WEBVTT 1 00:00:01.310 --> 00:00:02.740 Hi, guys, and welcome back. 2 00:00:02.740 --> 00:00:05.973 In this video, we're going to learn about errors in Python. 3 00:00:07.020 --> 00:00:10.060 What we've got here is a simple divide function, 4 00:00:10.060 --> 00:00:11.700 that takes in two parameters. 5 00:00:11.700 --> 00:00:13.100 If the divisor is equal to zero, 6 00:00:13.100 --> 00:00:15.740 it prints that it cannot be zero and then returns. 7 00:00:15.740 --> 00:00:19.480 Otherwise, it'll return the dividend divided by the divisor. 8 00:00:19.480 --> 00:00:22.780 If we run this code, you should see that you get a print, 9 00:00:22.780 --> 00:00:25.080 Divisor cannot be zero. 10 00:00:25.080 --> 00:00:28.420 However, if you use this function inside a programme, 11 00:00:28.420 --> 00:00:30.860 potentially, you may encounter a problem, 12 00:00:30.860 --> 00:00:34.640 which is that your programme expects the function to work 13 00:00:34.640 --> 00:00:37.690 and there's nothing in the function that says otherwise, 14 00:00:37.690 --> 00:00:38.960 other than a print statement, 15 00:00:38.960 --> 00:00:41.533 which really is meant for your users to see. 16 00:00:43.520 --> 00:00:46.200 For example, let's say that we've got a set of grades 17 00:00:49.170 --> 00:00:51.353 and we want to calculate the average, 18 00:00:54.790 --> 00:00:56.420 and we have a programme like this one. 19 00:00:56.420 --> 00:00:58.170 Welcome to the average grade programme, 20 00:00:58.170 --> 00:01:00.010 and the average is equal to divide 21 00:01:00.010 --> 00:01:01.500 of some grades by len(grades). 22 00:01:01.500 --> 00:01:03.830 Obviously, not the most useful programme, 23 00:01:03.830 --> 00:01:06.540 but just bare with me while I explain why errors 24 00:01:06.540 --> 00:01:08.450 are useful in Python. 25 00:01:08.450 --> 00:01:11.610 What we're gonna get here, if we run this programme 26 00:01:11.610 --> 00:01:16.153 and we print the average grade is average, 27 00:01:17.550 --> 00:01:20.850 is, of course, the average grade. 28 00:01:20.850 --> 00:01:23.610 It says the average grade is 90.5, that's it. 29 00:01:23.610 --> 00:01:24.603 Now, very good. 30 00:01:25.660 --> 00:01:30.660 However, if we want to print a nicer error message, 31 00:01:30.910 --> 00:01:34.160 we start having to do some pretty crazy stuff, 32 00:01:34.160 --> 00:01:36.510 if we are not using errors. 33 00:01:36.510 --> 00:01:39.780 For example, let's say that we don't have any grades yet. 34 00:01:39.780 --> 00:01:42.810 For example, imagine our programme adds the grades over time 35 00:01:42.810 --> 00:01:44.850 or something like that, and there are no grades. 36 00:01:44.850 --> 00:01:48.555 What you'll get is, welcome to the average grade programme, 37 00:01:48.555 --> 00:01:51.093 divisor cannot be zero, the average grade is none. 38 00:01:52.580 --> 00:01:57.160 Now, not the greatest functionality here in our programme, 39 00:01:57.160 --> 00:02:02.160 and this is caused by this function printing something out 40 00:02:03.210 --> 00:02:05.910 that is essentially just mathematical, 41 00:02:05.910 --> 00:02:09.660 it's unrelated to the context of the programme. 42 00:02:09.660 --> 00:02:12.050 This programme here talks about student grades, 43 00:02:12.050 --> 00:02:15.080 and that's what the user is expecting to read about. 