I am fairly new to ROS and I am working on a program that controls a robot arm. I am getting several errors related to various variables used in my nodes. I have been trying to find a way to print these variables like you do in python so that I can debug them, but I haven’t found a way to do so. I tried setting up a topic with the variables as the messages and a subscriber. I tried using rospy.loginfo and the console. I tried using plain print(). But none of these worked. I searched the ROS wiki tutorials but didn’t find anything. The code in question is supposed to control a robotic arm. Although the launch file runs a couple of other programs, I specifically want to print from one program called command_arm.py. The code for this program is:

(edit:) I created a node called printer which should print a simple string and then added it to the code. It still does not work. I ran a program with this same coding outside of ROS on my regular raspberry pi and it did work.

      #!/usr/bin/env python
    import rospy
    import traceback 
    import numpy as np
    # IMPORT the messages: 
    from sensor_msgs.msg import JointState
    from Adafruit_PWM_Servo_Driver import PWM
    #from std_msgs.msg import Float
    from std_msgs.msg import String

    #pins order is [base0 yaw, shoulder1 pitch, elbow2 pitch (relative to base), forearm3 roll, wrist4 pitch, finger5 roll]


    # PWM Controller set-up
    servo_pulse_frequency = 50
    servo_pulse_duration_us = 1.0e6/servo_pulse_frequency   # microseconds for each pulse
    servo_pulse_width_to_count_multiplier = 1./servo_pulse_duration_us*4095.0   # Counter is 12-bit, so 4096 levels (0-4095)

    # Initialise the PWM device using the default address
    pwm = PWM(0x40)   # Note if you'd like more debug output you can instead run:  pwm = PWM(0x40, debug=True)
    # Set the Frequency of all servo outputs
    pwm.setPWMFreq(servo_pulse_frequency)

    # Load parameters from rosparam to keep handy for the functions below: 
    # Matched lists of angles and microsecond commands
    map_angles_01 = np.radians(np.array(rospy.get_param('/rotational_angles_for_mapping_joint_01')))
    map_cmd_us_01 = np.array(rospy.get_param('/servo_cmd_us_for_mapping_joint_01'))
    map_angles_12 = np.radians(np.array(rospy.get_param('/rotational_angles_for_mapping_joint_12')))
    map_cmd_us_12 = np.array(rospy.get_param('/servo_cmd_us_for_mapping_joint_12'))
    map_angles_23 = np.radians(np.array(rospy.get_param('/rotational_angles_for_mapping_joint_23')))
    map_cmd_us_23 = np.array(rospy.get_param('/servo_cmd_us_for_mapping_joint_23'))
    map_angles_34 = np.radians(np.array(rospy.get_param('/rotational_angles_for_mapping_joint_34')))
    map_cmd_us_34 = np.array(rospy.get_param('/servo_cmd_us_for_mapping_joint_34'))
    map_angles_45 = np.radians(np.array(rospy.get_param('/rotational_angles_for_mapping_joint_45')))
    map_cmd_us_45 = np.array(rospy.get_param('/servo_cmd_us_for_mapping_joint_45'))
    map_angles_56 = np.radians(np.array(rospy.get_param('/rotational_angles_for_mapping_joint_56')))
    map_cmd_us_56 = np.array(rospy.get_param('/servo_cmd_us_for_mapping_joint_56'))


    # =============================================================================
    #   # No Publishers Needed 
    # =============================================================================

    def main(): 
        # =============================================================================
        #     # Launch a node called "command_arm"
        # =============================================================================
        rospy.init_node('command_arm', anonymous=False)
        rospy.init_node('printer', anonymous=False)
        # Set up subscriber that listens to "joint_states"
        sub_joint_states = rospy.Subscriber('/joint_states', JointState, command_arm)

        rospy.spin()

   def printer():
        print("something printed!!!!!!") 

    def command_arm(msg_in): 
        # unpack joint angle settings
        alpha0 = msg_in.position[0]
        beta1 = msg_in.position[1]
        beta2 = msg_in.position[2]
        beta2WORLD = beta1 + beta2
        gamma3 = msg_in.position[3]
        beta4 = msg_in.position[4]
        gamma5 = msg_in.position[5]    

        # Interpolate to find servo commands and send them
        cmd0 = interp_servo(map_angles_01, map_cmd_us_01, alpha0)
        command_servo(0,np.int(cmd0))

        cmd1 = interp_servo(map_angles_12, map_cmd_us_12, beta1)
        #rospy.loginfo("this is it:", cmd1)

        command_servo(1,np.int(cmd1))    

        cmd2 = interp_servo(map_angles_23, map_cmd_us_23, beta2WORLD)
        command_servo(2,np.int(cmd2))    

        cmd3 = interp_servo(map_angles_34, map_cmd_us_34, gamma3)
        command_servo(3,np.int(cmd3))    

        cmd4 = interp_servo(map_angles_45, map_cmd_us_45, beta4)
        command_servo(4,np.int(cmd4))    

        cmd5 = interp_servo(map_angles_56, map_cmd_us_56, gamma5)
        command_servo(5,np.int(cmd5))    


