1 00:00:01,060 --> 00:00:04,770 In this lecture, we will create our very own Roth publisher node. 2 00:00:04,780 --> 00:00:10,630 We'll make a simple publisher that publishes a string that says Hello World, followed by a number which 3 00:00:10,630 --> 00:00:13,240 increments up every time we publish. 4 00:00:13,450 --> 00:00:18,700 Then in a future lecture, we'll create a subscriber to receive the messages we publish. 5 00:00:20,290 --> 00:00:23,440 For now, let's open up our Visual Studio code editor. 6 00:00:24,010 --> 00:00:25,630 Go over to file. 7 00:00:27,240 --> 00:00:32,670 Open folder and we will open up our Ras two Pi workspace folder. 8 00:00:34,440 --> 00:00:35,910 Go ahead and trust the workspace. 9 00:00:37,210 --> 00:00:41,230 And for now we're going to skip configuring our C make environment. 10 00:00:41,230 --> 00:00:43,930 So for now, just go ahead and cancel that and hit escape. 11 00:00:43,930 --> 00:00:51,170 And now we see all of our folders from our project workspace directory here in the left side panel. 12 00:00:51,190 --> 00:00:54,790 Now notice it's also generated a hidden VS code folder. 13 00:00:54,790 --> 00:01:00,490 This contains all the setting configurations for our Visual Studio Code editor and you can see it within 14 00:01:00,490 --> 00:01:07,660 the file Explorer by hitting Control H or by heading over to the three dashes and checking or unchecking 15 00:01:07,660 --> 00:01:08,800 show hidden file. 16 00:01:08,800 --> 00:01:10,840 So now we can't see that hidden file. 17 00:01:10,840 --> 00:01:16,200 So the first thing I can go ahead and do is open up the source folder, which takes us into our RAS 18 00:01:16,210 --> 00:01:18,730 two package and we can go into the scripts folder. 19 00:01:18,730 --> 00:01:19,840 So currently we don't have any. 20 00:01:19,840 --> 00:01:25,810 So I'll go ahead and right click hit new file and I will just create a new script called Publisher Spy. 21 00:01:26,410 --> 00:01:33,430 Just a pre face all RAR to Python functionality is documented in the RCL py API documentation. 22 00:01:33,430 --> 00:01:36,310 Here you can review specific functionality of things. 23 00:01:36,310 --> 00:01:41,470 We are going to be using in the RAS two Python library, including additional arguments and how they 24 00:01:41,470 --> 00:01:42,100 work. 25 00:01:44,530 --> 00:01:47,890 But for now, I'm just going to go through and show you how this functionality works first hand. 26 00:01:47,890 --> 00:01:52,060 But if you have additional questions, feel free to reference that documentation page. 27 00:01:53,060 --> 00:01:58,650 In order to use raw features in Python, we need to import the ROS client library python module. 28 00:01:58,670 --> 00:02:01,070 This is abbreviated as rcl py. 29 00:02:01,340 --> 00:02:07,130 Within the script I will create a standard python if name is equal to main call and within it I will 30 00:02:07,130 --> 00:02:09,230 just call a function called main. 31 00:02:09,390 --> 00:02:12,560 You'll see my reasoning for doing this later in the course. 32 00:02:15,070 --> 00:02:17,350 So let's create the main function above. 33 00:02:19,270 --> 00:02:19,680 All right. 34 00:02:19,690 --> 00:02:24,760 The first thing we're going to do in our main function is initialize our Ros RDS communications. 35 00:02:24,790 --> 00:02:29,950 We can do this by calling our ROS client library and using the init function. 36 00:02:30,220 --> 00:02:35,980 This has two optional arguments which can be put in command line arguments and a context name we won't 37 00:02:35,980 --> 00:02:36,640 use either. 38 00:02:36,640 --> 00:02:39,850 So we'll just leave this function as is with no parameters in it. 39 00:02:40,290 --> 00:02:45,730 OC We are configured to be able to use ROS functionality, so let's actually create our node. 40 00:02:45,880 --> 00:02:51,280 The ROS client library contains a node class with all the different ROS functionality to implement in 41 00:02:51,280 --> 00:02:52,080 our code. 42 00:02:52,090 --> 00:02:57,790 So what we can do is create our own class which inherits all the functionality of that existing ROS 43 00:02:57,790 --> 00:02:58,810 node class. 44 00:02:59,220 --> 00:03:02,590 I'll create the class above and call it Hello World Publisher. 45 00:03:02,860 --> 00:03:07,870 Now I need to pass it in the node class to inherit its functionality, so I will first import it in 46 00:03:07,870 --> 00:03:09,430 from the ROS client library. 