1 00:00:04,620 --> 00:00:09,960 In this video we'll see how the debugger, built it into IntelliJ and PyCharm, 2 00:00:09,960 --> 00:00:14,190 can help us to understand what our code's doing. We've seen how to run the code by 3 00:00:14,190 --> 00:00:19,770 right-clicking in the editor pane, like so, and selecting run, but just below that 4 00:00:19,770 --> 00:00:24,480 there's this debug option. Now just before I click that, I want to talk about 5 00:00:24,480 --> 00:00:29,420 an option that might come up on the screen like you'll see here for Windows users. 6 00:00:29,420 --> 00:00:31,940 So if you see this dialog and you're on windows, 7 00:00:31,940 --> 00:00:35,800 either now or at some other point when using your IDE, then it's very 8 00:00:35,800 --> 00:00:39,760 important that you tick the box to allow access on private networks, as you can 9 00:00:39,760 --> 00:00:42,760 see checked on the screen. 10 00:00:43,400 --> 00:00:47,160 Make sure that's checked and then click on allow access. Failing to do that, well, 11 00:00:47,170 --> 00:00:51,340 you'll find that, basically, IntelliJ or your IDE whatever you're using won't be 12 00:00:51,340 --> 00:00:54,390 able to access the Internet. 13 00:00:54,450 --> 00:00:57,780 Now on a Mac, although I haven't got a screenshot for this, you may notice 14 00:00:57,780 --> 00:01:01,120 there's a little message that can appear down the bottom right-hand corner, 15 00:01:01,120 --> 00:01:05,239 suggesting you install a Cython extension. Now if you're using a 16 00:01:05,250 --> 00:01:09,030 different implementation of Python, other than what we've suggested you install in 17 00:01:09,030 --> 00:01:13,500 this course, you won't see that, but with the CPython implementation on a Mac, 18 00:01:13,500 --> 00:01:17,820 you should see a pop up at some point. But I suggest you ignore it and if you 19 00:01:17,820 --> 00:01:21,210 do that the dialog will disappear after a short while. Now in this particular 20 00:01:21,210 --> 00:01:24,869 extension on a Mac, it can be really useful if you've got a large code base but 21 00:01:24,869 --> 00:01:28,649 at the moment, the code in this course isn't complex enough to benefit from 22 00:01:28,649 --> 00:01:32,190 installing another extension. It's really just introducing another potential point 23 00:01:32,190 --> 00:01:37,080 of failure, so for that reason I suggest you ignore it. Alright, so back to the 24 00:01:37,080 --> 00:01:43,180 screen now. I'm going to click on Debug instead of Run. Now you can see what's 25 00:01:43,180 --> 00:01:48,820 happened there, we've got some text showing pydev debugger: process 4672 is connecting. 26 00:01:48,820 --> 00:01:51,440 Maybe we'll say connected instead of connecting. 27 00:01:51,440 --> 00:01:54,479 Now ignore the process ID. That number will probably be different for you, and in 28 00:01:54,479 --> 00:01:58,229 fact, will be different every time you're running it. Now notice that we've got a 29 00:01:58,229 --> 00:02:02,549 debug pane at the bottom of the screen, rather than a run pane, and that will 30 00:02:02,549 --> 00:02:06,600 come up when we're actually using this debugger. Now the debugger itself has got 31 00:02:06,600 --> 00:02:10,289 two panes. We've got the debugger and the console, and you can see at the moment, 32 00:02:10,289 --> 00:02:14,220 the console pain is checked and that just shows the output from your program, 33 00:02:14,220 --> 00:02:17,700 just like it did when we were running programs previously. 34 00:02:17,700 --> 00:02:22,460 It's basically the same output if you're running it using the run option, and we'll see the 35 00:02:22,460 --> 00:02:26,540 debugger tab in a moment. But what I'm going to do now is just enter my details. 