1 00:00:05,400 --> 00:00:07,580 In this video, we're going to take a break 2 00:00:07,580 --> 00:00:10,450 from databases, and we're gonna look at exceptions 3 00:00:10,450 --> 00:00:13,370 and exception handling in Python. 4 00:00:13,370 --> 00:00:16,650 Let's start by creating a new project. 5 00:00:16,650 --> 00:00:19,020 You should always select Python, as always. 6 00:00:19,020 --> 00:00:23,190 Next, and we're gonna call this project ExceptionHandling. 7 00:00:30,730 --> 00:00:33,480 Okay, and let's create a new file 8 00:00:34,450 --> 00:00:36,950 in the project, and we'll call that Examples. 9 00:00:36,950 --> 00:00:39,120 New Python file, Examples. 10 00:00:41,910 --> 00:00:44,060 Okay, let's go to full screen. 11 00:00:44,060 --> 00:00:46,570 All right, so let's experiment with some code. 12 00:00:46,570 --> 00:00:49,180 Now, there's two types of errors we can get. 13 00:00:49,180 --> 00:00:52,730 There's syntax errors and exceptions. 14 00:00:52,730 --> 00:00:54,750 Now, syntax errors, I'm sure you've seen before, 15 00:00:54,750 --> 00:00:57,360 are where we make a mistake in the code. 16 00:00:57,360 --> 00:00:58,800 As an example, if I type something like 17 00:00:58,800 --> 00:01:00,470 x equals 8 equals 5, 18 00:01:02,620 --> 00:01:05,120 instead of x equals 8 minus 5, 19 00:01:05,120 --> 00:01:06,700 we actually get a syntax error. 20 00:01:06,700 --> 00:01:09,280 If I actually try and run this, 21 00:01:10,260 --> 00:01:11,240 we get this error over here, 22 00:01:11,240 --> 00:01:14,320 which I'll just change to the bottom. 23 00:01:15,800 --> 00:01:19,230 We can see we got the error: can't assign to literal. 24 00:01:19,230 --> 00:01:21,720 Now, we can fix these types of errors, obviously, 25 00:01:21,720 --> 00:01:25,330 by fixing our code so that it's valid part in code, 26 00:01:25,330 --> 00:01:29,080 and then of course, the programme is gonna run. 27 00:01:31,330 --> 00:01:33,280 Okay, no surprises there, 28 00:01:33,280 --> 00:01:35,400 but when our programmes are running, though, 29 00:01:35,400 --> 00:01:37,860 we may actually get other errors as a result 30 00:01:37,860 --> 00:01:40,040 of flaws in the code's logic, 31 00:01:40,040 --> 00:01:42,650 or conditions that we couldn't predict. 32 00:01:42,650 --> 00:01:45,860 For example, we might attempt to create a new database, 33 00:01:45,860 --> 00:01:49,880 but there isn't enough space on the drive that we specify. 34 00:01:49,880 --> 00:01:52,500 These types of errors are exceptions. 35 00:01:52,500 --> 00:01:53,640 In the next few videos, we're going 36 00:01:53,640 --> 00:01:56,610 to look at dealing with them when they happen. 37 00:01:56,610 --> 00:01:58,600 Now, the thing to remember is if you don't handle 38 00:01:58,600 --> 00:02:01,490 an exception in your code when it occurs, 39 00:02:01,490 --> 00:02:03,210 then the programme actually crashes, 40 00:02:03,210 --> 00:02:05,210 and that's almost always not a good thing 41 00:02:05,210 --> 00:02:07,030 from a user's perspective. 42 00:02:07,030 --> 00:02:08,710 Now, it turns out that it is very easy 43 00:02:08,710 --> 00:02:11,270 to generate an exception. 44 00:02:11,270 --> 00:02:13,140 We can come up here, and on the second line, 45 00:02:13,140 --> 00:02:17,800 I can put something like y equals x divided by 0, 46 00:02:17,800 --> 00:02:20,050 and if I run that, we should see an error, 47 00:02:20,050 --> 00:02:22,800 an exception, rather, a ZeroDivisionError. 