WEBVTT 00:00.080 --> 00:06.410 Now that you know what parameters are, let's create one inside a Python node and see how to use it. 00:06.440 --> 00:11.810 So in this lesson we will directly dive into the code, add the parameters, and then see how to provide 00:11.810 --> 00:13.310 them at runtime. 00:13.310 --> 00:15.860 And for this I'm not going to create a new node. 00:15.860 --> 00:22.700 I'm going to come back to an existing node here to the number publisher node that we have created in 00:22.700 --> 00:25.160 an activity for the topics. 00:25.190 --> 00:25.610 All right. 00:25.640 --> 00:27.860 So make sure you have this node in this node. 00:27.860 --> 00:28.700 What do we do. 00:28.730 --> 00:31.010 Well very simply put we create. 00:31.100 --> 00:32.960 So we have a number variable here. 00:32.990 --> 00:34.790 The number was two by default. 00:34.790 --> 00:36.080 And then we have a publisher. 00:36.080 --> 00:39.560 We have a timer and we publish that number every. 00:39.590 --> 00:41.960 Well you see every one second. 00:41.990 --> 00:42.380 Okay. 00:42.410 --> 00:45.590 So by now you should be quite familiar with this code structure. 00:45.620 --> 00:49.700 Now what I'm going to do is you can see that we publish the number two. 00:49.700 --> 00:51.350 And two is hard coded here. 00:51.350 --> 00:55.760 What if I want to start the node and publish a different number? 00:55.790 --> 01:00.260 Or what if I want to start the node and use a different Around frequency or here. 01:00.260 --> 01:02.240 It's a period for the timer. 01:02.270 --> 01:02.780 Okay. 01:02.810 --> 01:03.920 I will need to. 01:03.950 --> 01:09.320 Well I will need to modify those values directly into the code and then maybe build the package again. 01:09.320 --> 01:11.840 So I'm going to use parameters for those two. 01:11.870 --> 01:12.230 All right. 01:12.230 --> 01:13.520 And how to use parameters. 01:13.520 --> 01:16.430 Well we're going to do that in the constructor here. 01:16.430 --> 01:19.250 And usually we do that before the rest. 01:19.250 --> 01:21.770 So before the publishers the timers etc.. 01:21.770 --> 01:25.010 And there are two things you need to do for each parameter okay. 01:25.010 --> 01:26.420 It's a two step process. 01:26.420 --> 01:30.140 You need to declare the parameter and then get the parameter value. 01:30.170 --> 01:32.810 Okay I'm going to come back with more details about this. 01:32.810 --> 01:38.030 So first you need to declare all the parameters that you want with self dot. 01:38.030 --> 01:45.500 So I'm using self to get the functionalities from the node class here declare parameter okay. 01:45.530 --> 01:47.570 You need to give it a name. 01:48.170 --> 01:51.110 So let's put number okay. 01:51.110 --> 01:56.780 So we will have a number parameter inside the number publisher node. 01:57.830 --> 02:02.030 And then there are different ways to create a parameter, but I think the best overall is just to give 02:02.060 --> 02:03.890 a default value. 02:03.890 --> 02:07.490 So for example, the default value is going to be two okay. 02:07.520 --> 02:11.960 That's going to be the default value if we don't provide any parameter as we will see when we start 02:11.960 --> 02:12.530 the node. 02:12.560 --> 02:12.920 All right. 02:12.920 --> 02:15.290 So what this will do. 02:15.320 --> 02:18.710 It will create a number parameter for the node. 02:18.740 --> 02:21.140 And the default value will be two. 02:21.170 --> 02:22.220 But that's not all. 02:22.220 --> 02:26.900 It will also set the data type for the parameter and how it's going to do that. 02:26.930 --> 02:31.280 Well it's going to use the data type you provide for the default value. 02:31.280 --> 02:32.870 So this is very important. 02:32.900 --> 02:34.760 You see this is an integer. 