36 00:02:26,540 --> 00:02:31,440 Down here I'm going to type in my name and an age, and you can see the program runs, 37 00:02:31,440 --> 00:02:35,130 just as it did before in the previous video. But where the debugger gets 38 00:02:35,130 --> 00:02:39,920 interesting, is when we set breakpoints in our code. A breakpoint tells the 39 00:02:39,920 --> 00:02:44,260 debugger to pause at that point. This allows us to see what the code's doing 40 00:02:44,260 --> 00:02:48,700 and step through, line by line. The good news is that setting a breakpoint's easy. 41 00:02:48,700 --> 00:02:51,930 All you need to do is just click in the left margin on the line you want to 42 00:02:51,930 --> 00:02:56,010 set the breakpoint at. So I'm going to set one over here on line 11, clicking in 43 00:02:56,010 --> 00:02:59,500 the margin there, and you can see when I did that, the line's highlighted and we've 44 00:02:59,500 --> 00:03:05,160 got now a red dot in the left margin. To clear a breakpoint, click it again, 45 00:03:05,160 --> 00:03:07,780 and you can toggler breakpoints on and off throughout your code. 46 00:03:07,780 --> 00:03:16,060 I'm gonna set that one back on line 11. Now that I've done that, I want to use the debug option again. 47 00:03:16,060 --> 00:03:20,140 The code will stop when it reaches that breakpoint. Now to get there, 48 00:03:20,140 --> 00:03:23,040 because we were executing the code at the moment above that, 49 00:03:23,040 --> 00:03:26,660 we're executing the code at the moment on line 1, so we need to enter some information until 50 00:03:26,660 --> 00:03:33,320 the code gets to that point. So I'm going to type in Tim and 30. 51 00:03:33,320 --> 00:03:37,620 You can see what happened when I pressed 30 there. The program's now got up to line 11, 52 00:03:37,620 --> 00:03:41,100 and it's waiting there for us to do something. Now in the debugger tab of the 53 00:03:41,100 --> 00:03:46,140 debug pane, you can see the value of our two variables there, on the right hand side. 54 00:03:46,140 --> 00:03:49,410 Now the debugger tab normally opens automatically, as you saw that it did 55 00:03:49,410 --> 00:03:52,590 here in my case, but you can switch between the debugger and the console 56 00:03:52,590 --> 00:03:57,560 tabs by clicking them. So I'll come over here and click on console, and there's the output. 57 00:03:57,560 --> 00:04:02,440 I'll go back to debugger for now. So looking at the left hand pane in the 58 00:04:02,459 --> 00:04:07,110 debugger tab, it can look a tad confusing. The good news is we can ignore all of 59 00:04:07,110 --> 00:04:10,470 that at the moment. Just make sure that you've got the top option blocks 60 00:04:10,470 --> 00:04:13,900 selected, and you can see at the top that it is selected for me. 61 00:04:13,900 --> 00:04:19,420 Now to the right of the console tab, note that we've got a mini toolbar with a few buttons. 62 00:04:19,420 --> 00:04:23,700 The first one, the three horizontal bars that represent lines of code, shows the 63 00:04:23,700 --> 00:04:27,960 current line that will be executed. Now where that is useful is in larger programs, 64 00:04:27,960 --> 00:04:31,950 or programs using several files. We've only got a few lines of code but we can 65 00:04:31,950 --> 00:04:35,720 demonstrate what it does. If I can do that by going back up to line 1, 66 00:04:35,720 --> 00:04:40,919 say to there. If our current execution line was off the screen, say, this button is a 67 00:04:40,920 --> 00:04:45,060 quick way to get back to it, so I wanna come down here and click on that button. 68 00:04:45,060 --> 00:04:48,980 You can see the cursor went down to line 11, not particularly useful as a 69 00:04:48,990 --> 00:04:52,620 small file, but very much useful if you've got larger files or multiple 70 00:04:52,620 --> 00:04:56,250 files that you're working on. Now we're going to talk about the last button next, 71 00:04:56,250 --> 00:05:00,030 this one over here. It's the last one that's available anyway. You can see that 72 00:05:00,030 --> 00:05:04,240 it looks a little bit like a calculator, and it lets you evaluate expressions. 