1 00:00:04,660 --> 00:00:09,400 In this video, we're going to add a bit more code to our simple jukebox program. 2 00:00:09,400 --> 00:00:13,070 We've got it printing the menu of albums, and now we need to add the menu of songs 3 00:00:13,070 --> 00:00:17,360 for a chosen album. We won't finish the program in this video, though. 4 00:00:17,360 --> 00:00:21,919 That's because I want to use this code to talk about constants in Python. I'll talk 5 00:00:21,920 --> 00:00:25,500 about them in a moment. First though, let's tidy up our code and add a bit more. 6 00:00:25,500 --> 00:00:31,080 We don't need that second for loop, so I'm going to remove that. So our menu should only 7 00:00:31,080 --> 00:00:34,420 show the album name. At the moment, we're printing all the details for an album, 8 00:00:34,420 --> 00:00:37,820 which makes the menu a bit cluttered. So let's remove the extra 9 00:00:37,820 --> 00:00:45,280 placeholders on line 6, so that we've only got a total of 2 there. 10 00:00:45,280 --> 00:00:50,920 We're only going to output index + 1 plus title, so we're going to remove artist, year and songs 11 00:00:50,920 --> 00:00:55,900 from line 7, like so. Now that's shortened the line quite 12 00:00:55,900 --> 00:00:59,080 considerably, and in fact, that'll fit on a single line now, so let's go back 13 00:00:59,080 --> 00:01:03,460 and do that, like so. Alright, so let's run that just to make 14 00:01:03,469 --> 00:01:07,490 sure that I haven't broken anything. It's a good idea, actually, to run your program 15 00:01:07,490 --> 00:01:11,320 often, when developing it. That way, if you get an error, you only have to look at 16 00:01:11,320 --> 00:01:14,869 the small amount of code that you've just changed. If you type in thousands of 17 00:01:14,869 --> 00:01:18,140 lines of code before running it, you could have all sorts of bugs and it'll be very hard 18 00:01:18,140 --> 00:01:20,460 to try and figure out where the problem is. 19 00:01:20,460 --> 00:01:23,360 Alright, so you can see there when I ran it, we get our albums menu printed out, 20 00:01:23,360 --> 00:01:26,899 which is just what we wanted. Next what we need to do, is get some input from the 21 00:01:26,899 --> 00:01:32,149 user, then check that it's a valid choice. So let's add some code for that. 22 00:01:32,149 --> 00:01:36,890 So I'm gonna start down here on line 8. I'm gonna type choice - and I'm indented to 23 00:01:36,890 --> 00:01:43,790 the same level as the for loop - choice equals int parentheses, and within 24 00:01:43,790 --> 00:01:47,509 the parentheses, input left and right parentheses. So obviously, 25 00:01:47,509 --> 00:01:51,020 as always we've got our closing right parenthesis. Then on line 9, I'm going to 26 00:01:51,020 --> 00:02:00,440 type if 1 is less than equal to choice, less then equal to len albums, 27 00:02:00,440 --> 00:02:08,259 right parenthesis : On the next line, songs_ list is equal to albums 28 00:02:08,259 --> 00:02:11,980 left square brackets, and within the square brackets, choice 29 00:02:11,990 --> 00:02:15,830 - 1, then we've got our right square bracket then the three in square 30 00:02:15,830 --> 00:02:20,900 brackets, after that. Then on the next line - line 11, indented to the same 31 00:02:20,900 --> 00:02:26,940 level as the if, I'm going to type else : then go down to line 12 and indent break 32 00:02:26,940 --> 00:02:31,400 to the next level. Alright, so on line 8, we get input 33 00:02:31,400 --> 00:02:35,510 from the user and convert it into an integer. Our program will crash if the 34 00:02:35,510 --> 00:02:39,050 user types anything other than digits. As I said, we'll be looking at how to deal with 35 00:02:39,050 --> 00:02:42,770 that, later in the course. Line 9's checking that the value they entered is 36 00:02:42,770 --> 00:02:48,860 within the required range. Our valid choice is from 1 to the length of the albums list. 37 00:02:48,860 --> 00:02:52,500 We've got four albums, so any value greater than four is invalid. 