02:34.760 --> 02:39.890 So we have declared a number parameter of type integer. 02:39.890 --> 02:41.840 And the default value is two. 02:41.870 --> 02:43.400 Now let's create another one. 02:43.400 --> 02:48.890 So actually let's declare another one self dot declare parameter. 02:48.890 --> 02:52.940 And let's call it timer period. 02:52.970 --> 02:54.890 That's what we're going to use here. 02:55.040 --> 02:57.860 And this one you see we have a float number. 02:57.890 --> 02:59.690 I'm going to use 1.0. 02:59.690 --> 03:03.290 So we create a time period parameter. 03:03.320 --> 03:06.110 Its default value is 1.0. 03:06.110 --> 03:09.740 And thus this is a float number. 03:09.830 --> 03:10.040 Okay. 03:10.070 --> 03:13.070 So those two parameters have a different data type. 03:13.100 --> 03:13.490 All right. 03:13.490 --> 03:15.740 So we have declared two parameters. 03:15.770 --> 03:16.370 That's great. 03:16.370 --> 03:18.260 But now it's not going to do anything. 03:18.260 --> 03:22.130 So those parameters are going to be created kind of inside the node. 03:22.130 --> 03:27.050 But we don't use them in the code and we don't get the value in the code. 03:27.050 --> 03:30.560 So after you declare a parameter you need to get its value. 03:30.560 --> 03:36.050 And to get the value, we can use the get parameter method from the node class. 03:36.050 --> 03:37.550 And where we do this. 03:37.580 --> 03:47.060 Well for example here I have self number instead of two I will do self dot get parameter like this. 03:47.060 --> 03:48.920 And I will need to provide the name. 03:49.130 --> 03:51.620 And the name is number okay. 03:51.650 --> 03:53.300 That's the exact same name here. 03:53.300 --> 03:57.840 So I declare it and then I get its value, then that's quite important. 03:57.870 --> 04:00.390 This is actually not the value. 04:00.420 --> 04:06.780 This is a parameter object and you need to do dot value okay to get the property. 04:06.780 --> 04:14.400 So to get the value property inside the parameter object from the CLP parameter class. 04:14.400 --> 04:19.560 And very important here you might be tempted to add parentheses but there is no parentheses. 04:19.560 --> 04:20.940 This is not a method. 04:20.940 --> 04:23.070 This is a property okay. 04:23.100 --> 04:24.210 So this is quite tricky. 04:24.210 --> 04:26.250 And this is a source of errors. 04:26.250 --> 04:28.170 Make sure you don't put any parentheses. 04:28.740 --> 04:29.040 Okay. 04:29.070 --> 04:31.380 So we self get parameter. 04:31.470 --> 04:35.850 And then the name of the parameter dot value you get the parameter value. 04:35.850 --> 04:38.580 And I will explain a bit more about this when we run the node. 04:38.610 --> 04:43.260 Now let's let's actually create a new attribute here. 04:43.260 --> 04:46.440 Let's call it timer period. 04:46.440 --> 04:55.320 You could name it as you want self get parameter with timer period Okay. 04:55.500 --> 04:57.810 And then dot value. 04:58.020 --> 04:59.220 As simple as that. 04:59.250 --> 05:01.800 And where do we use that. 05:01.830 --> 05:05.640 Well we use that in here in the create timer function. 05:05.670 --> 05:09.390 So self dot timer period. 05:09.660 --> 05:11.730 So now the code is complete. 05:11.730 --> 05:14.970 As you can see for each parameter we declare it. 05:14.970 --> 05:16.350 And then we get it. 05:16.380 --> 05:19.140 And then well we save it inside an attribute. 05:19.140 --> 05:23.700 And for example we use the self number here to publish the number with the publisher. 05:23.700 --> 05:30.000 And we use the timer period to create the timer period for the timer here to publish the number. 