03. Python principles, ROS Publisher, ROS Subscriber

Lecture

Python principles

Interpreted, high-level programming language
Name tribute to the comedy group Monty Python
Powerful, still easy to learn, easy to use
Readability
Whitespace indentation

Dynamically-typed
Garbage colector and reference counting
Object oriented programming
Used in: AI, web applications, scientific computing, and many other areas
python3

Python syntax

import numpy as np
import math

class A:
    def __init__(self, name):
        self.name = name

    def do_something(self):
        # will do something
        print(self.name + " is doing something.")

    def count_to(self, n):
        # count to n, tell if the number is odd or even
        for i in range(n):
            if i % 2 == 0:
                print(i + ", it's even.")
            else:
                print(i + ", it's odd.")



if __name__ == "__main__":
    a = A("John")
    a.do_something()
    a.count_to(10)

Practice

1: Move the turtle in a straight line

Let's write a ROS node that moves the turtle forward along a straight line for a given distance. Let's open a terminal. Let's create ~/ros2_ws/src/ros2_course/ros2_course the turtlesim_controller.py file in our directory:
```
cd ros2_ws/src/ros2_course/ros2_course
touch turtlesim_controller.py
```

Add a new entry point in the setup.py file:

'turtlesim_controller = ros2_course.turtlesim_controller:main',

Copy the skeleton of the program into turtlesim_controller.py:

import math
import rclpy
from rclpy.node import Node


class TurtlesimController(Node):

    def __init__(self):
        super().__init__('turtlesim_controller')


    def go_straight(self, speed, distance):
        # Implement straght motion here


def main(args=None):
    rclpy.init(args=args)
    tc = TurtlesimController()

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    tc.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Let's start a turtlesim_node and then examine the topic, with which we can control. In two separate terminal windows:

ros2 run turtlesim turtlesim_node

ros2 topic list
ros2 topic info /turtle1/cmd_vel
ros2 interface show geometry_msgs/msg/Twist

Or userqt_gui:

ros2 run rqt_gui rqt_gui

Import the message type geometry_msgs/msg/Twist and create the publisher in turtlesim_controller.py:

from geometry_msgs.msg import Twist

#...

# In the constructor:
self.twist_pub = self.create_publisher(Twist, '/turtle1/cmd_vel', 10)

We implement the go_straight method. Let's calculate how long it takes, so that the turtle covers the given distance at the given speed. Publish a message with which we set the speed, then wait for the calculated time, after that send another message to reset the speed. A little help for using the API:

# Create and publish msg
vel_msg = Twist()
if distance > 0:
    vel_msg.linear.x = speed
else:
    vel_msg.linear.x = -speed
vel_msg.linear.y = 0.0
vel_msg.linear.z = 0.0
vel_msg.angular.x = 0.0
vel_msg.angular.y = 0.0
vel_msg.angular.z = 0.0

# Set loop rate
loop_rate = self.create_rate(100, self.get_clock()) # Hz

# Calculate time
# T = ...

# Publish first msg and note time when to stop
self.twist_pub.publish(vel_msg)
# self.get_logger().info('Turtle started.')
when = self.get_clock().now() + rclpy.time.Duration(seconds=T)

# Publish msg while the calculated time is up
while (some condition...) and rclpy.ok():
    self.twist_pub.publish(vel_msg)
    # self.get_logger().info('On its way...')
    rclpy.spin_once(self)   # loop rate

# turtle arrived, set velocity to 0
vel_msg.linear.x = 0.0
self.twist_pub.publish(vel_msg)
# self.get_logger().info('Arrived to destination.')

Build and run the node:

cd ros2_ws
colcon build --symlink-install
ros2 run ros2_course turtlesim_controller

2: Draw shapes

Let's implement the method for turning with a given angle a in turtlesim_controller.py, similar to straight motion.
```
def turn(self, omega, angle):
        # Implement rotation here
```
Let's implement the straight movement method of drawing a square with a turtle and using the methods that perform the turn.
```
def draw_square(self, speed, omega, a):
```
Let's implement the method of drawing any regular shape with a turtle using the methods that perform straight movement and turning.
```
def draw_poly(self, speed, omega, N, a):
```

3: Go to function

Let's examine the topic on which turtlesim_node publishes its current position.

ros2 topic list
ros2 topic info /turtle1/pose
ros2 interface show turtlesim/msg/Pose

Or use rqt_gui:

ros2 run rqt_gui rqt_gui

Let's define a subscriber for the topic and write the callback function.

# Imports
from turtlesim.msg import Pose

    # Constructor
    self.pose = None
    self.subscription = self.create_subscription(
        Pose,
        '/turtle1/pose',
        self.cb_pose,
        10)

    # New method for TurtlesimController
    def cb_pose(self, msg):
        self.pose = msg

We implement the go_to method. Let's test it, call it from main.

# ...

# Go to method
    def go_to(self, speed, omega, x, y):
        # Wait for position to be received
        loop_rate = self.create_rate(100, self.get_clock()) # Hz
        while self.pose is None and rclpy.ok():
            self.get_logger().info('Waiting for pose...')
            rclpy.spin_once(self)

        # Stuff with atan2


# Main
def main(args=None):
    rclpy.init(args=args)
    tc = TurtlesimController()

    tc.go_to(1.0, 20.0, 2, 8)
    tc.go_to(1.0, 20.0, 2, 2)
    tc.go_to(1.0, 20.0, 3, 4)
    tc.go_to(1.0, 20.0, 6, 2)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    tc.destroy_node()
    rclpy.shutdown()

Extra: Advanced go to

Write a go to function that uses a proportional controller.