38 00:02:52,500 --> 00:02:57,520 Zero is also invalid. If they enter an invalid choice, we break out of the loop 39 00:02:57,520 --> 00:03:03,420 via the else statement. Line 10 is binding a variable called songs_list 40 00:03:03,420 --> 00:03:06,740 to the list of songs for the selected album. If you find yourself 41 00:03:06,740 --> 00:03:11,500 struggling with nested indexing like this, a good technique is to print the values out. 42 00:03:11,500 --> 00:03:16,700 Let's do that. We're going to indent the code, starting on line 14, to the same level as the if. 43 00:03:16,700 --> 00:03:21,980 So print parentheses albums, then left and right square brackets and 44 00:03:21,980 --> 00:03:27,000 within the square brackets, we're going to type choice - 1, then we've got our 45 00:03:27,000 --> 00:03:31,160 closing right square bracket and right parenthesis. On the next line, 46 00:03:31,160 --> 00:03:36,020 also indented at the same level, print parentheses songs_list and 47 00:03:36,020 --> 00:03:42,800 finally, line 16, print, and just a set of parentheses to print an empty line. 48 00:03:42,800 --> 00:03:48,920 Run the program, and what we might do just to see this a little bit clearer, we're going to 49 00:03:48,920 --> 00:03:53,510 change the View Mode again, as I've done previously, to Window. So this time we're 50 00:03:53,510 --> 00:03:56,990 going to select option 1, Welcome to my Nightmare. We can see that 51 00:03:56,990 --> 00:04:04,850 we've got our menu printing out. So you can see, we've got the tuple for 52 00:04:04,850 --> 00:04:09,620 album zero printed out on screen, there. Choice is 1, and we're subtracting one 53 00:04:09,620 --> 00:04:15,770 from that on line 14. Now on line 10, we bind songs_list to 54 00:04:15,770 --> 00:04:20,149 the fourth item in the tuple. We're using an index of 3 which gives us the 55 00:04:20,149 --> 00:04:24,940 fourth item - that's the list of songs - and that's what line 15 is printing out, 56 00:04:24,940 --> 00:04:28,940 and we can see that showing in the output pane. So choose some other 57 00:04:28,940 --> 00:04:32,539 options, and make sure that you're happy with how songs_list gets 58 00:04:32,539 --> 00:04:36,919 the correct list of songs for each album, before continuing. So pause the video and 59 00:04:36,919 --> 00:04:42,110 come back when you've finished experimenting. Alright, so before we write 60 00:04:42,110 --> 00:04:46,780 the rest of the code, I want to talk about constants in Python. Now this is 61 00:04:46,780 --> 00:04:50,720 going to be quite a short program, but often you'll be working with hundreds, or 62 00:04:50,720 --> 00:04:54,650 even thousands of lines of program code. With large programs, it can be very 63 00:04:54,650 --> 00:05:00,020 difficult to remember what everything is. So looking at line 10, for example, we 64 00:05:00,020 --> 00:05:04,700 have to remember that the list of songs is at index position 3, in albums. 65 00:05:04,700 --> 00:05:09,100 That would be much easier if we gave 3 a more meaningful name. We could call it 66 00:05:09,100 --> 00:05:15,540 songs list. So let's go ahead and actually do that, and exit out of our program. 67 00:05:15,540 --> 00:05:20,620 Back up here, where the while loop currently is, I'm going to type on line 3, 68 00:05:20,620 --> 00:05:24,680 songs_list equals 3 - put an extra 69 00:05:24,680 --> 00:05:29,630 blank line there. Now we can use the variable, instead of remembering that we 70 00:05:29,630 --> 00:05:34,880 want the value 3. The problem with that is, the value must always be 3. It's a 71 00:05:34,880 --> 00:05:39,080 constant that refers to the fourth item in the list. If you go changing the 72 00:05:39,080 --> 00:05:44,120 value that songs list is bound to, our code won't do what we expect. We want 73 00:05:44,120 --> 00:05:48,140 songs list to represent a constant value. 