48 00:02:22,800 --> 00:02:24,610 You can see we've got just that on line two, 49 00:02:24,610 --> 00:02:27,580 ZeroDivisionError: division by zero. 50 00:02:27,580 --> 00:02:29,960 If we don't handle an exception, 51 00:02:29,960 --> 00:02:32,260 it is called an unhandled exception, 52 00:02:32,260 --> 00:02:34,770 and execution of the programme stops, 53 00:02:34,770 --> 00:02:37,070 as you can see has happened in this case. 54 00:02:37,070 --> 00:02:39,960 Python then prints out the details of the exception 55 00:02:39,960 --> 00:02:43,200 and a stack trace showing where the exception occurred. 56 00:02:43,200 --> 00:02:45,900 In our case, as you can see, it was on line two, 57 00:02:45,900 --> 00:02:48,280 and the exception details are, again, 58 00:02:48,280 --> 00:02:51,570 ZeroDivisionError: division by zero. 59 00:02:51,570 --> 00:02:54,380 This first part over here before the colon 60 00:02:54,380 --> 00:02:58,150 is the type of the exception, its name, if you like, 61 00:02:58,150 --> 00:03:02,340 but the details after the colon are the extra information 62 00:03:02,340 --> 00:03:05,480 added to the exception when it occurs. 63 00:03:05,480 --> 00:03:08,440 In this particular case, it's quite a simple description, 64 00:03:08,440 --> 00:03:10,370 but you can add the values of variables 65 00:03:10,370 --> 00:03:12,050 to the exception, if you want. 66 00:03:12,050 --> 00:03:13,160 We'll be seeing how to do that 67 00:03:13,160 --> 00:03:17,090 when we raise our own exceptions a little bit later. 68 00:03:17,090 --> 00:03:19,060 You can get a list of all the built-in exceptions 69 00:03:19,060 --> 00:03:20,890 by going to this link. 70 00:03:26,500 --> 00:03:28,610 There's quite a few exceptions as you can see here, 71 00:03:28,610 --> 00:03:29,970 so I'm scrolling down. 72 00:03:29,970 --> 00:03:30,890 I'm certainly not going to attempt 73 00:03:30,890 --> 00:03:34,580 to go through them all now, but this page is useful 74 00:03:34,580 --> 00:03:35,980 because it goes into the mechanics 75 00:03:35,980 --> 00:03:38,370 of exception handling in great detail, 76 00:03:38,370 --> 00:03:41,060 but I recommend you don't read through it just yet, 77 00:03:41,060 --> 00:03:44,940 because much of it will be quite confusing at first. 78 00:03:44,940 --> 00:03:46,980 It is interesting to skip through or skim through 79 00:03:46,980 --> 00:03:50,150 the list of exceptions in section 5.2. 80 00:03:51,220 --> 00:03:52,470 Down here. 81 00:03:52,470 --> 00:03:53,940 Really just to, more or less, get a feel 82 00:03:53,940 --> 00:03:56,650 for the sorts of exceptions that are available 83 00:03:56,650 --> 00:03:59,060 for you to check in your code. 84 00:03:59,060 --> 00:04:03,150 Now, as I scroll through, one interesting one, 85 00:04:03,150 --> 00:04:07,640 as we scroll through here, is this RecursionError. 86 00:04:07,640 --> 00:04:11,570 Now, back in section 11, we looked at recursive functions, 87 00:04:11,570 --> 00:04:15,050 and wrote a recursive function to generate factorials. 88 00:04:15,050 --> 00:04:17,390 Now, if you tried to run that with large numbers, 89 00:04:17,390 --> 00:04:20,870 above 1,000 would've done it, then the stack overflows, 90 00:04:20,870 --> 00:04:23,250 and you end up with a RecursionError. 91 00:04:23,250 --> 00:04:25,590 Let's try and do that again. 