44 00:02:15.080 --> 00:02:17.030 This programme here, or this bit of code here, 45 00:02:17.030 --> 00:02:18.210 talks about mathematics, 46 00:02:18.210 --> 00:02:21.320 whether you can divide a number by zero or not. 47 00:02:21.320 --> 00:02:23.840 Though they are related, but you don't wanna be 48 00:02:23.840 --> 00:02:27.190 printing divisor cannot be zero, when the user is expecting 49 00:02:27.190 --> 00:02:29.023 an average of a bunch of grades. 50 00:02:30.310 --> 00:02:33.820 However, it is the only way that we can do something 51 00:02:33.820 --> 00:02:37.910 unless we check for the divisor being zero out here. 52 00:02:37.910 --> 00:02:42.070 So if we say, if len(grades) equal zero, 53 00:02:42.070 --> 00:02:44.190 then you can say you don't have any grades. 54 00:02:44.190 --> 00:02:46.263 You don't have grades yet. 55 00:02:48.150 --> 00:02:50.660 Or we could do this instead, and then, 56 00:02:50.660 --> 00:02:53.440 if you want, you can also put that same check here, 57 00:02:53.440 --> 00:02:54.800 you don't need this return. 58 00:02:54.800 --> 00:02:56.700 But as you can see, this is getting a little bit messy now, 59 00:02:56.700 --> 00:03:01.700 because you are removing logic from a division function 60 00:03:01.740 --> 00:03:03.240 that really should be in there, 61 00:03:03.240 --> 00:03:04.453 or it could be in there, 62 00:03:05.370 --> 00:03:06.880 and putting it in your main programme. 63 00:03:06.880 --> 00:03:08.400 It's making your main programme longer, 64 00:03:08.400 --> 00:03:10.010 it's making your function shorter, 65 00:03:10.010 --> 00:03:11.683 but it's doing a bit less. 66 00:03:13.033 --> 00:03:16.840 This function here would check for division by zero, 67 00:03:16.840 --> 00:03:18.400 and then do something a bit more useful, 68 00:03:18.400 --> 00:03:19.800 rather than print think out. 69 00:03:21.290 --> 00:03:24.580 So, here come the errors. 70 00:03:24.580 --> 00:03:28.440 In Python, errors are often used for flow control, 71 00:03:28.440 --> 00:03:30.430 very much like if statements. 72 00:03:30.430 --> 00:03:34.260 Though, by allowing your functions to raise errors, 73 00:03:34.260 --> 00:03:36.380 you're going to then be able to catch them 74 00:03:36.380 --> 00:03:39.240 in your code and handle them. 75 00:03:39.240 --> 00:03:41.963 Here's what we're gonna do, if divisor == 0, 76 00:03:43.420 --> 00:03:45.630 now we are going to raise something 77 00:03:45.630 --> 00:03:47.410 called a ZeroDivisionError, 78 00:03:47.410 --> 00:03:49.220 which is a error that comes with Python, 79 00:03:49.220 --> 00:03:52.433 and you're gonna say divisor cannot be zero. 80 00:03:53.360 --> 00:03:57.440 Now, this message here inside the brackets means 81 00:03:57.440 --> 00:04:01.153 that this error is going to have a message attached to it. 82 00:04:02.150 --> 00:04:04.200 And what we're doing here is we're saying raise, 83 00:04:04.200 --> 00:04:05.490 the keyword raise and a space, 84 00:04:05.490 --> 00:04:08.100 and then the exception name or the error name, 85 00:04:08.100 --> 00:04:10.230 exception and errors are the same thing, 86 00:04:10.230 --> 00:04:11.280 and then the bracket. 87 00:04:11.280 --> 00:04:14.880 So we're creating an exception object in here. 88 00:04:14.880 --> 00:04:17.833 This is another class that is defined inside Python. 89 00:04:18.980 --> 00:04:22.800 So if we try now to divide the sum of grades 90 00:04:22.800 --> 00:04:25.850 by the len of grades, knowing that there are no grades, 91 00:04:25.850 --> 00:04:28.040 what you're gonna get is, 92 00:04:28.040 --> 00:04:30.190 welcome to the average grade programme. 93 00:04:30.190 --> 00:04:34.720 And then you're going to get a very nice traceback. 