1 00:00:05,140 --> 00:00:09,130 Alright, so let's talk now about handling invalid arguments. What I'm 2 00:00:09,130 --> 00:00:13,320 going to start off by doing, is deleting the last few lines from the previous video, 3 00:00:13,320 --> 00:00:15,800 because I don't want to leave you with code that you shouldn't write. 4 00:00:15,800 --> 00:00:19,020 So I'm going to go ahead and delete that code 5 00:00:19,020 --> 00:00:25,260 Alright, so looking back up, or scrolling back up to line three now, in our banner_ text function. 6 00:00:25,260 --> 00:00:29,660 We're checking that the string we want to print will fit in a specified width. 7 00:00:29,660 --> 00:00:33,520 We haven't tested that part of our function yet, so we'd better do that now. Rather 8 00:00:33,520 --> 00:00:36,850 than messing about with all our strings in the main code, what I'll do is just 9 00:00:36,850 --> 00:00:42,860 reduce the width on line two. We're gonna set that to 50 instead of 80. 10 00:00:42,860 --> 00:00:46,880 Alright, so now that I've done that, I'll run the program again. 11 00:00:47,620 --> 00:00:54,380 And you can see, looking at that output, it's absolutely horrible - I'm sure you'd agree. 12 00:00:54,380 --> 00:00:58,840 A few of the lines are too big to fit in 50 characters. So what can we do 13 00:00:58,840 --> 00:01:03,309 in situations like this. If you recall, I chose to print a warning message and you 14 00:01:03,309 --> 00:01:07,930 can see that there in the output. It's obvious from the output that something's 15 00:01:07,930 --> 00:01:11,229 gone wrong, and the message explains what happened; the text is too long to fit 16 00:01:11,229 --> 00:01:16,220 in the specified width. That's not a bad approach here but it does have drawbacks. 17 00:01:16,220 --> 00:01:20,100 If we were setting this output to a machine that automatically prints 18 00:01:20,100 --> 00:01:24,440 plastic signs, then the result wouldn't be very good. The best we could hope for 19 00:01:24,440 --> 00:01:28,760 is that one of the plastic sheets is wasted. The worst that could happen 20 00:01:28,760 --> 00:01:33,000 is that the laser burns a hole in the operators shoe, when our code sends it 21 00:01:33,009 --> 00:01:36,880 past the end of the sheet. Python's a good language for controlling robots and 22 00:01:36,880 --> 00:01:41,380 machinery, but if your resume says Wrote a program that burnt people's feet off, I'm 23 00:01:41,380 --> 00:01:45,100 sure you'd agree, it wouldn't look good. So what I'm going to do instead, is cause 24 00:01:45,100 --> 00:01:51,360 the function to crash, and we can do that by raising an exception. 25 00:01:51,860 --> 00:01:55,350 So what I'm going to do, is close down the output pane, 26 00:01:55,350 --> 00:01:59,700 and I'll actually enter the text first - the code - and then we'll talk about it 27 00:01:59,700 --> 00:02:04,140 once we've seen it working. So I'm gonna start off by deleting these two lines here, 28 00:02:04,140 --> 00:02:09,039 lines four and five now. And instead, I'll tap over there and replace that 29 00:02:09,039 --> 00:02:14,420 code with this; so I'm going to type raise ValueError, so space and a Value 30 00:02:14,420 --> 00:02:18,580 Error and a capital V. You can see there that IntelliJ is doing that for us. 31 00:02:18,580 --> 00:02:22,760 In parentheses and in double quotes I'm going to type String, using a replacement 32 00:02:22,760 --> 00:02:33,360 field zero, so curly braces with a zero in it, is larger than specified width. 33 00:02:33,360 --> 00:02:39,400 Another replacement field, the second one, one - so one in curly braces - closing double quote. 34 00:02:39,400 --> 00:02:47,760 On the next line we'll do a dot format parentheses text comma screen_width, 35 00:02:47,760 --> 00:02:51,540 and we've got our two closing right parentheses there. Alright, so now 36 00:02:51,540 --> 00:02:55,480 that I've done that, if we run the program, 37 00:02:55,480 --> 00:02:59,120 remembering what we got last time. This time when we run it, we've now got a crash, 38 00:02:59,120 --> 00:03:04,040 as you can see on the screen there. So a program that crashes isn't ideal, 39 00:03:04,040 --> 00:03:07,819 but it's definitely preferable to burning someone's foot off with a laser. 40 00:03:07,819 --> 00:03:11,970 Sometimes there's nothing else your functions can do. At least we've made the 41 00:03:11,970 --> 00:03:16,020 programmer aware that they've made a mistake when calling our function. 42 00:03:16,020 --> 00:03:19,620 Of course, most of the time that programmer will be you, but you'll also write 43 00:03:19,620 --> 00:03:23,250 functions that other programmers will use. It's quite common for different 44 00:03:23,250 --> 00:03:26,940 programmers to work on parts of a large program. Alright, so have a look at the 45 00:03:26,940 --> 00:03:31,290 error message in the output. It's quite descriptive, and tells us which string 46 00:03:31,290 --> 00:03:35,100 caused the problem and what the specified width is: 50. We can scroll over 47 00:03:35,100 --> 00:03:39,170 and we can see the complete error there. 