92 00:04:28,370 --> 00:04:29,610 Now, it's only a few lines of code, 93 00:04:29,610 --> 00:04:31,170 so I'm just going to type it in again, 94 00:04:31,170 --> 00:04:33,410 but if you've got the code from section 11 95 00:04:33,410 --> 00:04:36,710 in the video Name Spaces: More On Scope and Recursion, 96 00:04:36,710 --> 00:04:39,610 you can grab it from there if you prefer not to type it in. 97 00:04:39,610 --> 00:04:43,780 I'm gonna type it in, def factorial(n):, 98 00:04:45,670 --> 00:04:47,700 and then we wanna do just a reminder 99 00:04:47,700 --> 00:04:51,030 that the factorial n can also be defined 100 00:04:53,090 --> 00:04:55,010 as n times n, take one, 101 00:05:01,520 --> 00:05:03,930 and calculates n recursively. 102 00:05:06,760 --> 00:05:08,030 This is essentially what we're doing here. 103 00:05:08,030 --> 00:05:09,890 All right, so in terms of the code, 104 00:05:09,890 --> 00:05:14,050 it's if n is less than or equal to one, return one, 105 00:05:15,370 --> 00:05:17,870 else return n times factorial, 106 00:05:22,900 --> 00:05:23,820 n take one. 107 00:05:25,170 --> 00:05:26,020 Now, we can actually try this 108 00:05:26,020 --> 00:05:30,190 by doing a print factorial, and let's start with 50. 109 00:05:34,580 --> 00:05:36,390 That actually runs without any error, 110 00:05:36,390 --> 00:05:40,480 so let's change that to a larger number, say 900. 111 00:05:43,500 --> 00:05:45,030 We can see we've now got a huge number here, 112 00:05:45,030 --> 00:05:46,600 but it has still worked. 113 00:05:46,600 --> 00:05:49,090 Now, this may or may not work on your system. 114 00:05:49,090 --> 00:05:50,990 It really does depend on how much memory 115 00:05:50,990 --> 00:05:52,940 you've allocated for IntelliJ. 116 00:05:52,940 --> 00:05:56,050 If you get a RecursionError already, that's fine, 117 00:05:56,050 --> 00:05:59,120 but on my system, obviously, it's working with 900, 118 00:05:59,120 --> 00:06:01,010 and we obviously have this huge number printed. 119 00:06:01,010 --> 00:06:02,830 I'll come back to that in a moment. 120 00:06:02,830 --> 00:06:06,430 All right, so let's actually try a thousand now. 121 00:06:06,430 --> 00:06:08,570 Gonna run that again. 122 00:06:08,570 --> 00:06:09,950 We actually get this error now, 123 00:06:09,950 --> 00:06:13,250 RecursionError: maximum recursion depth 124 00:06:13,250 --> 00:06:15,790 exceeded in comparison. 125 00:06:15,790 --> 00:06:19,150 Now, in this case, there's not really a lot we can do 126 00:06:19,150 --> 00:06:21,430 to fix this problem, unless we rewrite 127 00:06:21,430 --> 00:06:23,230 our factorial function, of course, 128 00:06:23,230 --> 00:06:26,720 but what we can do is prevent the programme from crashing. 129 00:06:26,720 --> 00:06:29,230 The way we wanna do that is come down here, 130 00:06:29,230 --> 00:06:32,650 and on line nine, you type in try, colon, 131 00:06:33,570 --> 00:06:35,770 then we add our print on that line. 132 00:06:35,770 --> 00:06:40,110 I'm going to initially change that back to 900. 133 00:06:40,110 --> 00:06:43,830 Then I'll come down here and type except, 134 00:06:43,830 --> 00:06:45,500 then RecursionError. 135 00:06:47,380 --> 00:06:50,470 Print, "This programme cannot calculate 136 00:06:51,540 --> 00:06:53,540 "factorials that large." 137 00:06:57,920 --> 00:07:02,000 Then here I'll do a print, "Programme terminating." 138 00:07:03,960 --> 00:07:06,740 All right, so what I've done there is put the code 139 00:07:06,740 --> 00:07:10,730 that's likely to raise an exception into a try block. 140 00:07:10,730 --> 00:07:13,910 We did that by starting this new block with the try keyword. 141 00:07:13,910 --> 00:07:17,840 Now, the code that's in between the try and the except 142 00:07:17,840 --> 00:07:20,410 is executed, and runs as per normal. 