94 00:04:34.720 --> 00:04:37.260 So let me just make that a bit bigger for you. 95 00:04:37.260 --> 00:04:41.160 A traceback shows you where the problem happened. 96 00:04:41.160 --> 00:04:45.760 And here it says that the last set of lines here 97 00:04:45.760 --> 00:04:48.460 tells you where the error originated, 98 00:04:48.460 --> 00:04:50.740 and it says it originated in code.py, 99 00:04:50.740 --> 00:04:52.250 which is our file, line three, 100 00:04:52.250 --> 00:04:55.730 which is this one here, in the divide function. 101 00:04:55.730 --> 00:04:58.540 And the problem was that you raised the error yourself. 102 00:04:58.540 --> 00:05:01.190 So because you raised an error the error happened. 103 00:05:01.190 --> 00:05:02.480 If you didn't raise the error 104 00:05:02.480 --> 00:05:03.940 the error wouldn't have happened, 105 00:05:03.940 --> 00:05:05.835 so that it what it's telling you there. 106 00:05:05.835 --> 00:05:08.510 And then it's saying that the next thing up, 107 00:05:08.510 --> 00:05:11.220 what called this divide function, 108 00:05:11.220 --> 00:05:13.720 was this line here, line 11. 109 00:05:13.720 --> 00:05:15.000 And it's showing you the line there 110 00:05:15.000 --> 00:05:17.870 that caused this function to be called, 111 00:05:17.870 --> 00:05:19.220 which then caused the exception. 112 00:05:19.220 --> 00:05:21.150 So you can see how it bubbles up, 113 00:05:21.150 --> 00:05:23.070 and in our case it's only got two lines here, 114 00:05:23.070 --> 00:05:24.870 but sometimes these can be really long, 115 00:05:24.870 --> 00:05:26.500 and they can tell you, this called that 116 00:05:26.500 --> 00:05:27.567 and that called that, and that called that, 117 00:05:27.567 --> 00:05:29.890 and so on, and eventually an error happened. 118 00:05:29.890 --> 00:05:30.990 And what you've got at the very end 119 00:05:30.990 --> 00:05:33.670 is the error itself, a ZeroDivisionError, 120 00:05:33.670 --> 00:05:34.943 divisor cannot be zero. 121 00:05:36.020 --> 00:05:38.630 Clearly we don't wanna be showing our users this, 122 00:05:38.630 --> 00:05:41.930 but this is excellent and really helpful 123 00:05:41.930 --> 00:05:42.860 for when you're coding. 124 00:05:42.860 --> 00:05:45.640 So errors help you track down problems 125 00:05:45.640 --> 00:05:47.690 and where and why they happened. 126 00:05:47.690 --> 00:05:51.540 So errors give developers a great tool for debugging, 127 00:05:51.540 --> 00:05:53.170 or finding where the problem is. 128 00:05:53.170 --> 00:05:56.840 So that's one great plus of using errors. 129 00:05:56.840 --> 00:06:00.220 The other thing is that now you can catch the error. 130 00:06:00.220 --> 00:06:03.430 You can say, okay, if an error happens in this line 131 00:06:03.430 --> 00:06:05.440 I wanna do something. 132 00:06:05.440 --> 00:06:09.300 And the syntax for that in Python is a try-except block. 133 00:06:09.300 --> 00:06:11.740 So we're going to try to do that, 134 00:06:11.740 --> 00:06:14.790 and then if a ZeroDivisionError happens, 135 00:06:14.790 --> 00:06:18.713 then we can print, there are no grades yet in your list. 136 00:06:21.450 --> 00:06:24.270 So, try, and then in an indented block, 137 00:06:24.270 --> 00:06:25.800 we put the code that we wanna try, 138 00:06:25.800 --> 00:06:27.940 this can be one line or it can be many lines. 