    # Function to interpolate using calibrated angle-to-servo-command mappings
    def interp_servo(angle_range,us_range,angle):
        # just use the same function arguments to call an Interpolation function np.interp
        return np.interp(angle,angle_range,us_range)


    # Function to command any servo with a given pulse width in microseconds
    def command_servo(servo_number, pulse_width_us):
        pulse_width_count = int(round(pulse_width_us*servo_pulse_width_to_count_multiplier))
        pwm.setPWM(servo_number, 0, pulse_width_count)


    # Function to shut down all the servos by sending them zero pulse width (same as no command)
    def shutdown_servos():
        for ii in range(16):
            command_servo(ii, 0)


    if __name__ == '__main__':
        try: 
            main()
        except: 
            traceback.print_exc()
            shutdown_servos()
            pass

When the launch file runs, the output is:

pi@Expt2019_A:~ $ roslaunch /home/pi/catkin_ws/me439robotarm/src/robot_operate.launch
... logging to /home/pi/.ros/log/9ac03a82-b3a3-11e9-b8de-b827eb90a835/roslaunch-Expt2019_A-1328.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

xacro: Traditional processing is deprecated. Switch to --inorder processing!
To check for compatibility of your document, use option --check-order.
For more infos, see http://wiki.ros.org/xacro#Processing_Order
xacro.py is deprecated; please use xacro instead
started roslaunch server http://Expt2019_A:43895/

SUMMARY
========

PARAMETERS
 * /command_frequency: 50
 * /endpoint_offset_in_frame_6: [0.037, 0.0, -0.035]
 * /endpoint_speed: 0.05
 * /frame_offset_01: [0.0, 0.0, 0.1026]
 * /frame_offset_12: [0.031, 0.0, 0.0]
 * /frame_offset_23: [0.118, 0.0, 0.0]
 * /frame_offset_34: [0.1335, 0.0, 0.02]
 * /frame_offset_45: [0.0, 0.0, 0.0]
 * /frame_offset_56: [0.0, 0.0, 0.0]
 * /path_svg_file: /home/pi/catkin_w...
 * /robot_description: <?xml version="1....
 * /rosdistro: kinetic
 * /rosversion: 1.12.14
 * /rotational_angles_for_mapping_joint_01: [-114.0, 0.0, 60]
 * /rotational_angles_for_mapping_joint_12: [-169.0, -90.0, -...
 * /rotational_angles_for_mapping_joint_23: [-11.0, 0.0, 129.0]
 * /rotational_angles_for_mapping_joint_34: [-112.0, 0.0, 82.0]
 * /rotational_angles_for_mapping_joint_45: [-85.0, 0.0, 90.0]
 * /rotational_angles_for_mapping_joint_56: [-116.0, 0.0, 72.0]
 * /rotational_limits_joint_01: [-90, 90]
 * /rotational_limits_joint_12: [-160, -24]
 * /rotational_limits_joint_23: [0, 136]
 * /rotational_limits_joint_34: [-84, 110]
 * /rotational_limits_joint_45: [-85, 85]
 * /rotational_limits_joint_56: [-87, 96]
 * /servo_cmd_us_for_mapping_joint_01: [600, 1616, 2281]
 * /servo_cmd_us_for_mapping_joint_12: [2483, 1657, 1020]
 * /servo_cmd_us_for_mapping_joint_23: [649, 744, 1656]
 * /servo_cmd_us_for_mapping_joint_34: [601, 1620, 2500]
 * /servo_cmd_us_for_mapping_joint_45: [2500, 1620, 715]
 * /servo_cmd_us_for_mapping_joint_56: [540, 1622, 2380]
 * /use_gui: True
 * /vertical_offset: 0.02
 * /y_rotation_sign: 1

NODES
  /robot_simulate/
    forward_kinematics (me439robotarm/forward_kinematics.py)
    robot_arm_animator (me439robotarm/robot_arm_animator.py)
  /
    robot_state_publisher (robot_state_publisher/state_publisher)
    rviz (rviz/rviz)
  /robot_control/
    command_arm (me439robotarm/command_arm.py)
    inverse_kinematics (me439robotarm/inverse_kinematics.py)
    printer (me439robotarm/printer.py)
    set_waypoints_from_svg (me439robotarm/set_waypoints_from_svg.py)
    smooth_waypoint_seeker (me439robotarm/smooth_waypoint_seeker.py)

