WEBVTT 0 00:00.630 --> 00:04.580 In this lesson, I'll show you how you can generate your own exceptions 1 00:04.580 --> 00:07.470 in case your code encounters unexpected situations. 2 00:08.250 --> 00:13.140 So these are the four keywords are most important when it comes to error 3 00:13.140 --> 00:15.360 handling and catching exceptions. 4 00:15.870 --> 00:20.790 The final keyword that is related is one called raise. 5 00:21.450 --> 00:25.890 And what this allows us to do is to raise our own exceptions. 6 00:27.090 --> 00:27.930 For example, 7 00:27.960 --> 00:32.730 let's say I decide that at the very end of all of our file, 8 00:32.970 --> 00:37.140 no matter if this file went through or if it didn't, if there were an error 9 00:37.140 --> 00:38.160 if there wasn't error, 10 00:38.520 --> 00:43.110 I'm still going to raise an exception. And to raise an exception 11 00:43.140 --> 00:47.460 all you have to do is tap into one of the known exception classes. 12 00:47.670 --> 00:52.020 So let's say I just decided to raise a key error. Now, when I hit run 13 00:52.050 --> 00:56.250 you can see no matter what happened, and in fact, there is no error. 14 00:56.490 --> 01:00.270 I decided to raise this error and that's what I see right here. 15 01:00.570 --> 01:02.760 I could change that to a type error, 16 01:03.150 --> 01:05.640 and that is what will crash my code. 17 01:06.150 --> 01:10.230 And I can even specify a message along with this error. 18 01:10.230 --> 01:14.670 So I could say raise a type error and let's just say the message is, 19 01:17.250 --> 01:21.510 and you can see now when it runs, it hits that type error. And it says, 20 01:21.570 --> 01:26.490 this is an error that I made up. So when might you want to raise errors? 21 01:27.210 --> 01:27.630 Well, 22 01:27.630 --> 01:31.620 let's comment out all the code that we have because we have that type error at 23 01:31.620 --> 01:34.320 the end so it's always going to crash the code no matter what. 24 01:34.590 --> 01:37.200 So let's comment it out and let's start from scratch. 25 01:37.560 --> 01:41.550 Let's say that I decided to calculate the body mass index or the BMI of 26 01:41.550 --> 01:46.080 somebody's and I'm getting them to input their height as a float. 27 01:46.140 --> 01:50.340 So this is going to be their height in meters and their weight as an integer. 28 01:50.880 --> 01:55.880 Now I can calculate the BMI by taking the weight / height * height. 29 02:00.360 --> 02:05.360 And you can either do this and wrap it inside some parentheses, 30 02:05.700 --> 02:09.240 or you can use the power function or even better 31 02:09.240 --> 02:13.950 you can actually use the builtin exponent, so height to the power of 2. 32 02:15.270 --> 02:18.870 No matter which way we use, we can print out the BMI. 33 02:19.140 --> 02:24.140 But if I provide a height that is just an unrealistic non-human height, 34 02:24.270 --> 02:27.180 lets say there were 45 meters tall, 35 02:27.270 --> 02:29.880 like some sort of four storey building. 36 02:30.390 --> 02:35.070 And then I gave them a weight, so let's say they are 67 kilos. 37 02:35.400 --> 02:39.360 Then obviously a mistake has been made somewhere in the height, right? 38 02:39.360 --> 02:42.210 Because this is just not within the normal human range. 39 02:42.210 --> 02:45.990 This is like Godzilla's height. If I go ahead and hit enter, 40 02:46.260 --> 02:50.190 you can see that we get at BMI and it is calculated correctly 41 02:50.520 --> 02:54.870 and there are no errors because everything is perfectly valid other than the 42 02:54.870 --> 02:59.040 fact that this height should not really ever go over 3 meters. 43 02:59.740 --> 03:03.100 In this case, we might want to raise our own exception. 44 03:03.520 --> 03:08.520 So we can go ahead and say that if the height is greater than 3 meters, 45 03:11.050 --> 03:13.720 well, in that case, it's probably not a valid height. 46 03:14.020 --> 03:17.560 So we can go ahead and raise a value error. 47 03:17.950 --> 03:22.210 So this is an error that says that whatever value was entered 48 03:22.570 --> 03:25.210 as the argument is probably wrong. 49 03:25.720 --> 03:28.300 And we can accompany that with a message. 50 03:29.410 --> 03:33.310 Human height should not be over 3 meters. 51 03:34.780 --> 03:38.920 Now, when we run our code and the user mistakenly types 52 03:38.980 --> 03:43.540 a wrong height, then it's going to actually give us this error 53 03:43.570 --> 03:47.290 and it won't proceed to give them an inaccurate BMI. 54 03:47.980 --> 03:51.310 So this is how you might raise your own exceptions 55 03:51.640 --> 03:55.360 when there are certain things that are not caught by the code because it's 56 03:55.360 --> 04:00.340 perfectly valid code, but it's in fact going to generate the wrong results. 57 04:01.960 --> 04:05.950 Now that we've seen all of these aspects of exceptions, 58 04:06.400 --> 04:10.300 I want you to have a go at catching some exceptions that can commonly occur. 59 04:10.810 --> 04:13.840 So head over to the next lesson where I've got an exercise for you.