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.