WEBVTT 0 00:00.560 --> 00:06.170 Now, before you start getting really committed to print and start making friendship bracelets for this 1 00:06.170 --> 00:12.470 print statement, there's another tool that I really want to talk about, and it's a tool that's bigger, 2 00:12.470 --> 00:14.540 it's fancier, it's more complex. 3 00:14.540 --> 00:15.860 So what is it? 4 00:15.860 --> 00:17.060 It's a debugger. 5 00:17.420 --> 00:20.060 And we've already seen debuggers a little bit. 6 00:20.270 --> 00:23.150 So I've already shown you pythontutor.com, 7 00:23.150 --> 00:28.280 and you've also seen us use the debugger in the Thonny editor right here. 8 00:28.280 --> 00:37.770 Now in the starting code, we have a function called mutate() that takes a list as the input and then 9 00:37.770 --> 00:40.440 prints another list at the end. 10 00:40.530 --> 00:48.480 So what happens is we start out with this b_list being an empty list, and we have a new_item that starts 11 00:48.480 --> 00:49.860 out being 0. 12 00:49.860 --> 00:57.300 Then we loop through each of the items in the a_list that comes in through the input, and we multiply 13 00:57.300 --> 01:01.860 it by 2 and turn that into the new_item. 14 01:02.040 --> 01:10.380 And then we modify the new_item even further by adding to it a random number between 1 and 3. 15 01:10.530 --> 01:19.580 Finally, we use a function called add() in our Maths Module, which we created right here, 16 01:19.580 --> 01:26.000 and it does the simple thing that you created before, which is just to add one number to another. 17 01:26.000 --> 01:32.840 So we add the new_item that we've got up to now to the item that we're currently looping through, 18 01:32.840 --> 01:35.870 and finally we add it all to our b_list, 19 01:35.870 --> 01:44.290 and hopefully, we are able to modify the a_list with this step-by-step approach to end up with a new 20 01:44.290 --> 01:49.450 list where each of the numbers is modified according to our rules. 21 01:50.200 --> 01:54.640 Now, unfortunately, at the moment it does not do what we expect it to do, 22 01:54.640 --> 01:58.060 it just prints out a single number in our list. 23 01:58.060 --> 02:04.600 So in order to debug and figure out what is wrong with our code to be able to fix it, we're going to 24 02:04.600 --> 02:08.030 cover how to use the debugger in PyCharm. 25 02:08.240 --> 02:16.910 Now, debuggers are pretty similar in most of these Intelligent Development Environments such as PyCharm. 26 02:16.910 --> 02:22.550 So I'm going to use this opportunity to show you a lot of things that will be reusable, even if later 27 02:22.550 --> 02:26.330 on down the line you choose to use a different code editor. 28 02:26.600 --> 02:31.760 Now what we do first is we can define what's called a breakpoint. 29 02:31.760 --> 02:35.870 So a breakpoint can be created in the gutter of our code. 30 02:35.870 --> 02:38.300 And you might have done it accidentally before. 31 02:38.300 --> 02:44.150 So when I hover over these line numbers in the gutter, you can see this red dot appears. 32 02:44.150 --> 02:51.350 And if I decide to go on to line nine and click on that red dot, I've now created a breakpoint and 33 02:51.350 --> 02:55.250 you can see that breaking line is now highlighted. 34 02:55.320 --> 03:01.890 So now I'm going to go into the debug run mode by clicking on this little bug here, 35 03:01.890 --> 03:05.160 and it's going to run my current file, 36 03:05.160 --> 03:11.220 and it's going to take into account the fact that now I have a defined breakpoint. 37 03:11.250 --> 03:12.810 So what is a breakpoint do. 38 03:12.840 --> 03:18.960 Well it basically puts a break on the code at that particular line. 39 03:19.150 --> 03:29.290 So here is where I'm setting the new_item from 0 to a new value, which is the value of the item 40 03:29.290 --> 03:30.940 multiplied by 2. 41 03:30.970 --> 03:36.700 Now you'll see a couple of things show up already in line in our code, which is kind of helpful. 42 03:36.700 --> 03:44.500 It's showing us at this point, where the code is at, where we've stopped the code on Line 9, 43 03:44.500 --> 03:52.360 the current value of a_list is this, this big list which we inputted right here. 44 03:52.360 --> 03:55.420 So this is the input for this function, 45 03:55.420 --> 03:57.310 and that's showing up right there. 46 03:57.310 --> 04:00.850 And then it's showing us b_list is still an empty list, 47 04:00.850 --> 04:02.680 new_item is 0. 48 04:02.680 --> 04:10.700 An item which is the first item in the a_list that we're looping through right here is equal to 1. 49 04:10.940 --> 04:18.890 So now what we can do is use the first of the step buttons in the debug console. 50 04:18.920 --> 04:25.820 So if I click on this one which is called, Step Over, what it's going to do is it's going to execute 51 04:25.820 --> 04:30.860 my code line by line, stopping at the very next line. 52 04:30.890 --> 04:38.300 So now that we've executed this line, which we've broken, it's now happened, 53 04:38.300 --> 04:47.000 and we can see that what's updated is the new_item is no longer equal to 0, but it's now equal to 54 04:47.000 --> 04:54.020 2, because we took the item which is one, multiplied it by 2, and then set it equal to new_item. 55 04:54.020 --> 04:57.390 So new item is now equal to 2 instead of 0. 