47 00:03:09,430 --> 00:03:10,300 Like so. 48 00:03:12,980 --> 00:03:17,120 Now I can pass this node class into our Hello world publisher for it to inherit it. 49 00:03:17,940 --> 00:03:22,530 The first thing to do in any Python class object is to create our init function. 50 00:03:24,180 --> 00:03:26,950 And the first thing we will do is initialize the node. 51 00:03:26,970 --> 00:03:33,570 So we will use Python's super function to gain access to the init method from that node object we inherited 52 00:03:33,570 --> 00:03:35,070 from ROS client library. 53 00:03:35,890 --> 00:03:40,440 And just as a side note, by default, Vee's code will set the spaces to four. 54 00:03:40,450 --> 00:03:43,540 I'd like to prefer using tabs instead of spaces. 55 00:03:43,810 --> 00:03:48,940 So if I go ahead and highlight this, you can actually see individual dots which represent the spaces 56 00:03:48,940 --> 00:03:50,770 for which our code is organized. 57 00:03:50,770 --> 00:03:55,390 So if you want to change that, I'll just go ahead and hit shift tab and re tab that back in. 58 00:03:55,390 --> 00:03:59,950 And now instead of these spaces we are seeing the actual tab marks and I'll go ahead and do that for 59 00:03:59,950 --> 00:04:01,240 these lines as well. 60 00:04:02,990 --> 00:04:06,440 So this is totally personal preference, if you like, spaces versus tabs. 61 00:04:06,440 --> 00:04:10,820 But I figured it was worth noting for those of you who are new to Visual Studio Code. 62 00:04:11,300 --> 00:04:13,160 Now back to this node in IT object. 63 00:04:13,160 --> 00:04:17,030 So the node object can take in several different optional parameters and I can actually see that by 64 00:04:17,030 --> 00:04:20,270 going ahead and deleting these parentheses right now and recreating them. 65 00:04:20,270 --> 00:04:24,460 And here we see VS Code gives us a dialog window of all the optional parameters we can put in here. 66 00:04:24,470 --> 00:04:28,580 Now of course you could also check the SQL Pie Docs page for more information. 67 00:04:28,580 --> 00:04:33,170 However, the only required arguments is what we want the name of the node to be. 68 00:04:33,170 --> 00:04:34,760 In this case, let's name it. 69 00:04:34,790 --> 00:04:36,290 Hello World Pub Node. 70 00:04:36,530 --> 00:04:38,500 Now our node is initialized. 71 00:04:38,510 --> 00:04:40,010 Let's create our publisher. 72 00:04:40,220 --> 00:04:46,430 I will create a class member variable called pub and set it equal to the self dot create publisher function 73 00:04:46,430 --> 00:04:47,750 which we inherited. 74 00:04:47,750 --> 00:04:53,750 When we initialize the node above the create publisher Command takes in three required arguments. 75 00:04:53,750 --> 00:04:57,260 The first is the ROS message type we want for our publisher. 76 00:04:57,260 --> 00:05:02,540 Since we want to publish a hello world message, we can use the ROS string message type. 77 00:05:02,540 --> 00:05:06,020 We will learn more about ROS message types in future lectures. 78 00:05:06,020 --> 00:05:11,810 For now I will just import the string message from the standard messages ROS library and use it in our 79 00:05:11,810 --> 00:05:13,250 create publisher function. 80 00:05:14,390 --> 00:05:17,870 The second parameter is the topic name we want to publish over. 81 00:05:17,870 --> 00:05:20,180 I will just call it Hello World. 82 00:05:20,690 --> 00:05:26,750 Note that the topic nor node name can have spaces in them, which is why I use underscores to separate 83 00:05:26,750 --> 00:05:27,530 words. 84 00:05:28,380 --> 00:05:35,340 Now, the last argument for our publisher is the shows profile size cross stands for quality of Service. 85 00:05:35,590 --> 00:05:40,920 COS serves a similar purpose as the queue size did in Ross, one in which we are setting a queue depth 86 00:05:40,920 --> 00:05:42,600 to save old messages. 87 00:05:42,600 --> 00:05:47,760 That way, if our robot is on an unstable network connection between other raw systems, when we regain 88 00:05:47,760 --> 00:05:50,950 connection, we can get a backlog of the published messages. 89 00:05:50,970 --> 00:05:54,390 In this case, I will just set our publisher queue size to ten. 90 00:05:54,570 --> 00:05:57,320 With that, our publisher is actually created. 