0.1)
    if rlist:
        key = sys.stdin.read(1)
    else:
        key = ''

    termios.tcsetattr(sys.stdin, termios.TCSADRAIN, settings)
    return key

speed = .2
turn = 1

def vels(speed,turn):
    return "currently:\tspeed %s\tturn %s " % (speed,turn)

if __name__=="__main__":
    settings = termios.tcgetattr(sys.stdin)
    
    rospy.init_node('mbot_teleop')
    pub = rospy.Publisher('/cmd_vel', Twist, queue_size=5)

    x = 0
    th = 0
    status = 0
    count = 0
    acc = 0.1
    target_speed = 0
    target_turn = 0
    control_speed = 0
    control_turn = 0
    try:
        print(msg)
        print(vels(speed,turn))
        while(1):
            key = getKey()
            if key in moveBindings.keys():
                x = moveBindings[key][0]
                th = moveBindings[key][1]
                count = 0
            elif key in speedBindings.keys():
                speed = speed * speedBindings[key][0] 
                turn = turn * speedBindings[key][1]   
                count = 0

                print(vels(speed,turn))
                if (status == 14):
                    print(msg)
                status = (status + 1) % 15
            elif key == ' ' or key == 'k' :
                x = 0
                th = 0
                control_speed = 0
                control_turn = 0
            else:
                count = count + 1
                if count > 4:
                    x = 0
                    th = 0
                if (key == '\x03'):
                    break

            target_speed = speed * x
            target_turn = turn * th

            if target_speed > control_speed:
                control_speed = min( target_speed, control_speed + 0.02 )
            elif target_speed < control_speed:
                control_speed = max( target_speed, control_speed - 0.02 )
            else:
                control_speed = target_speed

            if target_turn > control_turn:
                control_turn = min( target_turn, control_turn + 0.1 )
            elif target_turn < control_turn:
                control_turn = max( target_turn, control_turn - 0.1 )
            else:
                control_turn = target_turn

            twist = Twist()
            twist.linear.x = control_speed; 
            twist.linear.y = 0; 
            twist.linear.z = 0
            twist.angular.x = 0; 
            twist.angular.y = 0; 
            twist.angular.z = control_turn
            pub.publish(twist)

    except:
        print(e)

    finally:
        twist = Twist()
        twist.linear.x = 0; twist.linear.y = 0; twist.linear.z = 0
        twist.angular.x = 0; twist.angular.y = 0; twist.angular.z = 0
        pub.publish(twist)

    termios.tcsetattr(sys.stdin, termios.TCSADRAIN, settings)

It is probably to add () to the print output statement on lines 78, 79, 93, 95 and 141.