WEBVTT 00:00.240 --> 00:07.410 Let's talk about a very important topic if you want to be a great programmer, and that is called debugging. 00:07.620 --> 00:11.000 You see, nobody writes perfect programs. 00:11.010 --> 00:13.410 It's honestly impossible. 00:13.440 --> 00:17.430 We're always going to make mistakes and programs are constantly being improved. 00:17.640 --> 00:25.320 We constantly improve our code as we find more and more mistakes because programs are full of errors 00:25.320 --> 00:29.730 or what we call bugs or exceptions at runtime when the code is running. 00:30.300 --> 00:38.790 You see, the act of finding and removing these bugs or errors from our code is called debugging. 00:39.000 --> 00:46.470 In a big chunk of our time as programmers is debugging code, whether it's our own code or other people's 00:46.470 --> 00:49.560 code, and figuring out, hey, how come this doesn't work? 00:50.010 --> 00:54.690 And the senior developers are really, really good at debugging. 00:55.380 --> 01:01.320 So let's talk about in this video about some of my recommendations for debugging and what some good 01:01.320 --> 01:02.370 practices are. 01:02.850 --> 01:07.380 Now, keep in mind that you get better at debugging code as you code more and more. 01:07.380 --> 01:12.150 So the best thing you can do is to just keep practicing, keep writing Python code. 01:13.230 --> 01:14.940 So what are some of my tips? 01:14.970 --> 01:19.320 Well, first, you definitely want to use some lending. 01:19.380 --> 01:21.250 And what is lending? 01:21.270 --> 01:25.070 Well, we've talked about it right when we set up our developer environment. 01:25.080 --> 01:31.260 Lending allows us to detect as we code some issues with our code. 01:31.290 --> 01:40.290 For example, if I do num plus four here and I get a red underline and you see here that the Pi Flakes 01:40.290 --> 01:46.320 lender notices before I even run my code that hey, there's an undefined named NUM. 01:46.530 --> 01:51.510 So lending allows us to find these errors before we even run our code. 01:51.870 --> 01:59.550 And we saw that we can use IDs like pi charm that has lending already built in or use sublime text or 01:59.550 --> 02:06.540 Visual Studio code and install packages and extensions like PI lent to actually have lending in our 02:06.540 --> 02:06.990 code. 02:07.020 --> 02:09.570 That is a really, really important tool. 02:09.600 --> 02:15.480 Next, I recommend always using some sort of an ID or. 02:16.380 --> 02:23.040 Ed, because these editors and IDs, especially the ones that are specific for Python like Pi char, 02:23.070 --> 02:29.370 have all these built in tools for us to use, such as auto formatting based on Pep eight. 02:29.370 --> 02:34.410 Remember the style official style guide that the Python community has? 02:34.590 --> 02:42.600 It allows us to detect errors in our code with, let's say, highlighting and formatting of our code. 02:43.440 --> 02:47.670 So it becomes easy for us to detect any errors before we even run the code. 02:48.240 --> 02:52.050 Finally, you want to be able to learn to read errors. 02:52.350 --> 02:57.030 And this is something that I try to practice with you as we've learned, Python. 02:57.330 --> 03:04.260 For example, if I do four plus, let's say gibberish here and I click Run. 03:06.230 --> 03:06.950 Right away. 03:06.950 --> 03:07.410 I see. 03:07.430 --> 03:10.080 Hey, on line six. 03:10.100 --> 03:13.370 There's an error in my main PI file. 03:13.580 --> 03:19.310 There's a type error, and I see here that I'm trying to add int and a string. 03:19.950 --> 03:24.030 So we're able to read errors and understand what they mean. 03:25.380 --> 03:28.410 Maybe I do something like this. 03:28.440 --> 03:29.790 If I click Run. 03:30.490 --> 03:31.900 I get a syntax error. 