91 00:05:57,330 --> 00:06:02,040 Now we want to tell our node to keep publishing our Hello world message at a certain rate. 92 00:06:02,040 --> 00:06:07,140 This used to be done in Ross one using a rate object and a while loop to have your nodes keep completing 93 00:06:07,140 --> 00:06:12,930 the action within the while loop at the desired rate in Ross to a timer functionality was added to help 94 00:06:12,930 --> 00:06:17,610 simplify this so we can create a timer variable and set it equal to our nodes. 95 00:06:17,610 --> 00:06:18,930 Create timer function. 96 00:06:20,370 --> 00:06:25,590 This function takes in two parameters the rate of our timer in seconds and the callback function We 97 00:06:25,590 --> 00:06:30,900 want to run at that rate in this case, we're going to want to create a callback function to publish 98 00:06:30,900 --> 00:06:32,550 our Hello world message. 99 00:06:32,670 --> 00:06:37,140 So we will create a new function in our class called Publish Hello World. 100 00:06:37,530 --> 00:06:43,650 You can see I set the rate to 0.5, which means that every 0.5 seconds this function will run. 101 00:06:43,860 --> 00:06:45,990 So let's go ahead and create this function below. 102 00:06:49,490 --> 00:06:53,540 The first thing we need to do before we publish any data is to create the message object we're going 103 00:06:53,540 --> 00:06:54,320 to be using. 104 00:06:54,500 --> 00:06:59,360 In this case, we are using a string from the Standard Messages library so we can initialize it like 105 00:06:59,360 --> 00:07:04,010 so this string object only has one attribute which is called data. 106 00:07:04,160 --> 00:07:08,720 So all we need to do to load our data into this variable is to set the string message data attribute 107 00:07:08,720 --> 00:07:10,350 to the data we want to send. 108 00:07:10,370 --> 00:07:12,890 In this case, a string saying Hello world. 109 00:07:13,010 --> 00:07:19,160 Now that our message data is loaded, let's publish the message by using our pub variables publish function 110 00:07:19,160 --> 00:07:20,870 which takes in our message. 111 00:07:21,080 --> 00:07:21,650 Great. 112 00:07:21,650 --> 00:07:24,020 So let's quickly review what's going on in this code. 113 00:07:24,020 --> 00:07:30,950 So far we created a class called Hello World Publisher, which inherited the functionality of the node 114 00:07:30,950 --> 00:07:33,200 object from the ROS client library. 115 00:07:33,350 --> 00:07:39,950 We initialized our node, created a publisher which publishes string messages over the Hello world topic 116 00:07:40,100 --> 00:07:44,000 and then created a timer which will run our publish Hello world function. 117 00:07:44,000 --> 00:07:44,930 Within our publish. 118 00:07:44,930 --> 00:07:50,510 Hello world function we create a string message load our hello world string data into the message and 119 00:07:50,510 --> 00:07:53,630 then tell our publisher to publish this message. 120 00:07:53,930 --> 00:08:00,110 Now, one more thing I will add to this is a counter to count up every time we publish a message so 121 00:08:00,110 --> 00:08:02,900 we can see some variation in the data coming over this topic. 122 00:08:02,900 --> 00:08:06,680 So I will create a counter variable when we initialize our class. 123 00:08:07,010 --> 00:08:12,770 Then in our publish Hello world function, include it within our message data and add one after we publish 124 00:08:12,770 --> 00:08:13,610 the message. 125 00:08:16,430 --> 00:08:17,030 With that. 126 00:08:17,030 --> 00:08:18,860 I am now done with my class. 127 00:08:18,860 --> 00:08:22,430 So back in my main function I can create an instance of this class. 128 00:08:22,580 --> 00:08:26,930 I will just call it my pub then to make it look a little bit nicer. 129 00:08:26,930 --> 00:08:30,710 When we run this in terminal, let's print out that our publisher node is running. 130 00:08:31,430 --> 00:08:35,810 Now we are going to want this Python script to keep running in the terminal until we tell it to stop 131 00:08:35,810 --> 00:08:36,860 by hitting control. 132 00:08:36,860 --> 00:08:43,370 C So to stop our Python scripts from finishing right after we create our My Pub variable, I will use 133 00:08:43,370 --> 00:08:48,950 the ROS client library spin function, which we will run until there is a keyboard interrupt. 