1 00:00:04,980 --> 00:00:09,600 As we discussed in the last video, not all functions return a useful value. 2 00:00:09,600 --> 00:00:12,980 Sometimes, you'll want to write a function that does something but doesn't 3 00:00:12,990 --> 00:00:17,040 produce a result. I'd like to thank Aubrey Love, a student in this course, for 4 00:00:17,040 --> 00:00:21,419 asking the question that led to this next function. Aubrey wanted to print 5 00:00:21,419 --> 00:00:24,750 some instructions for an adventure game, and wanted them to be neatly formatted 6 00:00:24,750 --> 00:00:29,189 as a banner, when the program started. I'll create a new Python file called 7 00:00:29,189 --> 00:00:37,400 banner.py for this example. 8 00:00:37,450 --> 00:00:41,320 So we'll start by seeing the sort of output we want to produce. What I'm going 9 00:00:41,320 --> 00:00:45,340 to do is paste in some code and run the program, so that we can see what we want 10 00:00:45,340 --> 00:00:48,070 to produce. So don't type this code in. We're going 11 00:00:48,070 --> 00:00:52,780 to write a function to take care of the formatting for us. Okay we'll run the 12 00:00:52,780 --> 00:00:57,330 output, or run the program rather, to get the output, 13 00:00:58,620 --> 00:01:03,040 and there's the output on-screen. So formatting your code in the editor 14 00:01:03,040 --> 00:01:06,900 would be a bit tedious. You'd have to count the spaces that appear either side 15 00:01:06,900 --> 00:01:12,740 of the text, for each line. Each line is actually centered between the asterisks, 16 00:01:12,740 --> 00:01:17,600 and it's a bit boring counting spaces. We've got computers to do the boring stuff for us, 17 00:01:17,600 --> 00:01:21,700 so let's write a function to take care of formatting the text. What I'm 18 00:01:21,700 --> 00:01:27,300 going to do is delete all that code, and instead paste in a function. 19 00:01:27,659 --> 00:01:31,350 Alright, so there's the code. Alright, so pause the video while you type in the 20 00:01:31,350 --> 00:01:35,399 function. There's nothing complicated in this function, but I'll quickly run 21 00:01:35,399 --> 00:01:38,820 through what it's doing. We start by binding a variable to the 22 00:01:38,820 --> 00:01:42,509 width that we want our banner to be. That's currently 80, as you can see on 23 00:01:42,509 --> 00:01:47,320 line two. Using a variable makes it much easier to change the width if we want 24 00:01:47,320 --> 00:01:51,420 Next we check the length of the text. If it's too long, our banner won't look 25 00:01:51,420 --> 00:01:54,899 right. At this point in the course, we haven't covered everything we need to 26 00:01:54,900 --> 00:01:58,980 handle this situation, so rather than introducing something new, 27 00:01:58,980 --> 00:02:03,000 I've just printed a long and formatted message. That'll make it obvious that 28 00:02:03,000 --> 00:02:08,068 something's going wrong. What we really should do, is make the program crash. I'll 29 00:02:08,068 --> 00:02:11,940 do that when we test that bit of code, so you can see the correct way to handle 30 00:02:11,940 --> 00:02:15,800 invalid situations, but for now that message will do the job. 31 00:02:15,800 --> 00:02:20,340 The main body of the function prints a row of asterisks, if the calling code 32 00:02:20,340 --> 00:02:25,200 passed an asterisk to our function. That's the code on lines seven and eight. 33 00:02:25,200 --> 00:02:30,810 Otherwise, it prints the text with two asterisks at the start and end. We use 34 00:02:30,810 --> 00:02:35,489 the string center method to take care of centering the text. You can check the 35 00:02:35,489 --> 00:02:38,489 method in the string methods documentation. We've looked at that a few 36 00:02:38,489 --> 00:02:42,900 times now. 4 is subtracted from the screen width because we're printing 37 00:02:42,900 --> 00:02:48,080 4 asterisks - 2 each side of the text. We use normal string formatting with a 38 00:02:48,090 --> 00:02:53,370 placeholder, to print the centered string inside the asterisks. If that looks 39 00:02:53,370 --> 00:02:57,750 confusing, remember that you can split code up, and that can make it easier to 40 00:02:57,750 --> 00:03:01,820 understand when you're learning. For example, I could cut out text.center - 41 00:03:01,820 --> 00:03:07,440 this here, this entire call - including the parentheses screen_width take 4. 