73 00:05:04,240 --> 00:05:09,300 Now our code is about to execute line 11 and hasn't executed it yet. I'm just going to 74 00:05:09,300 --> 00:05:12,460 move that code down so you can see a bit more of it, and then I'm going to click on 75 00:05:12,460 --> 00:05:15,840 that little button, Evaluate expressions. I'm going to move 76 00:05:15,840 --> 00:05:19,620 this over to the right so we can still see the code and see this at the same time. 77 00:05:19,620 --> 00:05:21,620 And under expression I'm going to type 78 00:05:21,630 --> 00:05:28,200 age less than 18. I come down here, click on evaluate, and when I do that note that 79 00:05:28,200 --> 00:05:32,160 we've got result equals, any left and right clear braces, bool being the type, 80 00:05:32,160 --> 00:05:37,620 and then the value, in this case, false. So looking at our code, we can see now that, 81 00:05:37,620 --> 00:05:42,580 given that age less than 18 is false, that line 12 won't be executed. 82 00:05:42,580 --> 00:05:47,900 That's because the expression on line 11 is going to evaluate to false. I suggest you do this, 83 00:05:47,900 --> 00:05:51,600 when using the debugger, as a learning tool. Don't just click the next 84 00:05:51,600 --> 00:05:55,290 button and watch the code executing. Take the time to read the code and work out 85 00:05:55,290 --> 00:05:58,920 what it's going to do. When you think you've worked it out, then you can step 86 00:05:58,920 --> 00:06:02,940 through the code to see if you're right. A great way to learning, really solidify 87 00:06:02,940 --> 00:06:06,210 the information you're learning in this course. Now if you just mindlessly click 88 00:06:06,210 --> 00:06:08,880 the button to run the next line, which I'll talk about shortly, 89 00:06:08,880 --> 00:06:12,160 you won't really learn very much. You'll learn how to mindlessly click a button, 90 00:06:12,160 --> 00:06:15,580 but we're learning to become programmers here, not mindless button clickers. 91 00:06:15,580 --> 00:06:21,400 Alright, so moving forward, we know that line 12 won't be executed. Next comes line 13, 92 00:06:21,400 --> 00:06:26,060 though that's going to test the condition; age is equal to the value of 900. 93 00:06:26,060 --> 00:06:29,400 Now we really don't need the expression evaluated to work that one out, 94 00:06:29,400 --> 00:06:33,780 because we can clearly see age in the debugger variables pane there, 95 00:06:33,780 --> 00:06:37,800 at the bottom of the screen, showing the value of 30. But we're seeing how to use it 96 00:06:37,800 --> 00:06:43,660 so I'm going to enter the expression anyway; age equals equals 900. 97 00:06:43,660 --> 00:06:48,060 Note also, if I type a variable now that doesn't exist, we'll actually get an error. 98 00:06:48,060 --> 00:06:51,060 So it's a good way to confirm that what your typing is valid, so age is equal to 99 00:06:51,060 --> 00:06:57,240 equal, is equal to 900. I was typing 1800 instead of 900. Same do there, I can 100 00:06:57,240 --> 00:07:02,740 click on evaluate, and you can see the result down the bottom, again, is showing false. 101 00:07:02,740 --> 00:07:07,640 Just to confirm that, I could change this, result is equal to 30, evaluate. 102 00:07:07,640 --> 00:07:11,880 You can see the value there is true. So I can see how this is a very useful tool. Well I'll 103 00:07:11,880 --> 00:07:16,350 set that back to 900. Click on evaluate, we get false again. Basically, that's 104 00:07:16,350 --> 00:07:21,440 telling us also that the code on line 14 won't be executed either. 105 00:07:21,440 --> 00:07:25,220 Alright, so there's no other conditions in our short little program to check, and none of the 106 00:07:25,220 --> 00:07:28,940 ones we've checked were true. So the only thing left now is the else block, 107 00:07:28,940 --> 00:07:33,240 so that's what's ultimately going to be executed on line 16. So let's see 108 00:07:33,240 --> 00:07:37,960 if we're right. I'm gonna, firstly, close the Evaluate pop up. 109 00:07:37,960 --> 00:07:40,680 Let's have a look at some of these other buttons here, to the right of 110 00:07:40,680 --> 00:07:45,260 this little button here that we've already talked about, the Show Execution Point. 