Main not defined package python issue

Question

My apologies but I've run out of ideas. I'm very new to python and stuck trying to execute the following code from this tutorial:

#!/usr/bin/env python

import rospy
from geometry_msgs.msg import Twist

import RPi.GPIO as GPIO


# Set the GPIO modes
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)

_FREQUENCY = 20


def _clip(value, minimum, maximum):
    # """Ensure value is between minimum and maximum."""

    if value < minimum:
        return minimum
    elif value > maximum:
        return maximum
    return value

class Motor:
    def __init__(self, forward_pin, backward_pin):
        self._forward_pin = forward_pin
        self._backward_pin = backward_pin

        GPIO.setup(forward_pin, GPIO.OUT)
        GPIO.setup(backward_pin, GPIO.OUT)

        self._forward_pwm = GPIO.PWM(forward_pin, _FREQUENCY)
        self._backward_pwm = GPIO.PWM(backward_pin, _FREQUENCY)

    def move(self, speed_percent):
        speed = _clip(abs(speed_percent), 0, 100)

        # Positive speeds move wheels forward, negative speeds 
        # move wheels backward
        if speed_percent < 0:
            self._backward_pwm.start(speed)
            self._forward_pwm.start(0)
        else:
            self._forward_pwm.start(speed)
            self._backward_pwm.start(0)

class Driver:
    def __init__(self):
        rospy.init_node('driver')

        self._last_received = rospy.get_time()
        self._timeout = rospy.get_param('~timeout', 2)
        self._rate = rospy.get_param('~rate', 10)
        self._max_speed = rospy.get_param('~max_speed', 0.5)
        self._wheel_base = rospy.get_param('~wheel_base', 0.091)

        # Assign pins to motors. These may be distributed
        # differently depending on how you've built your robot
        self._left_motor = Motor(13, 15)
        self._right_motor = Motor(36, 32)
        self._left_speed_percent = 0
        self._right_speed_percent = 0

        # Setup subscriber for velocity twist message
        rospy.Subscriber(
            'cmd_vel', Twist, self.velocity_received_callback)

    def velocity_received_callback(self, message):
        # """Handle new velocity command message."""

        self._last_received = rospy.get_time()

        # Extract linear and angular velocities from the message
        linear = message.linear.x
        angular = message.angular.z

        # Calculate wheel speeds in m/s
        left_speed = linear - angular*self._wheel_base/2
        right_speed = linear + angular*self._wheel_base/2

        # Ideally we'd now use the desired wheel speeds along
        # with data from wheel speed sensors to come up with the
        # power we need to apply to the wheels, but we don't have
        # wheel speed sensors. Instead, we'll simply convert m/s
        # into percent of maximum wheel speed, which gives us a
        # duty cycle that we can apply to each motor.
        self._left_speed_percent = (100 * left_speed/self._max_speed)
        self._right_speed_percent = (100 * right_speed/self._max_speed)

    def run(self):
        # """The control loop of the driver."""

        rate = rospy.Rate(self._rate)

        while not rospy.is_shutdown():
            # If we haven't received new commands for a while, we
            # may have lost contact with the commander-- stop
            # moving
            delay = rospy.get_time() - self._last_received
            if delay < self._timeout:
                self._left_motor.move(self._left_speed_percent)
                self._right_motor.move(self._right_speed_percent)
            else:
                self._left_motor.move(0)
                self._right_motor.move(0)

            rate.sleep()

    def main():
        driver = Driver()

        # Run driver. This will block
        driver.run()

if __name__ == '__main__':
    main()

I get get the following error:

(classic)adeoduye@localhost:~/catkin_ws$ rosrun simple_robot_navigation driver_node_controller
    Traceback (most recent call last):
      File "/home/adeoduye/catkin_ws/src/simple_robot_navigation/src/driver_node_controller", line 117, in <module>
        main()
    NameError: name 'main' is not defined

BTW, it's ROS code which I have implemented in my custom catkin_ws/src/simple_robot_navigation package. I suspect it's a python issue but I very new at python and not sure how to fix it.

I would appreciate any help.

Answer 1

I suspect that there are some indentation error in the tutorial.

In the example code, the function main is defined as a function of the class Driver. However, as the actual code of main goes, it creates an instance of the class Driver and run it. So I think the function definition should be at the same indentation level as the class definition. What you need to do is to remove the indentation of the function definition of main. That is, remove four white spaces such that the function definition would be

def main():
    driver = Driver()
    driver.run()

if __name__ == '__main__':
    main()