1 00:00:03,270 --> 00:00:10,230 In the previous video, you saw that if the user enters edit and then instead of entering a number, 2 00:00:10,230 --> 00:00:12,130 they enter it to the item. 3 00:00:12,150 --> 00:00:18,810 The program will end abruptly and that is not a good user experience. 4 00:00:18,810 --> 00:00:25,950 So instead of showing this error to the user, instead of stopping the program, we want the program 5 00:00:25,950 --> 00:00:26,960 to continue. 6 00:00:26,970 --> 00:00:33,000 So what we want instead is if the user enters this, we will tell the user that. 7 00:00:34,560 --> 00:00:36,870 They entered the wrong. 8 00:00:37,850 --> 00:00:39,020 Command. 9 00:00:39,200 --> 00:00:45,830 And then we will let the user enter the command again by showing them this prompt. 10 00:00:48,100 --> 00:00:50,830 And that makes for a better user experience. 11 00:00:50,830 --> 00:00:57,280 So never break the programs like that for the user. 12 00:01:00,190 --> 00:01:07,630 So to not let the program and abruptly like this, we will use error handling. 13 00:01:07,630 --> 00:01:09,190 So what is error handling? 14 00:01:09,220 --> 00:01:09,940 Let me show you. 15 00:01:09,940 --> 00:01:10,750 That's to you. 16 00:01:12,550 --> 00:01:16,390 So here we are in the l if added case. 17 00:01:16,600 --> 00:01:21,580 So if the command starts with edit we get. 18 00:01:23,630 --> 00:01:26,150 The number from the commands. 19 00:01:26,150 --> 00:01:34,790 The number is supposed to come after edit, but sometimes the user will enter something else. 20 00:01:34,790 --> 00:01:39,770 So what we want to do is we want to first try out. 21 00:01:40,970 --> 00:01:44,390 We want to try to execute these lines. 22 00:01:44,390 --> 00:01:49,310 So everything under under the live condition, we want to try to execute it. 23 00:01:49,310 --> 00:01:56,630 So what you do is you want to select these lines and then press tab so you indent them. 24 00:01:57,650 --> 00:02:02,210 So now we have eight spaces and what do you do next? 25 00:02:02,210 --> 00:02:08,330 You go here and you press enter and then you say try and I column. 26 00:02:10,070 --> 00:02:20,510 So as you can see, I'm introducing a new code block, just like we had l f blocks, we had width blocks. 27 00:02:20,510 --> 00:02:22,550 We also had the case blocks. 28 00:02:22,550 --> 00:02:28,310 So everything under if under width is intended, the same goes for Troy. 29 00:02:28,310 --> 00:02:30,500 So everything under Troy is indented. 30 00:02:30,500 --> 00:02:35,470 You see all these lines, all of these are indented under trial. 31 00:02:35,600 --> 00:02:43,670 So we try to execute these and if there is an error. 32 00:02:44,520 --> 00:02:53,250 We go here in the same level with try to try is in the same vertical line as you can see and you say 33 00:02:53,250 --> 00:02:58,170 except followed by the error. 34 00:03:00,200 --> 00:03:03,440 The error in our case is value error. 35 00:03:04,520 --> 00:03:12,500 So you go back here and you say value with a capital V error with a capital E 36 00:03:15,260 --> 00:03:16,430 and a column. 37 00:03:22,290 --> 00:03:26,040 So if there is an error, then we want to print out. 38 00:03:29,780 --> 00:03:31,370 Your comment. 39 00:03:33,110 --> 00:03:35,180 It's not valid. 40 00:03:38,620 --> 00:03:45,430 And immediately after that, we want to give the user the opportunity to enter the command again. 41 00:03:46,780 --> 00:03:49,870 So we want to show this message again. 42 00:03:49,870 --> 00:03:54,880 And that message is produced by. 43 00:03:56,580 --> 00:03:59,460 These two lines, so I'll copy them. 44 00:04:00,000 --> 00:04:04,380 Go back here under the print function. 45 00:04:04,590 --> 00:04:09,780 I want to paste them in here and I'll execute the code now. 46 00:04:21,430 --> 00:04:24,610 So type and show edits complete or exit. 47 00:04:26,740 --> 00:04:37,570 Let's say Ed did add a new member presenter and what we get this time is not an error. 48 00:04:37,720 --> 00:04:43,570 It is this message Your command is not valid produced by that line. 49 00:04:43,570 --> 00:04:47,440 And then we have this prompt again. 50 00:04:47,440 --> 00:04:50,590 We can answer again the input. 51 00:04:51,700 --> 00:05:01,120 So if you enter again the wrong command, add a second member or anything like that. 52 00:05:01,340 --> 00:05:05,560 Presenter You get asked again for entering the input. 53 00:05:07,720 --> 00:05:09,010 Now let's try to enter. 54 00:05:09,010 --> 00:05:10,810 Edit one. 55 00:05:13,070 --> 00:05:22,310 So when you enter it, it's one we get this printed out under Troy, you see print number printed out 56 00:05:22,310 --> 00:05:23,270 number one. 57 00:05:23,480 --> 00:05:31,700 And here we enter the new to do add a new player show. 58 00:05:31,700 --> 00:05:35,990 And now as number one we have this adds a new player. 59 00:05:36,590 --> 00:05:39,080 It's working, but it's not ideal. 