143 00:07:20,410 --> 00:07:23,750 If everything works fine, then the code that appears 144 00:07:23,750 --> 00:07:27,410 after this except line is skipped, 145 00:07:27,410 --> 00:07:29,680 and the programme continues to run. 146 00:07:29,680 --> 00:07:32,980 However, if an exception is raised inside the try clause, 147 00:07:32,980 --> 00:07:36,400 execution will jump straight to this code that's immediately 148 00:07:36,400 --> 00:07:39,400 after the except line, or the except clause. 149 00:07:39,400 --> 00:07:40,930 Now, once the code in the except clause 150 00:07:40,930 --> 00:07:44,050 is finished executing, the programme will then continue 151 00:07:44,050 --> 00:07:46,070 with the code after the trial, 152 00:07:46,070 --> 00:07:49,040 so it should print, "Programme terminating," in this case. 153 00:07:49,040 --> 00:07:51,570 We'll just do a test of this to run it, 154 00:07:51,570 --> 00:07:53,550 to make sure it does do that, 155 00:07:53,550 --> 00:07:55,110 and you can see that we've got the normal output 156 00:07:55,110 --> 00:07:58,110 we had previously with the value of 900, 157 00:07:58,110 --> 00:08:00,140 and we've also got this extra message 158 00:08:00,140 --> 00:08:02,460 that the programme is terminating. 159 00:08:02,460 --> 00:08:04,470 Let's change this number back to a thousand, 160 00:08:04,470 --> 00:08:08,650 that we now know will actually generate an exception. 161 00:08:08,650 --> 00:08:10,230 If we run that now, 162 00:08:12,890 --> 00:08:15,090 you can see what's happened down here now. 163 00:08:15,090 --> 00:08:17,690 The programme still raises an exception, 164 00:08:17,690 --> 00:08:20,830 but because the code is now inside the try clause, 165 00:08:20,830 --> 00:08:23,150 the programme hasn't crashed this time. 166 00:08:23,150 --> 00:08:26,580 Instead, what's happened is it's executed the code 167 00:08:26,580 --> 00:08:28,920 in the except clause, and we get the message: 168 00:08:28,920 --> 00:08:32,659 "This programme cannot calculate factorials that large." 169 00:08:32,659 --> 00:08:34,280 Execution then continues on, 170 00:08:34,280 --> 00:08:36,530 and we see the programme terminating message, 171 00:08:36,530 --> 00:08:40,230 which we also got when the programme worked successfully. 172 00:08:40,230 --> 00:08:41,730 That really, that's basically all 173 00:08:41,730 --> 00:08:43,650 there is to exception handling. 174 00:08:43,650 --> 00:08:45,370 You wrap your code that's likely 175 00:08:45,370 --> 00:08:48,350 to raise an exception inside try and except. 176 00:08:48,350 --> 00:08:49,810 You then decide what action 177 00:08:49,810 --> 00:08:52,910 to take if an exception is raised. 178 00:08:52,910 --> 00:08:55,620 Now, in this particular case, we just printed a message, 179 00:08:55,620 --> 00:08:58,870 but if you're updating records in a database, for example, 180 00:08:58,870 --> 00:09:00,750 then you could roll back the transaction, 181 00:09:00,750 --> 00:09:04,060 and we're actually going to be doing that soon. 182 00:09:04,060 --> 00:09:07,280 Exception handling's a really very useful feature. 183 00:09:07,280 --> 00:09:09,250 At the very least, you can prevent your code 184 00:09:09,250 --> 00:09:12,720 from crashing when something unexpected happens. 185 00:09:12,720 --> 00:09:15,290 In many situations, you can allow the programme 186 00:09:15,290 --> 00:09:17,780 to recover from whatever caused the error, 187 00:09:17,780 --> 00:09:19,780 and maybe even retry the operation 188 00:09:19,780 --> 00:09:22,430 that failed, if that was appropriate. 