05:30.000 --> 05:32.010 So I'm going to save that. 05:32.010 --> 05:34.410 And let's see let's run it. 05:34.410 --> 05:38.430 So let's see how this works because maybe it's still a bit confusing for you. 05:38.460 --> 05:41.370 When we run it it's going to make much more sense. 05:41.370 --> 05:50.490 So let's go back to our workspace and let's do a call on build just in case with packages. 05:50.490 --> 05:56.430 Select let's build a Python package with Simulink install. 05:58.290 --> 05:58.800 Okay. 05:58.800 --> 06:00.570 And here let's source. 06:01.800 --> 06:11.880 So let's source the workspace and let's run that node Ros to run my pi PG with number publisher. 06:12.150 --> 06:17.100 So I just add it like this Ros to run package executable as always. 06:17.100 --> 06:18.780 And well it's working. 06:18.990 --> 06:19.980 It's the same thing. 06:19.980 --> 06:22.440 So nothing really changed here. 06:22.440 --> 06:28.980 And now let's let's do this here Ros two param list. 06:30.360 --> 06:30.600 Okay. 06:30.630 --> 06:34.980 So you can list all the parameters that exist for a node with Ros two param list. 06:35.010 --> 06:39.240 As you can see we had Ros two service Ros two topics for each functionality. 06:39.240 --> 06:42.150 Basically we have a new Ros2 command line. 06:42.150 --> 06:44.490 And you see we have our number publisher node. 06:44.490 --> 06:46.740 And we have a number parameter. 06:46.740 --> 06:48.870 And also the timer period parameter. 06:48.870 --> 06:50.280 We also have two more parameters. 06:50.280 --> 06:55.290 I'm going to come back to those later, those are going to be started for every node that you create. 06:55.320 --> 06:59.130 Okay, so you can see we already had two parameters and we can add more parameters. 06:59.130 --> 07:06.180 So every time we declare a parameter then you see we have a parameter kind of registered inside the 07:06.180 --> 07:06.780 node. 07:07.050 --> 07:15.840 And now if I do ros2 if I do Ros two topic equal with the number topic, you see that we are going to 07:15.870 --> 07:18.570 publish two every one second. 07:18.600 --> 07:22.590 You could verify the frequency also, but it's one second here. 07:22.590 --> 07:24.120 So what happened. 07:24.270 --> 07:29.520 Well you declare a parameter with a default value okay. 07:29.550 --> 07:31.110 That's going to create the parameter. 07:31.110 --> 07:35.250 So that's why you see the parameter here when you do Ros two paramlist for that node here. 07:35.250 --> 07:41.310 But then we get the parameter value because we haven't provided anything here. 07:41.340 --> 07:46.950 Then we are going to use the default value which is two and one here. 07:46.980 --> 07:52.350 Now let's stop that node And let's run it again. 07:52.350 --> 07:54.480 But this time I'm going to add. 07:54.480 --> 07:57.000 So let's put it a bit bigger here. 07:57.780 --> 08:01.740 I'm going to add dash dash rows args okay. 08:01.770 --> 08:06.780 Just like when we wanted to rename a node, remap a topic or add parameters. 08:06.780 --> 08:10.680 Here you need to add ros args and this only once. 08:10.710 --> 08:16.560 Then if you want to provide a parameter value, you can do dash p and then you provide the name of the 08:16.560 --> 08:22.380 parameter which is number colon equal the value. 08:22.380 --> 08:24.780 So let's say three. 08:24.810 --> 08:25.260 All right. 08:25.260 --> 08:30.630 So what we do here is we start the node and we set the parameter value to three. 08:30.630 --> 08:34.920 So in this case the default value here is not going to be used. 08:34.950 --> 08:37.830 What we're going to use is the value we provide here. 08:37.830 --> 08:41.400 And I can add as many parameters as I want with another dash p. 08:41.580 --> 08:46.290 And once again only once ros args and then as many p as you want. 08:46.320 --> 08:50.850 And the other one is timer Period. 