WEBVTT 1 00:00:00.380 --> 00:00:01.940 Hi guys and welcome back! 2 00:00:01.940 --> 00:00:03.120 In this video, we're going to learn 3 00:00:03.120 --> 00:00:06.513 about Custom Error Classes and how they can be helpful. 4 00:00:08.140 --> 00:00:09.658 Let's say we've got a class book 5 00:00:09.658 --> 00:00:13.760 that defines an init method and sets three properties, 6 00:00:13.760 --> 00:00:16.780 the name, the page count, and the pages read. 7 00:00:16.780 --> 00:00:19.120 Note that I have used type hinting here 8 00:00:19.120 --> 00:00:21.150 to tell me that the name should be a string 9 00:00:21.150 --> 00:00:23.150 and the page count should be an integer. 10 00:00:24.660 --> 00:00:27.040 I'm going to also add a couple other methods here 11 00:00:27.040 --> 00:00:29.600 just to review what we've already learned. 12 00:00:29.600 --> 00:00:33.200 The repr method that we return some information 13 00:00:33.200 --> 00:00:36.451 about the book in a format that will allow me 14 00:00:36.451 --> 00:00:38.560 to then recreate this book, 15 00:00:38.560 --> 00:00:41.260 so it contains all of the information about this book. 16 00:00:42.580 --> 00:00:44.800 And by the way, you can put a return statement 17 00:00:44.800 --> 00:00:48.300 in multiple lines by using brackets and like that 18 00:00:48.300 --> 00:00:50.190 if you want to return a longer string 19 00:00:50.190 --> 00:00:52.390 and you don't want your code to be too long. 20 00:00:53.860 --> 00:00:54.790 Something like that. 21 00:00:54.790 --> 00:00:56.010 If I put it in a single line, 22 00:00:56.010 --> 00:00:58.100 it would be too long and I would have to scroll 23 00:00:58.100 --> 00:00:59.100 in order to read it. 24 00:01:01.340 --> 00:01:03.250 And I'm also going to make a read function 25 00:01:03.250 --> 00:01:06.590 that allows me to read a bunch of pages. 26 00:01:06.590 --> 00:01:09.370 So I will allow it to take in that argument 27 00:01:09.370 --> 00:01:12.970 and say self.pages read plus equal pages. 28 00:01:12.970 --> 00:01:15.167 And then I will print out a message saying, 29 00:01:15.167 --> 00:01:18.747 "You have now read that number of pages." 30 00:01:20.320 --> 00:01:21.240 Perfect! 31 00:01:21.240 --> 00:01:24.450 Note this is a simple book class that allows me 32 00:01:24.450 --> 00:01:25.530 to create new books. 33 00:01:25.530 --> 00:01:29.830 For example, Python 101 can be a book called "Python 101" 34 00:01:29.830 --> 00:01:31.520 and it may have 50 pages. 35 00:01:31.520 --> 00:01:34.660 And then I can do python101.read 35 36 00:01:36.160 --> 00:01:38.000 and then I'm going to also read 50 pages. 37 00:01:38.000 --> 00:01:39.890 So clearly, you can't do that, right? 38 00:01:39.890 --> 00:01:43.760 Because it only has 50 pages, you can't read 85 pages. 39 00:01:43.760 --> 00:01:45.733 But what happens if we press play? 40 00:01:47.600 --> 00:01:50.580 Well you have now read 35 pages out of 50 41 00:01:50.580 --> 00:01:53.689 and then you have now read 85 pages out of 50. 42 00:01:53.689 --> 00:01:55.340 Not great! 43 00:01:55.340 --> 00:01:57.850 So we are going to use arrows instead 44 00:01:57.850 --> 00:02:00.920 in order to prevent that from happening. 45 00:02:00.920 --> 00:02:03.820 Note here in the read method, 46 00:02:03.820 --> 00:02:06.420 just before we add anything to pages read, 47 00:02:06.420 --> 00:02:10.290 we're gonna say if self.pages read plus pages 48 00:02:10.290 --> 00:02:11.950 is greater than self.page, 49 00:02:13.770 --> 00:02:15.767 then we're going to raise an error and we're gonna say, 50 00:02:15.767 --> 00:02:19.390 "Too many pages read error." 51 00:02:19.390 --> 00:02:21.380 And in here, we're going to put in the message 52 00:02:21.380 --> 00:02:23.563 that will say something like this. 53 00:02:25.410 --> 00:02:28.220 Of course, this error is not built into Python 54 00:02:28.220 --> 00:02:31.410 because it's a bit too specific for our programme logic. 55 00:02:31.410 --> 00:02:34.670 We could use something like value error. 56 00:02:34.670 --> 00:02:38.630 After all, this page is the wrong amount 57 00:02:38.630 --> 00:02:40.610 given the context of our programme 58 00:02:40.610 --> 00:02:42.330 but it's not really specific enough. 