189 00:09:22,430 --> 00:09:23,290 Okay, so let's have a look 190 00:09:23,290 --> 00:09:26,500 at that exception clause now in more detail. 191 00:09:26,500 --> 00:09:28,420 Now, in its simplest form, 192 00:09:28,420 --> 00:09:30,590 it can just be the keyword except, 193 00:09:30,590 --> 00:09:32,730 and we'll have a look at that in a minute. 194 00:09:32,730 --> 00:09:35,360 In our programme, we actually specified 195 00:09:35,360 --> 00:09:37,490 which exception we wanted to handle. 196 00:09:37,490 --> 00:09:41,790 A RecursionError, in this case, this is the code on line 11. 197 00:09:41,790 --> 00:09:44,390 We don't expect any other type of exception 198 00:09:44,390 --> 00:09:47,150 to be raised by our factorial function. 199 00:09:47,150 --> 00:09:49,080 It's good practise to be explicit 200 00:09:49,080 --> 00:09:51,730 about which exceptions you're handling, 201 00:09:51,730 --> 00:09:54,850 and you can have several except clauses if you want to. 202 00:09:54,850 --> 00:09:58,540 Let's add a division by zero into our factorial function, 203 00:09:58,540 --> 00:10:01,220 so we can see what happens when we get an exception 204 00:10:01,220 --> 00:10:03,980 that we haven't specified in the except clause. 205 00:10:03,980 --> 00:10:06,910 I'll come down here, and let's just add a line here 206 00:10:06,910 --> 00:10:09,780 that we know is gonna generate an exception. 207 00:10:09,780 --> 00:10:13,260 We'll do a print n divided by zero, 208 00:10:13,260 --> 00:10:14,880 which we know a divided by zero will give us 209 00:10:14,880 --> 00:10:18,880 that exception we saw at the start of the video. 210 00:10:21,060 --> 00:10:24,190 We just run this to see what happens. 211 00:10:24,190 --> 00:10:25,210 You'll notice here that we have got 212 00:10:25,210 --> 00:10:28,040 a ZeroDivisionError: division by zero. 213 00:10:28,040 --> 00:10:30,530 Now, you wouldn't normally do something like I've just done. 214 00:10:30,530 --> 00:10:33,290 I've only added the code on line seven 215 00:10:33,290 --> 00:10:35,060 purely to raise an exception, 216 00:10:35,060 --> 00:10:36,780 so that we can see what happens, 217 00:10:36,780 --> 00:10:38,740 and more on how to handle 218 00:10:38,740 --> 00:10:41,000 more than one exception in our code. 219 00:10:41,000 --> 00:10:44,700 When we run the programme, we got this ZeroDivisionError. 220 00:10:44,700 --> 00:10:47,500 That's because we've only handled RecursionError, 221 00:10:47,500 --> 00:10:50,740 at the moment, and as a result the programme crashes. 222 00:10:50,740 --> 00:10:53,630 When an exception is raised in a try block, 223 00:10:53,630 --> 00:10:55,990 Python checks the except clauses to see 224 00:10:55,990 --> 00:10:59,180 if there's one that handles that particular exception. 225 00:10:59,180 --> 00:11:00,920 Now, in our case, we didn't have one, 226 00:11:00,920 --> 00:11:05,070 so the exception wasn't handled and the programme crashed. 227 00:11:05,070 --> 00:11:06,190 Now, as I mentioned, we can add 228 00:11:06,190 --> 00:11:08,130 more than one except clause though. 229 00:11:08,130 --> 00:11:10,700 Let's go ahead and do that. 230 00:11:10,700 --> 00:11:13,920 Come down here after the output we want executed 231 00:11:13,920 --> 00:11:17,580 in the case of the RecursionError exception. 