TypeError: las_callback() takes exactly 2 arguments (1 given)

asked 2020-12-12 21:58:27 -0500

zzdmw
1 ●1 ●1 ●1

I try to make spatial median filtering that uses in sensor_msgs/LaserScan.msg. this is my python code:

#!/usr/bin/env python
"""
"""

import rospy

from sensor_msgs.msg import LaserScan

def las_callback(self, msg):

    data_list=list(msg.ranges)

    self.median_ranges=[]

    self.median_filter_size=3

    for i in range(0, len(data_list)):

        if (i>= self.median_filter_size and i<(len(data_list)-self.median_filter_size)):

            media_call=median(data_list[i-self.median_filter_size:i+self.median_filter_size+1])

            self.median_ranges.append(median_call)

    print(media_call)


rospy.init_node('laser_readings')
sub = rospy.Subscriber('/base_scan', LaserScan, las_callback)
rospy.spin()

when I try to rosrun this code, it shows me a worrying message is:

[ERROR] [1607831425.437657, 1678.699000]: bad callback: <function las_callback at 0x7fb50adbddd0>
Traceback (most recent call last):
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/topics.py", line 750, in _invoke_callback
    cb(msg)
TypeError: las_callback() takes exactly 2 arguments (1 given)

How can I solve this problem? thanks for your help!

answered 2020-12-13 01:48:40 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

updated 2020-12-13 02:24:20 -0500

This is a Python problem more than a ROS one.

def las_callback(self, msg):
    [..]

You've defined a method here (the first argument is self), but it is not part of any class.

Remove the self argument, so las_callback(..) becomes a function.

Alternatively: create a Python class and make las_callback(..) a method of that class. You'll have to take care to create all the member variables as well in that case, or the callback will not run successfully.

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Comments

when I make"las_callback(self, msg):" become "def las_callback(msg):". It show a new error:

[ERROR] [1607846695.003072, 183.190000]: bad callback: <function las_callback at 0x7f8cda22ed50>
Traceback (most recent call last):
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/topics.py", line 750, in _invoke_callback
    cb(msg)
  File "/home/wg/catkin_ws/src/rob599_hw3/src/median_filter.py", line 13, in las_callback
    self.median_ranges=[]
NameError: global name 'self' is not defined

zzdmw ( 2020-12-13 02:06:07 -0500 )edit

That's because you are still referencing self in the body of your callback.

This is still a Python problem: did you copy the callback code from somewhere?

If you haven't yet, you might want to lookup some Python tutorials to get some more experience with Python before taking on ROS. Learning all these new things at the same time can be quite a challenge.

gvdhoorn ( 2020-12-13 02:23:24 -0500 )edit

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