111 00:07:45,260 --> 00:07:47,840 The two buttons we're going to look at at the moment, are this one here, 112 00:07:47,849 --> 00:07:51,180 which is a step over and step into. We not really going to look at this other 113 00:07:51,180 --> 00:07:55,289 one, step into my code, at this point. Specifically, it's these first two which 114 00:07:55,289 --> 00:07:59,520 appear to do the same thing, and we'll actually look at step into when 115 00:07:59,520 --> 00:08:02,220 we actually start writing our own functions. For now though, I'm going to come over here 116 00:08:02,220 --> 00:08:06,660 and click on step over, noting that we've got a shortcut key we could use instead 117 00:08:06,660 --> 00:08:11,240 of clicking, if we prefer. So I'm going to click that once. Notice when I do that, 118 00:08:11,240 --> 00:08:16,820 the code on screen has jumped to line 13. So line 12 didn't get executed, as we expected, 119 00:08:16,820 --> 00:08:21,540 and we're at the elif statement on line 13. I'm going to click the button again 120 00:08:21,540 --> 00:08:24,920 and see what happens with this expression, and you can see also we were 121 00:08:24,930 --> 00:08:28,289 right again. In this case, line 14 wasn't executed. Instead we've skipped 122 00:08:28,289 --> 00:08:33,929 straight down to the else code on line 16. Basically, we knew that both of those 123 00:08:33,929 --> 00:08:38,429 expressions evaluated to false, and we've got confirmation of that by 124 00:08:38,429 --> 00:08:43,289 clicking on the step over button. Now we can also confirm that nothing's been 125 00:08:43,289 --> 00:08:47,260 output, by clicking over here to the console tab so I'll do that now. 126 00:08:47,260 --> 00:08:50,940 And notice we've got the two bits of information that I typed in but we've 127 00:08:50,940 --> 00:08:56,720 got no other output at this time. But if I click on step over one more time now, 128 00:08:56,720 --> 00:08:59,640 you can see that caused line 16 to execute. We've got the message 'You're old 129 00:08:59,640 --> 00:09:02,880 enough to vote' printed it in the console. Clicking it one more time 130 00:09:02,880 --> 00:09:07,860 will execute the last line and then end the program. You can see that's what's happened. 131 00:09:07,860 --> 00:09:11,480 We've got the message 'put an X in the box' and the program terminates. 132 00:09:11,480 --> 00:09:15,080 Alright, so that's the debugger. Now we're going to be using the debugger a lot in 133 00:09:15,089 --> 00:09:19,050 the early sections. I don't recommend using one for real debugging unless 134 00:09:19,050 --> 00:09:23,010 you're totally stuck, but they're a great way to watch your code executing while 135 00:09:23,010 --> 00:09:26,490 you're learning about it. Experiment with setting breakpoints and stepping through 136 00:09:26,490 --> 00:09:30,750 your code to get used to using the debugger. Now before I finish, there's another 137 00:09:30,750 --> 00:09:34,350 button I want to show you. It's very useful if you do get stuck, if you step 138 00:09:34,350 --> 00:09:38,360 too far, for example, or end up debugging the Python libraries. 139 00:09:38,360 --> 00:09:43,860 So I'm going to run the program in the debugger one more time. 140 00:09:43,860 --> 00:09:47,880 So notice the Red Square over to the left hand side of the screen, down the bottom. 141 00:09:47,880 --> 00:09:51,100 That's used to stop execution, allowing you to start again if you 142 00:09:51,100 --> 00:09:54,880 get caught or get into a mess. You might get some errors when you stop the program, 143 00:09:54,880 --> 00:09:58,080 but I'm going to click on that now. Now I've only got a note there 144 00:09:58,089 --> 00:10:01,629 saying the process finished with exit code minus one, but if you do get some 145 00:10:01,629 --> 00:10:05,740 errors when you stop the program, that's fine. Ignore any errors in that scenario. 146 00:10:05,740 --> 00:10:08,460 That's completely okay when you're terminating the program, as we've done there, 147 00:10:08,460 --> 00:10:12,620 while stopping it. So with that said, I'm going to finish now, and in the next video 148 00:10:12,620 --> 00:10:18,380 what we're going to do is start taking a look at more on if, elif and else. 149 00:10:18,380 --> 00:10:21,420 See you in the next video.