232 00:11:17,580 --> 00:11:20,580 Here, we'll type except on the next line, 233 00:11:20,580 --> 00:11:24,750 and this time it's going to be ZeroDivisionError, colon, 234 00:11:25,830 --> 00:11:28,180 and we'll just print out some output, 235 00:11:28,180 --> 00:11:32,340 which will be, "What are you doing dividing by zero?" 236 00:11:36,010 --> 00:11:39,510 All right, so now when we run the programme, 237 00:11:40,420 --> 00:11:43,320 this time it doesn't crash, and you can see that we've got, 238 00:11:43,320 --> 00:11:46,830 "What are you doing dividing by zero?" showing as output. 239 00:11:46,830 --> 00:11:49,250 We're now handling the ZeroDivisionError, 240 00:11:49,250 --> 00:11:51,890 as well as the RecursionError. 241 00:11:51,890 --> 00:11:53,510 Now, in this particular case, 242 00:11:53,510 --> 00:11:55,220 assuming that the factorial function 243 00:11:55,220 --> 00:11:57,440 really could raise a ZeroDivisionError, 244 00:11:57,440 --> 00:11:59,870 we've probably taken the right approach. 245 00:11:59,870 --> 00:12:02,270 By using two separate except clauses, 246 00:12:02,270 --> 00:12:04,620 we can display a message that's appropriate 247 00:12:04,620 --> 00:12:08,080 for the specific exception that was raised, 248 00:12:08,080 --> 00:12:10,260 but there is an alternate way of doing this. 249 00:12:10,260 --> 00:12:12,270 We can handle more than one exception 250 00:12:12,270 --> 00:12:15,170 in a single except clause. 251 00:12:15,170 --> 00:12:16,960 We can modify our code here, 252 00:12:16,960 --> 00:12:19,320 and we can actually come along here, 253 00:12:19,320 --> 00:12:22,710 and add parenthesis around our exception. 254 00:12:22,710 --> 00:12:26,420 Left parenthesis, then I'm gonna put a comma, 255 00:12:26,420 --> 00:12:29,890 then ZeroDivisionError, right parenthesis, 256 00:12:29,890 --> 00:12:33,260 and I'm gonna leave the message that currently shows, 257 00:12:33,260 --> 00:12:36,230 and I'm gonna delete the second exception line. 258 00:12:36,230 --> 00:12:38,780 Even though if our ZeroDivisionError occurs, 259 00:12:38,780 --> 00:12:40,660 the output probably isn't going to be appropriate, 260 00:12:40,660 --> 00:12:42,880 because it's talking still about factorials. 261 00:12:42,880 --> 00:12:44,030 It is proof that we can do it. 262 00:12:44,030 --> 00:12:47,610 Let's actually test this out, so we'll run. 263 00:12:48,810 --> 00:12:51,960 We get the message, "The programme calculates," 264 00:12:51,960 --> 00:12:53,320 and that should be, "This programme 265 00:12:53,320 --> 00:12:56,950 "cannot calculate factorials that large." 266 00:12:56,950 --> 00:12:59,160 Let's just try running it again. 267 00:12:59,160 --> 00:13:00,980 All right, so we currently get that message, 268 00:13:00,980 --> 00:13:03,920 "This programme cannot calculate factorials that large," 269 00:13:03,920 --> 00:13:07,440 even though, in effect, the ZeroDivisionError 270 00:13:07,440 --> 00:13:11,350 was the error that we caught in this particular occasion. 271 00:13:11,350 --> 00:13:13,330 You can see the exception handler 272 00:13:13,330 --> 00:13:16,130 isn't really appropriate for the exception we got. 273 00:13:16,130 --> 00:13:18,300 Now, even with a small number, we'd see the message, 274 00:13:18,300 --> 00:13:21,660 "This programme cannot calculate factorials that large," 275 00:13:21,660 --> 00:13:25,550 when really the problem was the division by zero. 276 00:13:25,550 --> 00:13:29,960 It is useful to have one handler for multiple exceptions, 277 00:13:29,960 --> 00:13:32,140 but if you need to do something different 278 00:13:32,140 --> 00:13:35,770 for different exceptions, then use separate except clauses 279 00:13:35,770 --> 00:13:37,530 instead of what I've done here. 280 00:13:37,530 --> 00:13:39,880 Of course, we saw the separate except clauses 281 00:13:39,880 --> 00:13:42,140 a few minutes ago in the video. 