139 00:06:27.940 --> 00:06:31.410 And then in the same indentation level as the try, 140 00:06:31.410 --> 00:06:34.190 we put except and the type of exception 141 00:06:34.190 --> 00:06:35.850 that we wanna look out for. 142 00:06:35.850 --> 00:06:38.800 If this exception happens in this code, 143 00:06:38.800 --> 00:06:41.290 then we will run this stuff here. 144 00:06:41.290 --> 00:06:46.180 So now our code can handle this ZeroDivisionError, 145 00:06:46.180 --> 00:06:48.570 meaning that if we get a ZeroDivisionError, 146 00:06:48.570 --> 00:06:51.140 we're not going to print the traceback, 147 00:06:51.140 --> 00:06:53.810 we are not going to print divisor cannot be zero. 148 00:06:53.810 --> 00:06:56.530 We are going to print, there are no grades yet in your list. 149 00:06:56.530 --> 00:06:58.830 However, if you use the divide function 150 00:06:58.830 --> 00:06:59.980 somewhere else in your programme 151 00:06:59.980 --> 00:07:03.370 and you don't wrap it in a try-except block, 152 00:07:03.370 --> 00:07:05.510 the error will still happen and the divisor cannot be zero 153 00:07:05.510 --> 00:07:08.173 will still be printed out, and the traceback. 154 00:07:09.060 --> 00:07:12.040 So this allows you to handle these errors 155 00:07:12.040 --> 00:07:13.810 in different ways throughout your programme, 156 00:07:13.810 --> 00:07:16.250 which means that this divide function 157 00:07:16.250 --> 00:07:19.530 is concerned about the mathematics of division, 158 00:07:19.530 --> 00:07:21.120 this piece of code here is concerned 159 00:07:21.120 --> 00:07:22.750 about how to handle the mathematics 160 00:07:22.750 --> 00:07:24.550 in the context of your programme, 161 00:07:24.550 --> 00:07:26.850 which is there are no grades yet in your list. 162 00:07:27.760 --> 00:07:28.967 Notice that here in the except block 163 00:07:28.967 --> 00:07:32.680 you can do as e, and what this does in Python 164 00:07:32.680 --> 00:07:35.340 is it creates a variable called e, 165 00:07:35.340 --> 00:07:39.130 and puts inside it the value of the error, if there is one. 166 00:07:39.130 --> 00:07:42.970 So if the ZeroDivisionError is created with this message, 167 00:07:42.970 --> 00:07:44.990 then e will contain that. 168 00:07:44.990 --> 00:07:46.450 And e can be anything you want, 169 00:07:46.450 --> 00:07:48.060 so you can call it exception, 170 00:07:48.060 --> 00:07:50.910 or you can call it e, or you can call it Bob if you want. 171 00:07:51.760 --> 00:07:53.740 And we can print e as well, if you like, 172 00:07:53.740 --> 00:07:58.740 just to see what happens, and what you get is this stuff. 173 00:07:58.750 --> 00:08:01.490 So you get there are no grades yet in your list, 174 00:08:01.490 --> 00:08:04.123 and you get divisor cannot be zero. 175 00:08:06.100 --> 00:08:09.133 Afterwards, because there was an error in this line, 176 00:08:10.440 --> 00:08:12.780 the average variable was never created, 177 00:08:12.780 --> 00:08:14.660 because this caused the error, 178 00:08:14.660 --> 00:08:16.843 which means our code jumped directly here. 179 00:08:17.780 --> 00:08:21.913 So, when you run line 17, the variable does not exist. 180 00:08:22.890 --> 00:08:27.120 Therefore, you may wanna put your variable in the same block 181 00:08:27.120 --> 00:08:28.960 as your variable definition. 182 00:08:28.960 --> 00:08:31.940 Now running that will give you, 183 00:08:31.940 --> 00:08:33.210 welcome to the average grade programme, 184 00:08:33.210 --> 00:08:36.060 divisor cannot be zero, there are no grades in your list. 185 00:08:37.410 --> 00:08:40.430 If you remove this print e, then that will get rid of 186 00:08:40.430 --> 00:08:43.230 the divisor cannot be zero, and you'll get, 187 00:08:43.230 --> 00:08:44.310 Welcome to the average grade programme, 188 00:08:44.310 --> 00:08:46.540 there are no grades yet in your list. 189 00:08:46.540 --> 00:08:48.030 This is exactly what we wanted, 190 00:08:48.030 --> 00:08:50.870 and using errors allows our programme to handle 191 00:08:50.870 --> 00:08:52.860 both the maths of the operation 192 00:08:52.860 --> 00:08:55.303 and also the programme context. 