56 04:57.390 --> 05:01.170 So we can continue stepping over to the next line, 57 05:01.170 --> 05:06.240 and remember that the highlighted line is the one that's going to be executed next, 58 05:06.240 --> 05:10.260 and the line above it is the one that just got executed. 59 05:10.260 --> 05:15.390 So right now we can see that this Line 10 has just been executed, 60 05:15.390 --> 05:25.840 and we've generated a random integer between 1 and 3, and we've added that number to our new_item. 61 05:25.870 --> 05:29.980 So now our new_item is equal to 5. 62 05:30.070 --> 05:33.850 So while we're here we can do something else, 63 05:34.660 --> 05:38.050 we can look at another one of these buttons. 64 05:38.050 --> 05:46.690 So if I go ahead and rerun my debugger and I use instead of the Step Over button, I click on the Step 65 05:46.690 --> 05:54.910 Into button, so it's going to continue executing each line as it goes, line by line, 66 05:54.910 --> 06:01.810 but when it encounters a line such as Line 10 where we're using another module, 67 06:01.810 --> 06:09.970 so code in another file, such as this randint() function from the Random Module, when I click Step Into, 68 06:09.980 --> 06:18.950 it's going to go into the definition of that function, the randint() function in the Random Module from 69 06:18.950 --> 06:25.490 random.py, which is built in to Python, and it's going to show you how that function works. 70 06:25.490 --> 06:30.950 So in fact the randint() actually uses another function called randrange(). 71 06:30.950 --> 06:38.150 And if I continue stepping into it it will go and find this function randrange() and show me how that 72 06:38.150 --> 06:39.320 function works. 73 06:39.320 --> 06:46.280 Now once you're done exploring this, which can be really helpful because your code relies on code that's 74 06:46.280 --> 06:48.230 been written by other people, 75 06:48.230 --> 06:53.840 and if you want to know how they've defined it, how they've created randrange(), then you can read 76 06:53.840 --> 06:54.800 through their code. 77 06:54.800 --> 07:00.390 And sometimes it can provide helpful hints for us to figure out what is wrong with our code. 78 07:00.510 --> 07:06.420 Now, you don't want to continue stepping through other people's code, so when you want to exit, you 79 07:06.450 --> 07:12.990 click on this button, which is Step Out and we can continue stepping out until we get back to the code 80 07:12.990 --> 07:14.460 that we want to examine. 81 07:15.330 --> 07:20.280 So this can be really helpful where we're stepping through the functions, 82 07:20.280 --> 07:27.420 but more often than not we don't actually want to step into the library functions such as random because 83 07:27.420 --> 07:30.780 they work the way they do and we can't really modify it. 84 07:30.780 --> 07:34.920 So we probably haven't made a mistake there. 85 07:35.010 --> 07:41.610 And what we want to do sometimes, though, is we want to be able to step into code that is in other 86 07:41.610 --> 07:46.050 files, but only the code that is written by us, 87 07:46.050 --> 07:50.500 and we want to ignore these library functions written by the Python team. 88 07:50.500 --> 07:52.180 So how can we do that? 89 07:52.180 --> 07:54.490 Well, let's repeat from the beginning. 90 07:54.490 --> 08:00.340 So this button restarts our debug run and again pauses on our breakpoint. 91 08:00.340 --> 08:07.420 Instead of using this Step Into button, we're going to use the Step Into My Code button, which does 92 08:07.420 --> 08:13.540 exactly the same thing as step into going line by line, but it ignores the library functions. 93 08:13.540 --> 08:19.960 For example, that random() function right there, it doesn't take you over to the random.py file. 94 08:19.960 --> 08:28.540 But now if I click into step into this current Line 11, then it takes me into this file that I created, 95 08:28.540 --> 08:32.650 math.py, where I created this function called add(). 96 08:32.770 --> 08:40.070 And it shows me that right now I'm trying to add 4 to 2, which was passed over from the previous function 97 08:40.070 --> 08:40.610 call. 98 08:40.610 --> 08:48.320 And if I step out of that add() function in the Math Module, I can go back to examine my file. 99 08:48.800 --> 08:54.230 So this button can be more useful when we have multi file projects, 100 08:54.230 --> 09:00.480 and we're trying to examine the cause and the relationship of what could be going wrong in our code. 101 09:00.480 --> 09:06.180 Because very often as our code gets more and more complex, there's more and more relations that start 102 09:06.180 --> 09:07.020 happening. 103 09:07.020 --> 09:13.800 And we're sometimes using code that's been defined elsewhere, and chasing it around can be quite difficult. 104 09:14.100 --> 09:19.020 So those are some of the most important features of the debugger. 105 09:19.020 --> 09:21.930 And I want you to take a look 106 09:21.930 --> 09:28.170 also down here, we've got two tabs here, the threads and variables and the console. 