282 00:13:42,140 --> 00:13:44,720 All right, so looking at our factorial function, 283 00:13:44,720 --> 00:13:46,890 what else could go wrong with it? 284 00:13:46,890 --> 00:13:48,390 Well, if you've got someone like me 285 00:13:48,390 --> 00:13:51,010 performing division by zero all over the place, 286 00:13:51,010 --> 00:13:53,380 then the answer's pretty much anything, 287 00:13:53,380 --> 00:13:56,500 but with sensible code, one thing that might happen 288 00:13:56,500 --> 00:13:58,250 is that the resulting number 289 00:13:58,250 --> 00:14:01,100 gets too large for Python to handle. 290 00:14:01,100 --> 00:14:03,420 Now, when that happens, that is when a number 291 00:14:03,420 --> 00:14:06,060 is too big to be represented by the computer, 292 00:14:06,060 --> 00:14:08,400 you get something called overflow. 293 00:14:08,400 --> 00:14:11,160 Now, if you're used to languages such as C or Java, 294 00:14:11,160 --> 00:14:12,980 you may have come across this. 295 00:14:12,980 --> 00:14:15,720 In Python, though, it's extremely difficult 296 00:14:15,720 --> 00:14:19,390 to overflow an integer, but there is an exception for it 297 00:14:19,390 --> 00:14:22,780 if we wanna be really sure that our code won't crash. 298 00:14:22,780 --> 00:14:24,630 Let's go back to that list of exceptions 299 00:14:24,630 --> 00:14:27,600 in the browser that we looked at earlier. 300 00:14:27,600 --> 00:14:30,530 We've got this RecursionError that we've looked at, 301 00:14:30,530 --> 00:14:33,050 but there's also this OverflowError. 302 00:14:33,050 --> 00:14:35,960 The OverflowError is raised when the result 303 00:14:35,960 --> 00:14:39,870 of an arithmetic operation is too large to be represented. 304 00:14:39,870 --> 00:14:42,290 As the documentation states there, 305 00:14:42,290 --> 00:14:44,080 this can't occur for integers. 306 00:14:44,080 --> 00:14:46,560 You'd run out of memory first. 307 00:14:46,560 --> 00:14:49,830 This might seem really strange to see for Java programmers, 308 00:14:49,830 --> 00:14:53,140 but Python can handle incredibly large numbers 309 00:14:53,140 --> 00:14:57,200 without having to use special classes to do so. 310 00:14:57,200 --> 00:15:01,860 I'm gonna go back to the code, and let's remove that row, 311 00:15:01,860 --> 00:15:06,310 division by zero line, the one on line seven. 312 00:15:06,310 --> 00:15:08,910 I'm gonna go back and change that factorial to 900, 313 00:15:08,910 --> 00:15:12,420 so that it doesn't, or it's not crashing anymore. 314 00:15:12,420 --> 00:15:15,260 Let's just run this again. 315 00:15:15,260 --> 00:15:16,090 You can see the number 316 00:15:16,090 --> 00:15:19,830 that's outputted there is absolutely huge. 317 00:15:19,830 --> 00:15:21,290 What I'm going to do, because you can't really see it 318 00:15:21,290 --> 00:15:22,810 in the output window too well, 319 00:15:22,810 --> 00:15:26,970 so I'm just gonna load up an editor, a text editor. 320 00:15:28,660 --> 00:15:31,160 I'm gonna paste it into there. 321 00:15:33,130 --> 00:15:34,550 Now, obviously this is quite a huge number. 322 00:15:34,550 --> 00:15:37,750 We can only sort of zoom it up so far. 323 00:15:37,750 --> 00:15:39,550 Now, I want to put this huge number 324 00:15:39,550 --> 00:15:42,160 that Python can do within to context, 325 00:15:42,160 --> 00:15:44,250 and that's why I've pasted it into this editor. 