48 00:03:40,680 --> 00:03:45,160 Now that's generated from the format string, on line four of our code here. 49 00:03:45,160 --> 00:03:49,040 You shouldn't raise exceptions very often but when you do, make sure you include 50 00:03:49,049 --> 00:03:53,010 enough information to help the programmer who gets the error. So before 51 00:03:53,010 --> 00:03:57,349 we look at lines four and five in more detail, a quick word about the output. 52 00:03:57,349 --> 00:04:01,799 IntelliJ shares the output in error consoles in a single window. That means 53 00:04:01,800 --> 00:04:05,720 that you might see the exception text mixed up with the program's output. 54 00:04:05,720 --> 00:04:09,320 You won't get that effect when your program's executed normally. It's just the 55 00:04:09,329 --> 00:04:13,349 way things work in some IDEs. So don't worry if things are mixed up in there - 56 00:04:13,349 --> 00:04:16,350 that's completely normal. Alright, so let's have a look at the code 57 00:04:16,350 --> 00:04:20,459 that's raised the exception. That's on lines four and five on-screen now. 58 00:04:20,459 --> 00:04:24,150 I split the code over two lines to keep the line links within the PEP 8 59 00:04:24,150 --> 00:04:28,260 guidelines. It also makes them show up better on video. So we start with the 60 00:04:28,260 --> 00:04:33,060 keyword raise - that's how you raise an exception in Python. Next we specify the 61 00:04:33,060 --> 00:04:37,600 type of exception that we want to raise. ValueError is a good choice here, 62 00:04:37,600 --> 00:04:42,320 because the problem was caused by one of the values that were passed to our function. 63 00:04:42,320 --> 00:04:51,100 Now if we open a browser we can check out some of the available exceptions. 64 00:04:51,160 --> 00:04:57,600 Alright, so I'm gonna scroll down here until we get to the concrete exceptions area, down here. 65 00:04:57,600 --> 00:05:01,460 In here, you'll find an alphabetical list of exceptions that you might get when 66 00:05:01,460 --> 00:05:06,520 your code contains errors. You can also raise any of these exceptions yourself. 67 00:05:06,520 --> 00:05:10,120 There's probably only two of these that you'd normally use in your own function. 68 00:05:10,120 --> 00:05:15,260 The first is type error - I'll just scroll down a little bit so we can see that one. 69 00:05:16,599 --> 00:05:20,349 OK, you can see it on screen there now. That's raised when an operational 70 00:05:20,349 --> 00:05:25,689 function is applied to an object of inappropriate type. You may want to raise 71 00:05:25,689 --> 00:05:30,360 that if your function excepts an integer, but it gets passed a string, for example. 72 00:05:30,360 --> 00:05:33,980 And the one we used here, if we scroll down a little bit further, 73 00:05:34,610 --> 00:05:39,169 was a ValueError. That's raised when an operational function receives an 74 00:05:39,169 --> 00:05:43,819 argument that has the right type, but an inappropriate value. And that's what 75 00:05:43,819 --> 00:05:46,879 happened in our function - we got strings that were too long. So this 76 00:05:46,879 --> 00:05:51,740 exception was a good choice, in our case. You might want to also consider using 77 00:05:51,740 --> 00:05:56,500 the next one - the ZeroDivisionError. If your code checks that a divisor isn't 0, 78 00:05:56,500 --> 00:06:01,280 you raise this exception if it is. I said you may want to use this one, not that 79 00:06:01,280 --> 00:06:05,330 you will want to, because your attempt to divide by zero will probably raise it 80 00:06:05,330 --> 00:06:09,780 for you anyway. If you attempt to divide by zero, you'll get a ZeroDivisionError, 81 00:06:09,780 --> 00:06:14,300 without having to explicitly raise it yourself. So make sure you bookmark this 82 00:06:14,300 --> 00:06:18,680 page because it's useful, even if you're not raising your own exceptions. If your 83 00:06:18,680 --> 00:06:22,430 code crashes, you can look up the exception in here, and maybe get a bit more 84 00:06:22,430 --> 00:06:26,719 information about what caused the crash. Generally, though, the text of the error 85 00:06:26,719 --> 00:06:30,259 that you'll receive in the exception, will help you to work out what went 86 00:06:30,260 --> 00:06:35,980 wrong. Alright, so back to our code. Okay, so our function now informs the caller 87 00:06:35,980 --> 00:06:39,860 if they try to use a string that's too long. But maybe they want to print longer 88 00:06:39,860 --> 00:06:43,820 strings. They might have put the plastic sheet into the machine in landscape, 89 00:06:43,820 --> 00:06:48,240 giving a lot more width to print on. Our function shouldn't restrict them to only 90 00:06:48,259 --> 00:06:52,550 80 characters - or 50, as it is at the moment. You should really let them pass 91 00:06:52,550 --> 00:06:56,870 in the width, as another argument to the function. You may have noticed by now, 92 00:06:56,870 --> 00:07:02,349 that whenever I say something like you should, it's time for another challenge. 93 00:07:02,510 --> 00:07:06,620 Now the challenge is to 94 00:07:15,610 --> 00:07:21,150 I'll go over my solution to this challenge in the next video.