03:33.110 --> 03:36.440 And a syntax error I know means that. 03:36.440 --> 03:42.410 Hey, Python interpreter is trying to read my code, but what you just wrote is not Python. 03:42.410 --> 03:43.880 I have no idea what you're doing. 03:45.410 --> 03:45.890 Here. 03:45.890 --> 03:49.180 I see that it's end of line while scanning string literal. 03:49.190 --> 03:54.590 So I'm trying to read this string and all of a sudden the program ends like you're supposed to end the 03:54.590 --> 03:55.670 string, remember? 03:56.770 --> 04:01.060 So being able to read these errors is really, really important. 04:01.450 --> 04:07.390 And we've discussed some of them in this course, some of the most common ones like syntax error, where 04:07.390 --> 04:10.420 you're just writing something that Python interpreter doesn't understand. 04:10.780 --> 04:13.210 We have something like name error, right? 04:13.210 --> 04:16.720 Where we use a variable that doesn't exist. 04:17.710 --> 04:19.180 So Python doesn't understand. 04:19.180 --> 04:20.970 Hey, I have no idea what name is. 04:20.980 --> 04:22.810 What's what's the value of name? 04:24.000 --> 04:25.650 We have things like type air. 04:25.680 --> 04:31.470 That is when we mismatch data type and do let's say a string plus four. 04:32.220 --> 04:34.740 We have things like value error. 04:35.470 --> 04:41.800 We have things like key error where we're trying to access a key that doesn't exist or attribute error 04:41.800 --> 04:47.020 where we're trying to access the method of a object and that method doesn't exist. 04:47.710 --> 04:53.440 Now, these errors are something that we've been seeing throughout the course, and we're going to see 04:53.440 --> 04:54.190 a few more. 04:54.580 --> 05:01.780 I would say that there is about 15 to 20 errors that are really, really common that show up 90% of 05:01.780 --> 05:02.380 the time. 05:03.230 --> 05:07.370 And if you remember, you can always go to the Python documentation to learn about them. 05:07.370 --> 05:09.620 But this just comes with practice. 05:09.650 --> 05:14.420 If you see an error that you've never seen before, I recommend going to the documentation and reading 05:14.420 --> 05:16.260 up on it once you see it. 05:16.280 --> 05:19.490 You usually tend to remember when it comes up next. 05:20.080 --> 05:23.920 But let me show you my favorite way to debug code. 05:25.050 --> 05:30.570 And it's called PDB or Python Debugger. 05:31.080 --> 05:35.070 And PDB is a built in module in Python right here. 05:36.270 --> 05:36.750 Again. 05:36.750 --> 05:43.470 Remember, our built in module are part of the standard library that Python comes with and PDB is the 05:43.470 --> 05:46.200 Python debugger for interactive interpreters. 05:47.400 --> 05:51.180 And you can read about it here, but let me show you what it does. 05:52.460 --> 05:52.860 You see. 05:52.880 --> 05:58.070 PDB is extremely useful because it allows us to interact with the code. 05:58.550 --> 06:08.210 So previously when we had a function, let's say we have function ADD and we have num one, num two 06:08.630 --> 06:14.630 and then this function simply returns num one plus num two. 06:15.630 --> 06:23.460 Now before if we wanted to, let's say debug this code and let's say we're adding four plus some gibberish 06:23.460 --> 06:24.930 and it's giving us an error. 06:26.760 --> 06:33.870 The first step that I would take is most likely adding a print in here so I could say print. 06:33.870 --> 06:35.160 Hey, what's what's going on? 06:35.160 --> 06:35.610 Let me see. 06:35.610 --> 06:38.130 Number one and number two and what it's giving me. 06:38.970 --> 06:43.170 And print is a really, really quick way and easy way to debug your code. 06:43.530 --> 06:48.480 I use it all the time, but PDB gives us a little extra boost. 