74 00:06:30,760 --> 00:06:35,420 So looking at our code, we've reassigned songs_list on line 12. 75 00:06:35,420 --> 00:06:39,119 That's not good and it will cause our code to break. So many languages 76 00:06:39,119 --> 00:06:43,740 have some way of marking a value as constant. The keyword const, c o n s t, 77 00:06:43,740 --> 00:06:47,380 is used in C, for example. In Java, the keyword is final. 78 00:06:47,380 --> 00:06:50,520 Python, on the other hand, doesn't have any way of preventing us from changing 79 00:06:50,520 --> 00:06:54,980 the value of a constant. Instead, there's a convention that constants should have 80 00:06:54,990 --> 00:07:00,270 names that are in all capitals. So let's go ahead and make a change here, on line 3, 81 00:07:00,270 --> 00:07:04,319 and make it SONGS_LIST in uppercase, as you can see there, leaving 82 00:07:04,320 --> 00:07:08,660 it still to the value of 3. Whenever you see a name that's all in capitals, that 83 00:07:08,660 --> 00:07:13,020 means it represents a constant and you shouldn't change it. Now you'll find that 84 00:07:13,020 --> 00:07:20,440 convention is in the Python Style sheet, which I'll quickly open up now. 85 00:07:20,440 --> 00:07:25,140 The link, as always, is in the resources section. Note that this is only a convention, though. 86 00:07:25,140 --> 00:07:28,499 The Python compiler won't stop you reassigning a constant - 87 00:07:28,500 --> 00:07:31,560 it's ultimately your responsibility to make sure that you don't do that. 88 00:07:31,560 --> 00:07:37,500 Python 3.8 adds a Final annotation, and a final qualifier to the typing module. 89 00:07:37,500 --> 00:07:41,000 However, the compiler still doesn't prevent you from changing the value of a 90 00:07:41,009 --> 00:07:44,849 constant, so pay attention to this convention. Use uppercase for the names 91 00:07:44,849 --> 00:07:50,219 of your constants, and don't change any variable that's in uppercase. Alright, so 92 00:07:50,220 --> 00:07:56,200 back to the code. We can now use our constant on line 12, in place of the value 3. 93 00:07:56,200 --> 00:08:01,040 I'm gonna do just that: SONGS_LIST, as you can see there, 94 00:08:01,040 --> 00:08:04,580 Because variable names are case sensitive, SONGS_LIST in 95 00:08:04,580 --> 00:08:09,540 square brackets at the end of that line, represents a different value to songs_list 96 00:08:09,540 --> 00:08:13,100 at the start, which isn't in uppercase. If you want to be even 97 00:08:13,110 --> 00:08:16,849 clearer, you could come up here and refactor this. Let's go ahead and do that: 98 00:08:16,849 --> 00:08:25,720 Refactor, Rename, and we'll charge it from SONGS_LIST to SONGS_LIST_INDEX, 99 00:08:25,720 --> 00:08:27,800 and you can see the change has been reflected 100 00:08:27,809 --> 00:08:31,979 on line 12, as well as line 3. That's a good idea - now the name more accurately 101 00:08:31,980 --> 00:08:36,360 reflects what the constant's for. So that's constants in Python. 102 00:08:36,360 --> 00:08:39,779 There's nothing special about them - they're just variables. The compiler doesn't treat 103 00:08:39,779 --> 00:08:43,440 them any differently, to any other variable. We write them in capitals to 104 00:08:43,440 --> 00:08:47,220 let other humans know, that they represent a value that shouldn't be changed. 105 00:08:47,220 --> 00:08:51,560 Alright, so in the next video, we'll finish the code for our jukebox menu. 106 00:08:51,560 --> 00:08:54,560 See you in the next video.