134 00:08:48,950 --> 00:08:52,280 So I will wrap it in a try except statement like so. 135 00:08:57,950 --> 00:09:02,510 Now, when there is a keyboard interrupt, I'm going to want to stop my node by calling the destroy 136 00:09:02,510 --> 00:09:03,560 node function. 137 00:09:05,370 --> 00:09:09,720 With that, I have finished my publisher script, so let's save it, which you can do with either the 138 00:09:09,720 --> 00:09:14,450 control se shortcut key or by heading over to file and save. 139 00:09:14,460 --> 00:09:18,870 And then we can go ahead and open up a terminal and then we'll change it to the directory which our 140 00:09:18,870 --> 00:09:24,330 script is located in and run the Python three command, followed by the name of our script, which is 141 00:09:24,330 --> 00:09:26,460 publisher Pi OC. 142 00:09:26,490 --> 00:09:28,980 We ran the command and got that the publisher node running. 143 00:09:28,980 --> 00:09:30,180 Print it to the screen. 144 00:09:30,180 --> 00:09:33,000 We can check this by opening a new terminal tab. 145 00:09:33,720 --> 00:09:36,840 And using the Ross to node list command. 146 00:09:37,110 --> 00:09:38,040 Here we see. 147 00:09:38,040 --> 00:09:43,500 Hello World Pub node is one of the nodes running and that's the name we defined when we called our init 148 00:09:43,500 --> 00:09:46,350 method from the node class we inherited. 149 00:09:47,070 --> 00:09:51,570 You can also check for topics by using the ROS two topic list command. 150 00:09:51,870 --> 00:09:57,090 And here we see our default ROS out and parameter events topics which you'll see by default when ROS 151 00:09:57,090 --> 00:09:57,690 is running. 152 00:09:57,690 --> 00:10:02,880 And our Hello world topic which we defined for our publisher here. 153 00:10:05,060 --> 00:10:10,100 Last but not least, we can see the data coming in over the topic by running the Ross two topic Echo 154 00:10:10,100 --> 00:10:14,360 command, followed by the name of the topic, which as we know is Hello World. 155 00:10:15,930 --> 00:10:21,540 And here we see the Hello World data coming in, as well as this number incrementing up with each time 156 00:10:21,540 --> 00:10:22,480 it prints out. 157 00:10:22,500 --> 00:10:28,140 So I'll go ahead and stop that with control C And to stop your publisher node, you can click on that 158 00:10:28,140 --> 00:10:30,660 terminal tab and hit Control C as well. 159 00:10:30,660 --> 00:10:34,020 And we see the terminating nodes and now none of our nodes are running. 160 00:10:34,020 --> 00:10:41,280 And if we run across to node list, we see that indeed our Hello world hub node has stopped. 161 00:10:42,190 --> 00:10:43,330 Congratulations With that. 162 00:10:43,330 --> 00:10:47,710 You have successfully created your very own Ras to publisher in Python. 163 00:10:48,840 --> 00:10:54,880 In this lecture, we learned how to use the ROS client Library Python API to create a publisher node. 164 00:10:54,900 --> 00:10:59,970 We imported the ROS client library, node class and string message types. 165 00:11:00,450 --> 00:11:03,690 We'll talk a bit more about message types later on in the course. 166 00:11:04,140 --> 00:11:09,480 Then we created our own class, which is based off of the ROS client libraries Node object. 167 00:11:09,570 --> 00:11:14,520 We then use the super init call to initialize our node with a node name. 168 00:11:14,730 --> 00:11:19,210 We then use the create publisher function, which takes in the message type, topic, name and queue 169 00:11:19,230 --> 00:11:19,820 size. 170 00:11:19,830 --> 00:11:25,320 And lastly, we utilize a create timer function to run a callback function at a particular rate. 171 00:11:25,830 --> 00:11:31,950 In this callback function we initialize our message object, which in this case was our string message. 172 00:11:32,010 --> 00:11:36,450 We then loaded the data attribute of it with the data we wanted to send. 173 00:11:36,900 --> 00:11:41,790 Then we call the publish function of our publisher variable, which takes in the message object and 174 00:11:41,790 --> 00:11:43,740 publishes it over Ros. 175 00:11:44,190 --> 00:11:49,890 Lastly, within our main function we initialize our ROS communications and use the spin function to 176 00:11:49,890 --> 00:11:51,000 keep the node running. 177 00:11:51,000 --> 00:11:57,330 And then if an exception occurred, we then destroyed our node to stop our ROS communications.