06:49.780 --> 06:51.840 So we can use it like this. 06:51.850 --> 07:00.310 We simply import PDB and instead of using something like a print function, we can simply say, Let 07:00.310 --> 07:01.540 me add a space in here. 07:01.540 --> 07:08.650 We can simply say PDB for Python debugger and then say set trace. 07:09.310 --> 07:15.430 And set trace, I would say is the most useful method that we have on this PDB object that we imported. 07:16.520 --> 07:18.830 You see when I run a set method. 07:22.300 --> 07:28.900 It now gives me an interactive Python debugger that I can now type commands in here and actually test 07:28.900 --> 07:29.560 on my code. 07:30.290 --> 07:36.360 And here's the interesting part right now in this PDB, I'm right inside of line eight. 07:36.380 --> 07:41.630 So my code tried to run, but then it paused as soon as it said PDB set trace. 07:41.630 --> 07:47.510 And now I'm in this world and I can test out what's going on so I can actually say, Hey, num one, 07:47.510 --> 07:48.170 what is it? 07:48.200 --> 07:49.400 Well num one is four. 07:49.400 --> 07:50.420 What about num two. 07:50.750 --> 07:57.050 Well num two is a string and I can be like oh I'm trying to add a string and an int this, this doesn't 07:57.050 --> 08:00.050 work so I can detect errors this way as well. 08:00.680 --> 08:06.140 Now this PDB interactive actually has a ton of useful things. 08:06.140 --> 08:08.270 So if I type in help here. 08:09.670 --> 08:10.300 You see that? 08:10.300 --> 08:14.140 I have commands that I can use in the PDB. 08:14.260 --> 08:16.510 So I have a ton here. 08:16.510 --> 08:21.510 I have exit, which I'm assuming is going to exit me out of this PDB, Python debugger. 08:21.520 --> 08:23.020 But I also have. 08:23.780 --> 08:24.710 Things like. 08:25.490 --> 08:26.570 Let's say list. 08:26.570 --> 08:27.740 Let's see what list does. 08:27.740 --> 08:31.970 So when I type in list here and right now the website is really. 08:32.820 --> 08:35.940 Not showing me too much, but hopefully you can still see list. 08:37.110 --> 08:38.940 And I guess some sort of an output. 08:38.940 --> 08:46.410 I see that I get a list of my code and OC, but let's say I don't know exactly what list does well. 08:47.280 --> 08:50.970 PDB allows us to type, help and then list. 08:51.670 --> 08:57.700 And when we do that, it actually gives us the documentation, just like we have here. 08:58.720 --> 09:00.820 If we go to, let's say, list. 09:03.370 --> 09:03.910 Let's see. 09:03.910 --> 09:05.470 List, list, list, list. 09:06.130 --> 09:06.880 Right here. 09:06.880 --> 09:09.520 It gives me the documentation right in here. 09:09.520 --> 09:12.160 So it says list source for the current file. 09:12.610 --> 09:12.880 All right. 09:12.880 --> 09:15.940 So it just lists the source code that we wrote. 09:16.030 --> 09:17.080 Okay, that's cool. 09:17.080 --> 09:18.190 What else is there? 09:19.090 --> 09:25.330 Well, we also have something like step or se for short and step is really, really cool. 09:26.970 --> 09:29.910 Let's actually use clear to bring this up to the top. 09:29.910 --> 09:33.720 So I'm going to say clear so that it clears all breaks. 09:35.480 --> 09:36.230 Let's do. 09:37.400 --> 09:39.460 Let's make this a little bit bigger so you can see. 09:39.470 --> 09:39.970 There you go. 09:39.980 --> 09:40.580 That's better. 09:40.610 --> 09:40.880 All right. 09:40.880 --> 09:47.120 So now I can actually do something like step and step is going to allow us to go to the next line. 09:47.120 --> 09:49.850 So, again, if we make this a little bit bigger. 09:50.860 --> 09:53.440 Step is going to say, hey, go to line nine. 09:53.440 --> 09:54.370 So let's see what happens. 09:54.370 --> 09:55.660 I'm going to type in step. 09:57.630 --> 09:57.840 Oh. 09:57.840 --> 09:58.860 And I got step twice. 