59 00:02:42.330 --> 00:02:44.770 So when we are getting an error back 60 00:02:44.770 --> 00:02:46.760 in our console, it may be helpful to have 61 00:02:46.760 --> 00:02:48.163 a slightly nicer name. 62 00:02:49.090 --> 00:02:52.040 Note what we can do is before the class there, 63 00:02:52.040 --> 00:02:53.360 I usually like to define my errors 64 00:02:53.360 --> 00:02:55.060 at the top of the file, but it doesn't really matter. 65 00:02:55.060 --> 00:02:57.123 You can put them at the bottom if you want. 66 00:02:57.123 --> 00:03:01.070 We can say, "Too many pages read error," 67 00:03:01.070 --> 00:03:05.550 and we're gonna make it inherit from value error. 68 00:03:05.550 --> 00:03:08.130 You can inherit from any built-in exception or error 69 00:03:08.130 --> 00:03:11.890 if you like, in order to make it an exception class. 70 00:03:11.890 --> 00:03:16.890 If you don't inherit from an error or the exception class 71 00:03:17.100 --> 00:03:19.360 then you won't be able to raise it 72 00:03:19.360 --> 00:03:21.810 because these classes have some methods inside them 73 00:03:21.810 --> 00:03:25.210 that you need in order to be able to raise them as errors. 74 00:03:25.210 --> 00:03:27.360 Note you can create that and you actually don't have 75 00:03:27.360 --> 00:03:29.900 to do anything inside it because it already extend 76 00:03:29.900 --> 00:03:32.510 a message and all that, so you can just put pass. 77 00:03:32.510 --> 00:03:34.624 And this too many pages read error 78 00:03:34.624 --> 00:03:37.000 will essentially just be a copy of value error 79 00:03:37.000 --> 00:03:38.890 but with a different name. 80 00:03:38.890 --> 00:03:40.530 Note now that when we raise this error, 81 00:03:40.530 --> 00:03:42.870 we're going to get a nice message there 82 00:03:42.870 --> 00:03:45.187 that says, "Too many pages read error. 83 00:03:45.187 --> 00:03:46.457 "You tried to read 85 pages, 84 00:03:46.457 --> 00:03:48.307 "but this book only has 50 pages." 85 00:03:49.820 --> 00:03:52.250 In addition, you get your traceback what tells you 86 00:03:52.250 --> 00:03:54.300 where the problem was, which is that you tried 87 00:03:54.300 --> 00:03:58.010 to read 50 pages there but you'd already read some before. 88 00:03:58.010 --> 00:03:59.440 When you're looking back through your code, 89 00:03:59.440 --> 00:04:02.230 you can very easily determine where abouts 90 00:04:02.230 --> 00:04:03.290 in your code the problem happened 91 00:04:03.290 --> 00:04:05.030 and you can read through it and try to find out 92 00:04:05.030 --> 00:04:06.093 what the problem was. 93 00:04:07.140 --> 00:04:09.770 Now of course, this isn't great for users, 94 00:04:09.770 --> 00:04:12.440 so if you wanted to print a nicer message, 95 00:04:12.440 --> 00:04:15.983 you can always try to do that. 96 00:04:18.489 --> 00:04:22.527 And if not, except it, and print the message on its own. 97 00:04:25.110 --> 00:04:25.943 Just like that. 98 00:04:28.830 --> 00:04:30.630 That's it for creating custom classes. 99 00:04:30.630 --> 00:04:32.510 It's really very straightforward. 100 00:04:32.510 --> 00:04:34.070 You just inherit from the most 101 00:04:34.070 --> 00:04:36.220 relevant built-in error class, 102 00:04:36.220 --> 00:04:39.400 like value error, runtime error, or even type error 103 00:04:39.400 --> 00:04:41.420 and you name it as you would like. 104 00:04:41.420 --> 00:04:43.490 That gives you a slightly nicer name 105 00:04:43.490 --> 00:04:45.650 and the ability to find your problems 106 00:04:45.650 --> 00:04:46.870 a bit more easily. 107 00:04:46.870 --> 00:04:48.610 Most of the time, you don't have to put anything 108 00:04:48.610 --> 00:04:49.790 inside the error classes, 109 00:04:49.790 --> 00:04:52.080 just giving them a new name is good enough. 110 00:04:52.080 --> 00:04:54.350 But you can do, if you want. 111 00:04:54.350 --> 00:04:57.070 We're not gonna cover how to populate the body 112 00:04:57.070 --> 00:04:59.140 of your custom error classes in this course, though. 113 00:04:59.140 --> 00:05:00.890 That's a little bit more advanced. 114 00:05:00.890 --> 00:05:02.330 Thank you for joining me in this one 115 00:05:02.330 --> 00:05:04.080 and I'll see you in the next video.