193 00:08:56.680 --> 00:08:58.162 As well as ZeroDivisionErrors, 194 00:08:58.162 --> 00:09:00.440 there's a whole bunch more built-in errors, 195 00:09:00.440 --> 00:09:01.690 errors that come with Python, 196 00:09:01.690 --> 00:09:04.000 that you can create for different things. 197 00:09:04.000 --> 00:09:06.300 All errors are handled the same way, 198 00:09:06.300 --> 00:09:08.040 but when you see in your programme 199 00:09:08.040 --> 00:09:10.070 that a ZeroDivisionError has happened, 200 00:09:10.070 --> 00:09:11.540 it tells you that you probably tried 201 00:09:11.540 --> 00:09:13.350 to divide something by zero. 202 00:09:13.350 --> 00:09:14.630 Know there are other types of errors, 203 00:09:14.630 --> 00:09:17.300 so that you as the programmer can more easily tell 204 00:09:17.300 --> 00:09:19.010 what happened and what went wrong. 205 00:09:19.010 --> 00:09:21.650 Some examples are a TypeError, 206 00:09:21.650 --> 00:09:23.010 or when something was the wrong type, 207 00:09:23.010 --> 00:09:26.330 and you would raise a TypeError, that. 208 00:09:26.330 --> 00:09:28.900 You've got ValueError, or when something 209 00:09:28.900 --> 00:09:31.560 had the wrong value, or an unexpected value. 210 00:09:31.560 --> 00:09:35.350 You've got RuntimeError, or most other things. 211 00:09:35.350 --> 00:09:37.990 And you can also create your own errors, 212 00:09:37.990 --> 00:09:40.250 but we're going to be doing that in the next video. 213 00:09:40.250 --> 00:09:44.170 Finally, instead of putting this line of code here 214 00:09:44.170 --> 00:09:48.860 together with the try, you can actually put it afterwards, 215 00:09:48.860 --> 00:09:53.770 but tell Python to only run it if this succeeded. 216 00:09:53.770 --> 00:09:54.950 So the way you do that in Python 217 00:09:54.950 --> 00:09:58.220 is you put an else, actually, in there. 218 00:09:58.220 --> 00:10:01.180 And you put this line of code in here. 219 00:10:01.180 --> 00:10:04.420 So this is a different way of running that same code, 220 00:10:04.420 --> 00:10:07.800 and what happens here is you try to run this, 221 00:10:07.800 --> 00:10:10.420 if an error happened then you run this, 222 00:10:10.420 --> 00:10:13.600 and if no error happened that means that this line of code 223 00:10:13.600 --> 00:10:17.410 ran successfully, then you run this. 224 00:10:17.410 --> 00:10:20.330 So the reason why we may wanna use the else, 225 00:10:20.330 --> 00:10:22.550 instead of putting this line of code here, 226 00:10:22.550 --> 00:10:26.130 is if we want to catch some specific errors 227 00:10:26.130 --> 00:10:28.240 in a specific line of code, and we don't wanna put 228 00:10:28.240 --> 00:10:31.470 all of our code in the same block. 229 00:10:31.470 --> 00:10:34.110 Finally, if you wanna run a piece of code always, 230 00:10:34.110 --> 00:10:36.850 no matter whether there is an error, or no error, 231 00:10:36.850 --> 00:10:39.820 you can use the finally clause here 232 00:10:41.460 --> 00:10:42.790 to print something else out, 233 00:10:42.790 --> 00:10:44.110 or you need to do anything else. 234 00:10:44.110 --> 00:10:45.920 So these are the different clauses, 235 00:10:45.920 --> 00:10:50.313 try, except, else and finally, and they run in that order. 