326 00:15:44,250 --> 00:15:45,640 Now, you will struggle a little bit 327 00:15:45,640 --> 00:15:47,430 if you use Notepad on Windows, 328 00:15:47,430 --> 00:15:50,680 because the number's too big to fit on one line in Notepad, 329 00:15:50,680 --> 00:15:51,900 but you'll still see the effect 330 00:15:51,900 --> 00:15:54,140 if you're doing this on your computer. 331 00:15:54,140 --> 00:15:57,720 On the line below, I'm going to paste in another number. 332 00:15:57,720 --> 00:16:01,480 This is the number of atoms in the known universe. 333 00:16:01,480 --> 00:16:04,310 The upper bound for the number of atoms in the universe 334 00:16:04,310 --> 00:16:09,140 is estimated to be four times to the power of 82. 335 00:16:09,140 --> 00:16:11,140 I'll paste that in here. 336 00:16:12,940 --> 00:16:15,100 We'll just look at the results there. 337 00:16:15,100 --> 00:16:19,200 Python's calculated a result significantly larger than that. 338 00:16:19,200 --> 00:16:21,440 If I just scroll over, look how far 339 00:16:21,440 --> 00:16:23,640 I can actually scroll this number over, 340 00:16:23,640 --> 00:16:26,140 the number is absolutely huge. 341 00:16:27,830 --> 00:16:30,230 Absolutely huge number. 342 00:16:30,230 --> 00:16:33,090 The point here is that there's really no point checking 343 00:16:33,090 --> 00:16:35,660 for an overflow operation in our code. 344 00:16:35,660 --> 00:16:37,960 It's really just not going to happen, 345 00:16:37,960 --> 00:16:41,630 but if it were possible, then that would be a good case 346 00:16:41,630 --> 00:16:44,140 for using the same exception handler. 347 00:16:44,140 --> 00:16:46,330 If overflow was possible, it would be caused 348 00:16:46,330 --> 00:16:48,800 by too large a number, which is the same thing 349 00:16:48,800 --> 00:16:51,550 that results in our RecursionError. 350 00:16:51,550 --> 00:16:53,790 In other words, coming back to the code, 351 00:16:53,790 --> 00:16:55,620 we might end up doing something like 352 00:16:55,620 --> 00:16:58,500 putting an OverflowError in there, 353 00:16:58,500 --> 00:17:01,400 and combining those exceptions. 354 00:17:01,400 --> 00:17:03,440 In other words, here it would make sense 355 00:17:03,440 --> 00:17:06,760 to use the same handler for both exceptions. 356 00:17:06,760 --> 00:17:09,300 This also emphasises that is really is worth reading 357 00:17:09,300 --> 00:17:11,660 the documentation for the exceptions 358 00:17:11,660 --> 00:17:13,510 that you plan to handle. 359 00:17:13,510 --> 00:17:14,920 Now, if we really were concerned 360 00:17:14,920 --> 00:17:16,530 about the result getting too big 361 00:17:16,530 --> 00:17:18,970 before a RecursionError was raised, 362 00:17:18,970 --> 00:17:21,089 we might be better off handling MemoryError, 363 00:17:21,089 --> 00:17:23,740 as the documentation suggested on the other page, 364 00:17:23,740 --> 00:17:25,800 rather than the OverflowError. 365 00:17:25,800 --> 00:17:27,910 All right, that's the basics of exception handling, 366 00:17:27,910 --> 00:17:30,660 and there isn't really much more to it than that. 367 00:17:30,660 --> 00:17:33,550 There's some fine details that we should know about 368 00:17:33,550 --> 00:17:34,720 to give us a bit more control 369 00:17:34,720 --> 00:17:37,600 in how we actually handle exceptions, 370 00:17:37,600 --> 00:17:39,720 and we'll be looking at those next, 371 00:17:39,720 --> 00:17:41,610 but before all that in the next video, 372 00:17:41,610 --> 00:17:43,320 let's start off with a challenge. 373 00:17:43,320 --> 00:17:45,360 I'll see you in the next video.