107 09:28.170 --> 09:33.960 So the threads and variables can be really helpful, showing us the values of the variables that we've 108 09:33.960 --> 09:35.040 got going on. 109 09:35.040 --> 09:41.490 So for example, just as they've shown up here as these comments, you can also see them in the list 110 09:41.490 --> 09:42.150 right here. 111 09:42.150 --> 09:47.130 What's the current value of new_item at this particular time point? 112 09:47.130 --> 09:51.370 What is the current value of b_list at this particular time point? 113 09:51.370 --> 09:59.590 And as you can see that as I continue stepping through my code line by line, these values, they update 114 09:59.590 --> 10:02.680 as they get changed in the code. 115 10:02.680 --> 10:08.800 So we can actually trace what's happening to each of our variables in their intermediate stages. 116 10:08.800 --> 10:12.880 And especially for something like a for loop where everything is pretty much hidden, 117 10:12.880 --> 10:15.520 we don't know what's going on while it's looping. 118 10:15.520 --> 10:20.530 This can be really, really helpful to help us figure out what's wrong with our code. 119 10:20.890 --> 10:23.830 So the other tab here is the console. 120 10:23.830 --> 10:30.400 And you can see that it shows what gets printed during the run of the code. 121 10:30.400 --> 10:34.720 So for example, if I was to get rid of this breakpoint, 122 10:34.720 --> 10:41.960 so I can either disable it by clicking it once and it goes into this darker color, or I can keep it 123 10:41.960 --> 10:45.110 there and simply say mute breakpoints. 124 10:45.320 --> 10:52.850 Now if we continue running through our code and stepping it through, you'll see that eventually it 125 10:52.850 --> 10:56.450 ends the loop and goes to the print. 126 10:56.450 --> 10:59.390 And we can see that print happening in the console. 127 10:59.390 --> 11:03.800 So we get our console even when we're running in debug mode as well. 128 11:03.890 --> 11:09.260 So that's pretty much all there is to it, to the major features of a debugger. 129 11:09.260 --> 11:18.380 The most useful thing, I think, is just simply being able to see these values show up as you go through 130 11:18.380 --> 11:21.500 the code, stepping through each of the lines. 131 11:22.430 --> 11:29.490 So now it's your turn to use what you've learned about the PyCharm debugger and figure out what is wrong 132 11:29.490 --> 11:30.540 with our code. 133 11:30.540 --> 11:39.030 Why is it not printing out an entire list with 123456 items? 134 11:39.030 --> 11:44.880 And instead, why is it only modifying and printing out the first item in the list? 135 11:44.880 --> 11:48.960 So pause the video and complete this challenge. 136 11:56.230 --> 11:56.740 All right. 137 11:56.740 --> 12:01.480 So with our breakpoint in its current position let's do another debug run. 138 12:01.600 --> 12:10.000 And we are going to watch the value of the b_list and see if it's changing the way we expect it to. 139 12:10.030 --> 12:16.930 So as I step over each of these lines and we're looping through the list, one thing you'll notice is 140 12:16.930 --> 12:20.770 that the value of b_list is not changing at all. 141 12:20.770 --> 12:27.250 So at which point does it actually get populated with these new items? 142 12:27.250 --> 12:29.320 Well, it's on line 12. 143 12:29.320 --> 12:34.120 So let's disable this and put our breakpoint here instead. 144 12:34.120 --> 12:39.650 Let's go ahead and resume our program until it runs to this point. 145 12:39.650 --> 12:43.010 And we can see that b_list is still empty, 146 12:43.010 --> 12:47.180 and all that's happening is we're appending new_item. 147 12:47.180 --> 12:51.680 So new_item is currently equal to 42 as we can see here. 148 12:51.680 --> 12:58.370 And if I step over this line to execute it, you can see that 42 gets added to the b_list, 149 12:58.370 --> 13:06.710 and then if I step over once more you'll see that print statement execute and that 42 gets printed. 150 13:06.710 --> 13:10.370 And then that's the end of our code. 151 13:10.370 --> 13:18.020 So what's happening is that these new items are not being added to the b_list as they're created. 152 13:18.020 --> 13:22.730 So the only thing we need to do is just to add an indent. 153 13:22.730 --> 13:29.670 And so often my Python code fails because of an indent, or because of something silly like that, but 154 13:29.670 --> 13:34.260 with the debugger, it makes it a lot easier to figure out what is the problem. 155 13:34.260 --> 13:42.450 So if we do another debug run and you watch how this b_list changes now, every time we loop through the 156 13:42.450 --> 13:51.000 list, once we get to the end of it, that new_item gets added to the b_list, and you can see it's 157 13:51.000 --> 13:54.390 adding a new_item each time the loop runs. 158 13:54.390 --> 13:57.660 And that is what we expect to happen. 159 13:57.900 --> 14:00.840 So a debugger can be really useful. 160 14:00.840 --> 14:07.380 It's almost like you've put a print statement on all the variables in your code, and you can isolate 161 14:07.380 --> 14:11.670 a particular time point to see what each of the values are. 162 14:11.700 --> 14:17.520 And as we become even more experienced, we're going to be using the debugger more and more.