42 00:03:07,440 --> 00:03:11,900 I could cut that. We come up to the previous line and assign that expression 43 00:03:11,910 --> 00:03:16,400 to another variable. Let's do that, so I'm going to call this one centered 44 00:03:16,400 --> 00:03:21,580 text is equal to, and I'll paste it in again, and then we'll use that variable, 45 00:03:21,580 --> 00:03:26,520 centered_text. That can help with the confusion. So if you find that easier 46 00:03:26,520 --> 00:03:30,420 to understand, then it's fine to write your code that way. In time, you'll avoid 47 00:03:30,420 --> 00:03:34,050 the intermediate steps and start combining statements, such as those on 48 00:03:34,050 --> 00:03:39,000 lines 10 and 12. Alright, so let's write some code now, that calls our function, 49 00:03:39,000 --> 00:03:44,010 and come down here and start on lines 15, or line 15. Remember that we leave two 50 00:03:44,010 --> 00:03:47,940 blank lines after a function definition. Now you could use any text you want to 51 00:03:47,940 --> 00:03:51,900 create your banner. I'm going to paste in part of the Monty Python song, Always look on 52 00:03:51,900 --> 00:03:56,340 the bright side of life. Okay, so there's what I'll paste it in, 53 00:03:56,340 --> 00:04:00,320 noting that we're calling the banner text function, which is what we called 54 00:04:00,330 --> 00:04:05,190 our function on line one. So we're calling that instead of print. The banner 55 00:04:05,190 --> 00:04:09,120 text function takes care of printing, but also formats the text for us. 56 00:04:09,120 --> 00:04:12,200 So if we run the program now, 57 00:04:12,490 --> 00:04:16,510 you can see we've got our neatly formatted banner. So that's a lot easier 58 00:04:16,510 --> 00:04:21,380 than counting spaces. We've got a simple way to print neatly formatted output, 59 00:04:21,380 --> 00:04:26,760 just by passing a string to the banner underscore text function. So we pass an 60 00:04:26,770 --> 00:04:33,820 asterisk to the function on lines 15 and 25. When the function gets an asterisk, 61 00:04:33,820 --> 00:04:39,820 it prints a row of asterisks. Line 20 isn't very intuitive. We want to include a 62 00:04:39,820 --> 00:04:44,590 blank line, but we don't want a full row of asterisks. Passing a space as the 63 00:04:44,590 --> 00:04:49,480 argument produces the effect we want. The space gets centered, with asterisks at 64 00:04:49,480 --> 00:04:54,120 either end. And again, we'll run that to see the output. 65 00:04:54,120 --> 00:04:58,830 So that's not very obvious though - that functionality - to anyone who uses the 66 00:04:58,830 --> 00:05:03,210 function. If you want to print a blank line with a built-in print function, you 67 00:05:03,210 --> 00:05:08,160 call print without providing an argument. Ideally, our banner text function should 68 00:05:08,160 --> 00:05:12,600 work in the same way. We'll get to do that soon. Before that though, I want to 69 00:05:12,600 --> 00:05:16,590 finish this video with a comment about our function's return value. 70 00:05:16,590 --> 00:05:21,330 So basically, we're not using it. What I mean is, we wouldn't do something like 71 00:05:21,330 --> 00:05:28,710 this; result equals banner_text, 72 00:05:28,710 --> 00:05:32,190 Nothing is returned. 73 00:05:32,319 --> 00:05:35,979 But if I hover over the warning there, Function banner_text doesn't 74 00:05:35,979 --> 00:05:40,479 return anything. But that's not strictly true, in fact, as I've alluded to previously. 75 00:05:40,479 --> 00:05:45,309 Every Python function returns something. Functions return None if you don't 76 00:05:45,309 --> 00:05:50,320 specify a value to return. We can check that by printing out the result again again. 77 00:05:50,320 --> 00:05:54,200 And again, I know you've seen this previously, so result. 78 00:05:56,020 --> 00:05:58,790 We can see None there, down the bottom, 79 00:05:58,790 --> 00:06:05,340 as the last line of output. So our banner_text function is designed, 80 00:06:05,340 --> 00:06:10,580 or intended to perform an action, not to return a result. It returns None, because 81 00:06:10,580 --> 00:06:16,310 that's what Python does, but the return value is no use to us here. Now you know 82 00:06:16,310 --> 00:06:20,060 why you get None printed out in situations like this. 83 00:06:20,060 --> 00:06:26,160 And I'll type in an example here, so numbers is equal to square brackets; 84 00:06:26,160 --> 00:06:40,380 four two seven five eight three nine six one, print numbers dot sort. 85 00:06:41,540 --> 00:06:44,290 If we run this, 86 00:06:44,380 --> 00:06:48,340 hopefully, that now makes it clear why you get None printed out when you do 87 00:06:48,340 --> 00:06:51,820 something like I've just done there, on lines 30 and 31, or line 31 88 00:06:51,820 --> 00:06:56,920 specifically. The dot sort method sorts the list in place. So it's performing an 89 00:06:56,920 --> 00:07:01,360 action, sorting the list here, and doesn't return a useful value, and that's why 90 00:07:01,360 --> 00:07:05,110 we've got None in the output. Alright, so I'm gonna finish this video now with 91 00:07:05,110 --> 00:07:09,210 a quick summary. 92 00:07:52,020 --> 00:07:53,580 Let's move on now to the next video, 93 00:07:53,580 --> 00:07:56,540 where we'll start talking about handling invalid arguments. 94 00:07:56,540 --> 00:07:59,620 See you in the next video.