60 00:05:39,890 --> 00:05:48,950 The ideal solution would be not to have repetitive code because currently these two lines are the same 61 00:05:48,950 --> 00:05:50,930 as these two. 62 00:05:50,960 --> 00:05:54,830 That is not the best code optimization. 63 00:05:55,400 --> 00:06:03,770 So what we can do actually is we use the opposite of a break statement instead of using these. 64 00:06:04,790 --> 00:06:14,460 You remember the break statement breaks the loop and the continuous statement continues. 65 00:06:15,050 --> 00:06:20,210 Or run another cycle of the while loop. 66 00:06:21,530 --> 00:06:23,810 So what's going to happen now is this. 67 00:06:25,160 --> 00:06:27,290 Let's say the user enters. 68 00:06:27,950 --> 00:06:30,200 Edit Add a new member. 69 00:06:31,310 --> 00:06:33,980 The program will try to run these lines. 70 00:06:33,980 --> 00:06:38,900 And if there is an error, then these lines will be executed. 71 00:06:39,410 --> 00:06:45,290 Actually, if there is a value error, because there are different kinds of errors out there, it is 72 00:06:45,290 --> 00:06:52,310 important to specify the error here because you don't want to get confused with all kinds of errors. 73 00:06:52,310 --> 00:06:56,090 So you want to execute these only when this error occurs. 74 00:06:56,690 --> 00:07:01,010 So when that error occurs, you print out the message. 75 00:07:02,630 --> 00:07:03,800 In the command line. 76 00:07:03,800 --> 00:07:14,510 And then this statement here, what it will do is it will ignore everything that comes underneath here 77 00:07:14,660 --> 00:07:20,600 and it will go back again to the beginning of the loop here in line two. 78 00:07:21,230 --> 00:07:24,350 So it will execute this input function. 79 00:07:28,510 --> 00:07:29,560 Let's try it. 80 00:07:31,240 --> 00:07:32,380 Run the code. 81 00:07:34,130 --> 00:07:34,870 Edit. 82 00:07:37,630 --> 00:07:39,430 Add a new member. 83 00:07:40,600 --> 00:07:42,640 Your command is not valid. 84 00:07:42,670 --> 00:07:49,900 Type ads show edits, complete or exit edit Add a new member. 85 00:07:50,530 --> 00:07:53,410 Again we get your command is not valid. 86 00:07:54,250 --> 00:08:08,320 Type add show, edit or complete or exit edit to enter a new to do Update the OS press Enter show and 87 00:08:08,320 --> 00:08:11,410 update The OS is number two now. 88 00:08:17,490 --> 00:08:21,330 So that was an example of the tri accept block. 89 00:08:24,100 --> 00:08:30,880 Before I close the video, I want to create another tri accept block here in this code because we have 90 00:08:30,880 --> 00:08:36,880 another error here which might happen in the complete condition. 91 00:08:39,390 --> 00:08:43,919 Because if we run the code and the user tries to complete. 92 00:08:47,950 --> 00:08:51,040 A night in which is not in the list. 93 00:08:51,040 --> 00:08:58,840 For example, item 23, when all we have is a few items and in that case we will get an index error. 94 00:08:59,020 --> 00:09:01,810 It says that the list index is out of range. 95 00:09:06,210 --> 00:09:12,900 So that's the same error as if we had a least. 96 00:09:16,180 --> 00:09:27,010 For example, A and B, So with two items and you try to access item with index 23 or 24, so you will 97 00:09:27,010 --> 00:09:28,900 get that error again, index error. 98 00:09:29,770 --> 00:09:37,900 So let's quickly handle this error as well so that the user knows that he is out of the range of the 99 00:09:37,900 --> 00:09:38,680 items. 100 00:09:40,680 --> 00:09:43,860 Available in the list in the to do list. 101 00:09:46,540 --> 00:09:51,100 So again, we want to select all of these. 102 00:09:51,460 --> 00:09:58,510 So all the lines which are under live and we want to indent them by pressing tab and then we want to 103 00:09:58,510 --> 00:09:59,710 add try here. 104 00:10:01,360 --> 00:10:04,900 So we will try to execute all that code and then. 105 00:10:08,980 --> 00:10:12,310 Catch an index error. 106 00:10:12,310 --> 00:10:16,570 So that error there index. 107 00:10:19,020 --> 00:10:19,620 Error. 108 00:10:22,650 --> 00:10:25,050 So if that's the case, we want to print out. 109 00:10:26,860 --> 00:10:30,640 There is no item with that number. 110 00:10:34,240 --> 00:10:36,700 And then continue again. 111 00:10:36,850 --> 00:10:43,690 So which means that if the user enters a number which is out of range, we show that message and then 112 00:10:43,690 --> 00:10:46,810 we let the user enter. 113 00:10:47,980 --> 00:10:49,300 A new commands again. 114 00:10:49,300 --> 00:10:52,750 So continue will execute these lines again. 115 00:10:53,890 --> 00:11:00,600 So now if the user enters complete 23, they get this message. 116 00:11:02,610 --> 00:11:06,060 And they get the chance to enter command again. 117 00:11:07,260 --> 00:11:10,530 So that was the lecture about error handling. 118 00:11:11,710 --> 00:11:12,670 Let's keep going.