08:51.600 --> 08:55.500 And this is a float number let's say 0.5. 08:55.500 --> 09:00.090 So we want to publish the number three every 0.5 seconds. 09:00.120 --> 09:02.010 Let's press enter. 09:03.030 --> 09:03.480 Okay. 09:03.600 --> 09:05.250 Still working the same. 09:05.250 --> 09:07.470 And let's just verify that with Ros2. 09:07.500 --> 09:09.000 Topic equal number. 09:10.140 --> 09:14.730 And you can see now we publish three every 0.5 seconds. 09:15.090 --> 09:15.600 All right. 09:15.630 --> 09:17.070 Because what happened. 09:17.490 --> 09:23.280 We first declare the parameters so that the number parameter and time period parameter exist. 09:23.280 --> 09:28.440 And then we get the value that we provide at runtime. 09:28.440 --> 09:30.420 You see with this p argument. 09:30.420 --> 09:32.280 So we get the value from there. 09:32.310 --> 09:35.160 And we save that value inside the code. 09:35.160 --> 09:37.050 So then we can use that value. 09:37.080 --> 09:37.470 Okay. 09:37.500 --> 09:38.520 So two step. 09:38.520 --> 09:41.580 Once again you declare a parameter with a default value. 09:41.580 --> 09:43.650 And then you get the parameter value. 09:43.650 --> 09:48.310 And then if you provide the parameter value here that's going to be set in the code. 09:48.310 --> 09:52.810 If you don't, we're going to use the default value so we can. 09:52.840 --> 09:54.190 I'm going to stop that. 09:54.370 --> 09:55.990 And I can run the command again. 09:55.990 --> 09:58.840 But for example just provide one parameter. 09:58.840 --> 10:03.310 If I just provide one then let's see what happens. 10:03.970 --> 10:05.230 We're going to publish the number three. 10:05.230 --> 10:08.620 But you see every one second. 10:09.400 --> 10:11.380 Because if I go back here what's going to happen. 10:11.380 --> 10:12.940 We declare those two parameters. 10:12.940 --> 10:14.590 Then we get this one. 10:14.590 --> 10:16.330 We have provided it from the terminal. 10:16.330 --> 10:18.880 So the number is going to be three. 10:19.000 --> 10:22.750 But then the timer period we haven't provided it in the terminal. 10:22.750 --> 10:26.320 So we just get the default value which is one okay. 10:26.350 --> 10:28.030 So you can see it's quite modular. 10:28.030 --> 10:32.080 And the node now is more dynamic because you can start the node again. 10:32.110 --> 10:33.610 You don't need to compile anything. 10:33.610 --> 10:37.600 You can just start it and provide any value you want. 10:37.630 --> 10:41.410 Then just make sure that you respect the data type. 10:41.440 --> 10:50.980 For example, if I want to put 3.4 for the number, Then you see we get an error because the number 10:51.010 --> 10:56.440 parameter here, as I told you, the type is going to be defined by the default value. 10:56.440 --> 10:58.270 So the type is integer. 10:58.420 --> 11:01.990 And we provide a float number or a double number. 11:01.990 --> 11:09.670 And you can see the error we get is here number of type double expecting type integer. 11:09.670 --> 11:15.850 So make sure that when you start a node and you provide parameters, make sure that you provide a value 11:15.880 --> 11:17.650 that is valid. 11:17.650 --> 11:22.480 So with a correct data type that corresponds to the one you have set in the code. 11:22.480 --> 11:23.830 And now it's working. 11:23.830 --> 11:27.850 And now you can see I'm publishing the number seven. 11:29.290 --> 11:29.740 Great. 11:29.740 --> 11:34.510 And that's basically it for how to use parameters in your code. 11:34.510 --> 11:38.140 You just add one line to declare parameter with a default value. 11:38.140 --> 11:41.080 And then you get the parameter and you can just use it. 11:41.080 --> 11:46.300 And then to provide the value at runtime you just add the value like that.