WEBVTT 00:00.080 --> 00:06.830 So they are actually three languages you can use to write large files XML, YAML and Python. 00:06.830 --> 00:11.660 We have just seen XML version in the last video here with this large file. 00:11.690 --> 00:16.190 As for YAML, well first it's not really used by anyone. 00:16.190 --> 00:21.950 And second, I find the XML version much clearer and less prone to errors. 00:21.950 --> 00:23.960 So I'm going to skip YAML anyway. 00:23.960 --> 00:25.220 Nobody uses that. 00:25.220 --> 00:26.540 Then we have Python. 00:26.540 --> 00:33.110 And why I'm talking about Python here is because Python is used quite a lot for large files in ros2. 00:33.140 --> 00:36.590 In fact, that was the first choice at the beginning of ros2. 00:36.590 --> 00:40.940 And now things are slowly switching back to favor XML. 00:40.940 --> 00:44.930 And in this lesson you will understand why we want to use XML. 00:44.960 --> 00:50.870 I can give you a spoiler already, so I like simplicity and I dislike Overengineering. 00:50.870 --> 00:57.710 I am 100% personally for using XML launch files whenever you can instead of Python. 00:57.740 --> 01:01.430 Okay, so this video is going to be a bit opinionated. 01:01.430 --> 01:08.040 With that said, let's get started and let's create a Python launch file that does the exact same thing 01:08.040 --> 01:10.770 as what we did here with this XML one. 01:10.770 --> 01:15.150 So we want to start the number publisher node and the number counter node. 01:15.150 --> 01:21.000 And just a quick detail when I talk about Python here, I'm talking about writing a launch file in Python, 01:21.000 --> 01:24.570 not about the node that is written in Python or C plus. 01:24.570 --> 01:27.300 Plus those are completely different things okay. 01:27.330 --> 01:29.640 So how to create a Python launch file. 01:29.640 --> 01:34.380 Well you first have to create a package like we did my robot bring up. 01:34.380 --> 01:38.820 And to set it up with the install launch file instruction. 01:38.820 --> 01:40.230 But that is done already. 01:40.230 --> 01:42.660 So what we can do is just add a new file. 01:42.660 --> 01:45.900 We can add a new launch file inside our launch folder. 01:45.930 --> 01:46.410 Okay. 01:46.440 --> 01:50.190 You can mix XML and Python files as you want here. 01:50.190 --> 01:51.720 It's not going to be a problem. 01:51.720 --> 01:56.100 And because we're going to do the exact same thing I'm going to also name it number app. 01:56.190 --> 02:01.350 And then it's going to be dot launch and then dot pi okay. 02:01.380 --> 02:07.040 The only difference is that we have a dot pi extension at the end because that's a Python file. 02:07.040 --> 02:09.050 Now let's write this. 02:09.050 --> 02:20.210 And first we will need to create a function that's named generate launch description okay. 02:20.240 --> 02:23.000 You need to write exactly this okay. 02:23.030 --> 02:26.390 Generate launch description exactly like that. 02:26.390 --> 02:31.940 In this launch description we are going to create a launch description object. 02:31.940 --> 02:35.630 So we need to do from launch okay. 02:35.660 --> 02:45.320 There is a ros2 Python module called launch import launch description okay. 02:45.350 --> 02:49.730 If you ever write Python launch files I encourage you to have the auto completion. 02:49.760 --> 02:52.760 Otherwise that's going to be a complete nightmare. 02:52.760 --> 02:54.410 And let's create. 02:54.410 --> 02:58.700 So usually we do LD is equal to launch description. 02:58.700 --> 03:00.830 So we create a launch description object. 03:00.830 --> 03:03.470 And at the end so we could return it directly. 03:03.470 --> 03:05.670 But we're going to add stuff to it at the end. 03:05.700 --> 03:08.880 We're going to do return launch description. 03:09.120 --> 03:09.660 Okay. 03:09.660 --> 03:13.320 So that's basically the minimum code you need for a launch file. 03:13.470 --> 03:17.910 You have this function here generate launch description. 03:17.940 --> 03:21.810 Then you create a launch description object that you return. 03:21.870 --> 03:24.870 Now of course that's going to do nothing. 03:24.870 --> 03:30.120 So we are going to create some nodes and add the nodes inside the launch description. 03:30.120 --> 03:41.160 For that I will need to do from launch underscore Ros dot actions import node. 03:41.190 --> 03:45.060 If it doesn't make any sense well don't even try to make sense of that. 03:45.060 --> 03:47.430 I have completely given up at this point. 03:47.430 --> 03:53.400 And then so let's call it number publisher or number publisher node. 03:53.730 --> 03:55.770 That's going to be a node object. 03:55.800 --> 03:56.070 Okay. 03:56.100 --> 03:57.210 So we create a node object. 03:57.210 --> 04:00.300 But note this is a launch file node object okay. 04:00.330 --> 04:04.950 That's nothing to do with the node object we had in the code for the node itself. 