236 00:10:51.200 --> 00:10:56.200 You can have multiple except clauses if you want. 237 00:10:56.860 --> 00:10:59.770 For example, if this function here could raise 238 00:10:59.770 --> 00:11:02.440 a ZeroDivisionError, or potentially somewhere else, 239 00:11:02.440 --> 00:11:03.950 it could raise a ValueError, 240 00:11:03.950 --> 00:11:05.710 you can catch them separately, 241 00:11:05.710 --> 00:11:07.923 and handle them differently if you want. 242 00:11:09.090 --> 00:11:10.270 I'm just showing you the syntax there, 243 00:11:10.270 --> 00:11:13.840 but we're gonna look as some examples in just a moment. 244 00:11:13.840 --> 00:11:16.160 Imagine you have a list of students 245 00:11:16.160 --> 00:11:18.163 and each one has a bunch of grades, 246 00:11:18.163 --> 00:11:21.890 here is when using this function might come in handy. 247 00:11:21.890 --> 00:11:23.330 'Cause you've got Bob, Rolf, and Jen, 248 00:11:23.330 --> 00:11:25.730 and Rolf doesn't have any grades. 249 00:11:25.730 --> 00:11:29.380 Though we can try to iterate over the student's list, 250 00:11:29.380 --> 00:11:31.670 get the name and grades, and calculate the average 251 00:11:31.670 --> 00:11:35.250 for a given student, printing their average afterwards. 252 00:11:35.250 --> 00:11:38.090 Of course, Rolf's average will fail, 253 00:11:38.090 --> 00:11:40.600 though we can except in a ZeroDivisionRrror, 254 00:11:40.600 --> 00:11:44.220 and say, error, Rolf has no grades. 255 00:11:44.220 --> 00:11:46.440 Otherwise, if all of them have grades, 256 00:11:46.440 --> 00:11:48.770 we can print that all the student averages 257 00:11:48.770 --> 00:11:51.750 were calculated and finally at the end, no matter what, 258 00:11:51.750 --> 00:11:54.900 we can print the end of the student average calculation. 259 00:11:54.900 --> 00:11:56.800 So if we run this we will encounter an error, 260 00:11:56.800 --> 00:12:00.760 we will print Bob's average and then Rolf will error as out. 261 00:12:00.760 --> 00:12:03.710 Note, Bob averaged 82.5 and we get an error, 262 00:12:03.710 --> 00:12:05.110 Rolf has no grades. 263 00:12:05.110 --> 00:12:08.960 But if we put a grade for Rolf and we press play again, 264 00:12:08.960 --> 00:12:11.060 now you see that all of them run to completion 265 00:12:11.060 --> 00:12:12.510 and we get back a message saying 266 00:12:12.510 --> 00:12:14.790 that all student averages were calculated, 267 00:12:14.790 --> 00:12:17.120 because that's in our else. 268 00:12:17.120 --> 00:12:20.890 So to recap, a try allows you to run a piece of code, 269 00:12:20.890 --> 00:12:24.100 then you can catch or except some exception, 270 00:12:24.100 --> 00:12:25.760 or multiple exceptions if you want, 271 00:12:25.760 --> 00:12:28.660 handling them in that block of code. 272 00:12:28.660 --> 00:12:31.160 The else keyword, together with try and except, 273 00:12:31.160 --> 00:12:34.960 allows you to run some code if there are no errors. 274 00:12:34.960 --> 00:12:37.470 And finally, you can run a piece of code at the very end, 275 00:12:37.470 --> 00:12:40.340 no matter what, whether there's an error, or not, 276 00:12:40.340 --> 00:12:41.590 in case you wanna print a message, 277 00:12:41.590 --> 00:12:43.140 or do some clean up at the end. 278 00:12:44.030 --> 00:12:46.110 That's it for this video, thank you for joining me, 279 00:12:46.110 --> 00:12:47.760 and I'll see you in the next one.