ROS_MASTER_URI=http://localhost:11311

process[robot_state_publisher-1]: started with pid [1348]
process[rviz-2]: started with pid [1349]
process[robot_simulate/forward_kinematics-3]: started with pid [1350]
process[robot_simulate/robot_arm_animator-4]: started with pid [1351]
process[robot_control/inverse_kinematics-5]: started with pid [1352]
process[robot_control/smooth_waypoint_seeker-6]: started with pid [1353]
process[robot_control/set_waypoints_from_svg-7]: started with pid [1354]
process[robot_control/command_arm-8]: started with pid [1364]
ERROR: cannot launch node of type [me439robotarm/printer.py]: can't locate node [printer.py] in package [me439robotarm]
libEGL warning: DRI2: failed to authenticate
qt5ct: using qt5ct plugin
/home/pi/catkin_ws/src/me439robotarm/src/inverse_kinematics.py:109: RuntimeWarning: invalid value encountered in arccos
  phi = np.arccos( (L2**2 - L1**2 - H**2)/(-2*L1*H) )  # arccos will always return a positive value.
/home/pi/catkin_ws/src/me439robotarm/src/smooth_waypoint_seeker.py:73: RuntimeWarning: invalid value encountered in divide
  drhat = dr/drlength
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
inotify_add_watch("/home/pi/.config/qt5ct") failed: "No such file or directory"
qt5ct: D-Bus global menu: no
libpng warning: iCCP: known incorrect sRGB profile
Traceback (most recent call last):
  File "/home/pi/catkin_ws/src/me439robotarm/src/set_waypoints_from_svg.py", line 75, in talker
    msg_waypoint.xyz = waypoints[waypoint_number]
IndexError: index 43 is out of bounds for axis 0 with size 43
[robot_control/set_waypoints_from_svg-7] process has finished cleanly
log file: /home/pi/.ros/log/9ac03a82-b3a3-11e9-b8de-b827eb90a835/robot_control-set_waypoints_from_svg-7*.log
================================================================================REQUIRED process [rviz-2] has died!
process has finished cleanly
log file: /home/pi/.ros/log/9ac03a82-b3a3-11e9-b8de-b827eb90a835/rviz-2*.log
Initiating shutdown!
================================================================================
[robot_control/command_arm-8] killing on exit
[robot_control/smooth_waypoint_seeker-6] killing on exit
[robot_control/inverse_kinematics-5] killing on exit
[robot_simulate/robot_arm_animator-4] killing on exit
[robot_simulate/forward_kinematics-3] killing on exit
[rviz-2] killing on exit
[robot_state_publisher-1] killing on exit
Traceback (most recent call last):
  File "/home/pi/catkin_ws/src/me439robotarm/src/robot_arm_animator.py", line 82, in <module>
    listener()
  File "/home/pi/catkin_ws/src/me439robotarm/src/robot_arm_animator.py", line 31, in listener
    animate_robot_arm()
  File "/home/pi/catkin_ws/src/me439robotarm/src/robot_arm_animator.py", line 59, in animate_robot_arm
    plt.show()
  File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 253, in show
    return _show(*args, **kw)
  File "/usr/lib/python2.7/dist-packages/matplotlib/backend_bases.py", line 163, in __call__
    manager.show()
  File "/usr/lib/python2.7/dist-packages/matplotlib/backends/backend_tkagg.py", line 610, in show
    self.canvas.manager.window.attributes('-topmost', 1)
AttributeError: 'NoneType' object has no attribute 'attributes'
shutting down processing monitor...
... shutting down processing monitor complete
done

Any suggestions on strategies I could try or perhaps tutorials that could help?

cmd0 = interp_servo(map_angles_01, map_cmd_us_01, alpha0)
print("wow: 0")
command_servo(0,np.int(cmd0))

   header: 
      seq: 1944
      stamp: 
        secs: 1564669298
        nsecs: 494210004
      frame_id: ''
    name: [base_joint, shoulder_joint, elbow_joint, forearm_joint, wrist_joint, fingers_joint]
    position: [-0.07059167068719688, -1.1265526268264099, 2.1564758335387584, 0.0, -1.0299232067123485, 0.0]
    velocity: []
    effort: []
    ---
    header: 
      seq: 1945
      stamp: 
        secs: 1564669298
        nsecs: 514286994
      frame_id: ''
    name: [base_joint, shoulder_joint, elbow_joint, forearm_joint, wrist_joint, fingers_joint]
    position: [-0.07059167068719688, -1.1265526268264099, 2.1564758335387584, 0.0, -1.0299232067123485, 0.0]
    velocity: []
    effort: []