04:04.950 --> 04:08.960 And then in this we can provide a lot of different parameters. 04:08.960 --> 04:12.740 For example package okay. 04:12.770 --> 04:14.630 And the package is going to be. 04:14.630 --> 04:20.690 So if I want to start the same thing I'm going to use the my pi pkg. 04:20.990 --> 04:26.660 And then I have executable which is okay. 04:26.690 --> 04:34.100 It's this one executable which is number publisher okay. 04:34.160 --> 04:36.230 And then I can add more stuff if I want. 04:36.260 --> 04:38.270 I'm going to keep it like this for now. 04:38.270 --> 04:40.010 So that's my first node. 04:40.010 --> 04:43.160 Second node is going to be number counter. 04:43.160 --> 04:45.260 So I just create a variable for that. 04:46.130 --> 04:48.230 It's going to be node okay. 04:48.230 --> 04:51.230 And that's going to be the same thing package. 04:51.830 --> 04:57.920 Let's use the C plus plus package and execute table. 04:58.550 --> 05:04.040 That's going to be number counter okay. 05:04.040 --> 05:07.700 So we have created I mean we have our two nodes here. 05:07.700 --> 05:13.410 And then to say that we want to start them, because this is going to do nothing to say that we want 05:13.410 --> 05:16.920 to start them, we need to add them to the launch description. 05:16.920 --> 05:23.370 So ld dot add action, we need to use the add action function. 05:23.370 --> 05:27.810 And we need to provide number publisher. 05:27.810 --> 05:34.560 And then we need to do ld dot add action with the number counter. 05:35.310 --> 05:35.850 All right. 05:35.850 --> 05:38.130 So you create a launch description. 05:38.130 --> 05:39.900 Then you create all your nodes. 05:39.900 --> 05:45.540 You add your nodes to the launch description and then you return the launch description. 05:45.690 --> 05:46.770 So let's save. 05:46.770 --> 05:50.550 And that's basically the same thing as this launch file. 05:50.670 --> 05:52.380 You see here we have four lines. 05:52.530 --> 05:54.480 And here I have 20 lines. 05:54.480 --> 05:58.800 I could reduce the spaces but basically well you see it's much bigger. 05:58.800 --> 06:02.430 And that's just the basic functionality of starting a node. 06:02.460 --> 06:02.640 Okay. 06:02.670 --> 06:09.650 We haven't even tried to import other launch files or import configurations from somewhere else. 06:09.650 --> 06:12.050 In this case, we're going to have many, many more. 06:12.050 --> 06:12.710 Includes. 06:12.710 --> 06:14.510 It's going to become much more verbose. 06:14.510 --> 06:17.690 Well, I guess you see the point here already. 06:17.690 --> 06:19.550 And actually I thought at the beginning. 06:19.550 --> 06:24.680 So at the beginning of Ros2, I thought the Python launch files could be really great features, because 06:24.680 --> 06:27.530 then you can use Python code, you can use for loops. 06:27.530 --> 06:33.380 You can well, you can use just Python logic inside this description. 06:33.380 --> 06:39.020 But in the end what I realized is, well, this is the theory, but then this is what you actually do 06:39.050 --> 06:41.630 in the end and what everybody does for the project. 06:41.630 --> 06:44.990 And I found that we don't really use that logic that much. 06:44.990 --> 06:51.470 And for everything we want to do, even a condition, if you want to do if to start some arguments in 06:51.500 --> 06:55.370 a node, you will have to use some very complicated imports. 06:55.400 --> 06:55.670 Okay. 06:55.700 --> 07:03.200 And in the end, I feel like this Python launch, uh, functionality basically uses a complicated syntax 07:03.200 --> 07:05.870 just to provide a description. 07:05.900 --> 07:12.060 Okay, that's a description we are trying to recreate with Python, but that's basically why XML was 07:12.060 --> 07:16.710 created in the first place is to do a description and it does it pretty well. 07:16.710 --> 07:22.110 So I would recommend that any time you can use XML, just use XML. 07:22.140 --> 07:24.570 Now the thing is some functionality. 07:24.600 --> 07:30.240 So some more advanced functionalities may be only available for Python. 07:30.240 --> 07:34.650 So what if you absolutely need to start one functionality using Python. 07:34.680 --> 07:37.860 Do you then need to write the entire launch file in Python? 07:37.860 --> 07:41.520 Or also lots of existing launch files are written in Python? 07:41.520 --> 07:42.900 What if you need to import one? 07:42.900 --> 07:50.490 Well, the good news is that you can easily import a Python launch file inside an XML launch file. 07:50.580 --> 07:52.410 And I'm going to show you that right now. 07:52.410 --> 07:54.990 So I'm going to create a new launch file here. 07:55.020 --> 08:06.210 New XML launch file named I'm going to call it number app from Python dot launch dot XML okay so the 08:06.210 --> 08:07.680 title is quite long. 08:07.680 --> 08:10.410 But then you see we have the dot launch dot XML. 08:10.500 --> 08:18.010 And I'm going to start with my lounge tag that I open and that I close. 