09:58.860 --> 09:59.910 So let's right step. 10:01.730 --> 10:02.600 And look at that. 10:02.720 --> 10:06.770 I go to the next line in my code and it gets me. 10:06.800 --> 10:08.030 Hey, you got a typo? 10:08.030 --> 10:08.510 Error. 10:08.540 --> 10:09.520 The error that we got. 10:09.530 --> 10:11.250 So let's add another line in here. 10:11.270 --> 10:19.640 Let's say that we're trying to add four plus five in here or four multiply by five and assigning assign 10:19.640 --> 10:22.310 it to a variable. 10:24.170 --> 10:26.450 If I stop this and run this again. 10:28.570 --> 10:30.100 I'm now inside. 10:30.130 --> 10:30.820 Over here. 10:32.640 --> 10:35.310 If I type in tea, I get a name. 10:35.310 --> 10:35.760 Air. 10:35.880 --> 10:37.560 Tea is not defined because. 10:37.560 --> 10:42.630 Well, I'm inside the code here on line eight, but we haven't ran line nine yet. 10:42.630 --> 10:44.220 So let's do step. 10:45.120 --> 10:46.800 And now I'm on the next side. 10:47.940 --> 10:52.950 So if I do pt now I get hey t equals to 20. 10:53.190 --> 10:53.760 Awesome. 10:53.760 --> 10:57.570 So step I can step through the code which is really, really useful. 10:58.080 --> 11:03.720 I also have things like continue which lets me continue through the code until I return something and 11:03.720 --> 11:06.750 I continue and exit out of PDB and look at that. 11:06.750 --> 11:07.620 I get the error. 11:10.100 --> 11:11.570 Let's run this again. 11:11.600 --> 11:18.320 Another useful command is a and a actually gives me all the arguments of the current function that we're 11:18.320 --> 11:18.560 in. 11:18.560 --> 11:19.700 So look at that. 11:19.700 --> 11:22.610 I get all the arguments which again is really useful. 11:23.240 --> 11:25.190 There's also the command w. 11:25.860 --> 11:31.050 Which shows me the context of the current line it is executing. 11:32.200 --> 11:35.710 So you can see here where we are, we're in the ad. 11:36.240 --> 11:36.990 Function. 11:38.130 --> 11:43.740 We called it with these arguments, and we're now about to run this line. 11:45.970 --> 11:48.040 And then finally, as we're debugging this. 11:48.040 --> 11:51.460 All right, I think I finally see that we have an error. 11:51.460 --> 11:59.500 We should have given it a number so I can now just exit out of here and simply fix this to five. 11:59.620 --> 12:02.950 So now I can just remove my PDB and everything. 12:02.950 --> 12:05.410 Should be working fine with my code. 12:06.680 --> 12:07.280 Awesome. 12:07.760 --> 12:12.710 So PTB is awesome to step through your code and debug and try different things. 12:12.710 --> 12:19.520 It does require a bit of practice, but after a day of just playing around with PDB, you will become 12:19.520 --> 12:20.900 really, really good at debugging. 12:20.930 --> 12:28.730 So after this video, I recommend that you write PDB and hopefully you have a large enough Python file 12:28.730 --> 12:35.540 that you can just step over and experiment with different commands using help, using list to see what 12:35.540 --> 12:37.010 you have available to you. 12:38.060 --> 12:39.710 Actually, I want to show you one last thing. 12:40.340 --> 12:44.210 So let's say we go back to our code, our PDB set trace. 12:44.780 --> 12:45.800 I click run. 12:47.670 --> 12:53.430 Could I do something like this where I notice that I type A and I have. 12:53.430 --> 13:00.330 All right, my code is working, but could I change num two to now equal some gibberish? 13:02.230 --> 13:04.320 Well, let's see if I do next now. 13:06.100 --> 13:08.740 And let's do next to run this final line. 13:10.120 --> 13:11.140 I get an air. 13:11.200 --> 13:19.710 So I'm also able to change variables in here and write Python code to perhaps test out my functions. 13:19.720 --> 13:21.190 This is now working. 13:21.190 --> 13:25.300 I can test out, hey, what happens if we change this variable to another value? 13:26.220 --> 13:28.050 Again, very, very useful.