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.