08:18.460 --> 08:26.170 Now let's say that from that XML lounge file, I need to include this Python lounge file here. 08:26.260 --> 08:27.340 What do I do? 08:27.370 --> 08:34.660 Well, I can just use the include tag and file argument with okay. 08:34.690 --> 08:37.180 There is a function here that you can use. 08:37.180 --> 08:40.270 You're going to put dollar sign and parentheses. 08:40.300 --> 08:46.420 This function is named find dash pg dash share. 08:46.420 --> 08:49.510 So it's going to find the share folder inside the package. 08:49.510 --> 08:50.830 And you see the name share. 08:50.860 --> 08:53.860 Here we have the same in Cmakelists.txt. 08:54.280 --> 08:54.790 Okay. 08:54.790 --> 09:00.730 So that's important that you write share here for this folder because this function is going to find 09:00.730 --> 09:01.030 it. 09:01.030 --> 09:07.900 So we put find PG share and then the package name for my robot bring up. 09:07.930 --> 09:11.530 In this case we are including a launch file from the same package. 09:11.710 --> 09:13.060 It could be another one. 09:13.110 --> 09:13.620 Okay. 09:13.620 --> 09:19.380 So then you can do slash launch slash and then the name. 09:19.380 --> 09:24.630 So number app dot launch dot pi. 09:24.900 --> 09:27.210 And then you close the includes. 09:27.210 --> 09:31.650 So you see with one line you can include another file. 09:31.650 --> 09:33.780 And it doesn't have to be an XML file. 09:33.780 --> 09:37.950 You can include a Python launch file inside an XML launch file. 09:37.950 --> 09:40.860 The only thing to know is this syntax. 09:40.860 --> 09:42.330 We find pkg share. 09:42.330 --> 09:43.980 But that's going to be the same every time. 09:44.010 --> 09:45.720 All right so I can save that. 09:45.720 --> 09:51.690 And you can see that if I wanted to create an XML launch file with some stuff and then include that 09:51.690 --> 09:54.330 one, I can just do this. 09:54.330 --> 10:00.300 So then I can just write XML, launch files for everything and just include what I need. 10:00.300 --> 10:04.320 If I need to import another launch file which is written in a different language. 10:04.350 --> 10:04.740 All right. 10:04.740 --> 10:06.690 We haven't tested that already. 10:06.690 --> 10:15.610 So let's just open a terminal and let's go to our workspace and let's build click on build packages 10:15.610 --> 10:20.980 select with my robot bring up. 10:21.430 --> 10:25.270 So we need to build every time because that's going to install the folder. 10:25.270 --> 10:29.080 There's nothing to compile, but it's going to install the folder and the new files. 10:29.080 --> 10:33.040 Then I'm going to well let's open two terminals. 10:33.040 --> 10:35.260 So those are going to be sourced. 10:35.500 --> 10:41.380 And we can do Ros two launch my robot bring up. 10:41.380 --> 10:45.280 And let's start for example the Python launch file that we have created. 10:45.280 --> 10:53.770 So my uh no it's called number app dot launch dot Pi. 10:55.000 --> 10:59.410 And you can see it's starting just like the XML one. 10:59.410 --> 11:05.530 So exactly the same functionalities to stop it I can also press Ctrl C okay. 11:05.560 --> 11:06.670 And I stop the node. 11:06.670 --> 11:08.530 And now what if I want to start. 11:08.530 --> 11:10.180 So let's start this one. 11:10.270 --> 11:13.840 This XML launch file that includes a Python one. 11:14.230 --> 11:15.880 Let's start this one Ros two. 11:15.900 --> 11:23.130 So Rose two launch number app from Python that launch that XML. 11:24.630 --> 11:26.490 And you can see it's the same thing. 11:26.880 --> 11:27.960 Exactly the same thing. 11:27.960 --> 11:32.730 If I do a graph we have the same stuff. 11:32.730 --> 11:35.490 So the application was correctly started. 11:36.060 --> 11:36.510 All right. 11:36.510 --> 11:42.990 So in the following of this course what I'm going to do is I'm going to use XML okay. 11:43.890 --> 11:45.480 And I encourage you to do the same. 11:45.480 --> 11:47.580 This is going to make your life much easier. 11:47.580 --> 11:52.890 Then later on if you encounter some Python launch files well you know the basic structure. 11:52.890 --> 11:57.030 And then you might need to find existing codes on GitHub. 11:57.030 --> 11:59.970 Maybe if you need to modify one Python launch file. 11:59.970 --> 12:05.940 So there is nothing really to understand about all the imports, you just have to find them and basically 12:05.940 --> 12:08.460 copy paste from stuff that you find on GitHub. 12:08.460 --> 12:10.740 So that's if you need to work with Python launch files. 12:10.740 --> 12:15.090 And if you need to include a Python launch file in your application, you know that from an XML launch 12:15.090 --> 12:18.150 file, you can just do that and it's going to work.