1 00:00:04,710 --> 00:00:09,750 Alright, so let's talk about removing items from a list. So we're going to modify our 2 00:00:09,750 --> 00:00:14,190 program to allow the customer to change their mind, and remove items from their 3 00:00:14,190 --> 00:00:18,820 list. Our program's a bit limited because we're not using a GUI interface. 4 00:00:18,820 --> 00:00:22,770 Most likely our customer would be using their browser to purchase parts, and 5 00:00:22,770 --> 00:00:26,520 would have a drop-down to indicate how many of each item they want. So bear in 6 00:00:26,520 --> 00:00:29,820 mind that we're learning how to manipulate lists here - we're not learning 7 00:00:29,820 --> 00:00:33,570 how to write an online shopping cart. Having said that, though, the code for an 8 00:00:33,570 --> 00:00:37,680 online cart would contain all the techniques that we're using here. So the 9 00:00:37,680 --> 00:00:42,460 code we need to change, is this code on lines 18 through 21. 10 00:00:42,460 --> 00:00:46,820 When we get to that if block, we know which part the customer's chosen. At the 11 00:00:46,829 --> 00:00:51,360 moment we add that part. What we want to do, is check first to see if the part's 12 00:00:51,360 --> 00:00:56,430 already in their list. Let's have a go at doing that. So basically, under line 20 13 00:00:56,430 --> 00:01:03,120 here, we're going to type if chosen underscore part is in computer underscore 14 00:01:03,120 --> 00:01:07,590 parts : I'm just going to put a comment there for the moment to say, it's 15 00:01:07,590 --> 00:01:15,510 already in, so remove it, and we'll leave line 23 as is. Now ignore the error on 16 00:01:15,510 --> 00:01:19,409 line 22 at the moment - we saw that before in an earlier video. If you want to you 17 00:01:19,409 --> 00:01:22,770 could add a pass statement to fix that error - I'll just add a pass statement for 18 00:01:22,770 --> 00:01:29,220 now. All right, so how do we actually remove an item from a list. Well let's go 19 00:01:29,220 --> 00:01:34,140 back to that table that we looked at previously. If we scroll down and have a 20 00:01:34,140 --> 00:01:39,210 look here, you can see we've got a remove method. We tell it which item to remove 21 00:01:39,210 --> 00:01:43,590 and it removes it from the list. Now it's always worth reading any notes when 22 00:01:43,590 --> 00:01:47,549 you're checking how to use functions and methods. The remove method has Note 3 23 00:01:47,549 --> 00:01:53,130 in the last column, and you can see there under notes: raises ValueError when X is 24 00:01:53,130 --> 00:01:57,630 not found in s. So in other words, we'll get an error if the item isn't in the 25 00:01:57,630 --> 00:02:01,259 list. So that's something just to be aware of. Later in the course, we'll see 26 00:02:01,260 --> 00:02:05,180 how to handle errors like that. In this case we don't have to worry about it, 27 00:02:05,180 --> 00:02:08,280 and that's because - and I'll switch back to the code here - 28 00:02:08,288 --> 00:02:12,909 on line 21, we've already checked to see if the part is in the list. So that means 29 00:02:12,909 --> 00:02:17,290 we can safely remove it without worrying about getting an error. Alright, so 30 00:02:17,290 --> 00:02:21,489 let's replace this pass statement with the actual code, which will be computer 31 00:02:21,489 --> 00:02:27,340 underscore parts dot remove, and in parentheses, chosen underscore part. Okay, 32 00:02:27,340 --> 00:02:31,510 so if the chosen part isn't in the list then we want to add that, and we can do 33 00:02:31,510 --> 00:02:35,620 that by putting line 24 inside an else block. So we're gonna go ahead and do 34 00:02:35,620 --> 00:02:42,300 that - else : then on the next line, come back over here and indent that correctly, 35 00:02:42,300 --> 00:02:45,910 and that's all we need to do. It would be nice, though, to remind our customer of 36 00:02:45,910 --> 00:02:49,690 what's in their list, though, so I'm going to add a line and print the list out, and 37 00:02:49,690 --> 00:02:55,329 I'll do that outside of that block. So I'm going to type print, in parentheses, 38 00:02:55,329 --> 00:03:06,080 double quotes Your list now contains : left and right curly braces dot format, 39 00:03:06,080 --> 00:03:09,790 and in parentheses, computer underscore parts, and the two closing 40 00:03:09,790 --> 00:03:14,829 right parentheses there. Now be careful over the indentation for line 26. We want 41 00:03:14,829 --> 00:03:18,340 to print it out whether the customer added or removed an item, which means it 42 00:03:18,340 --> 00:03:21,340 mustn't be indented inside the else block, and you can see in my case that 43 00:03:21,340 --> 00:03:26,020 it's not. Alright, so I'm gonna run the program now and see how it behaves. 44 00:03:26,020 --> 00:03:33,240 I'll add option one first, a computer, then option two for a monitor. You can see 45 00:03:33,250 --> 00:03:36,970 that each time I'm doing that, we get the new item list printed out. Now it turns out 46 00:03:36,970 --> 00:03:39,459 that I've got a spare monitor kicking about, so I'm going to use option two 47 00:03:39,459 --> 00:03:43,569 again now, to remove the monitor from the list. The list now only contains 48 00:03:43,569 --> 00:03:47,230 computer, so that's working well. It did say it was adding to when it really 49 00:03:47,230 --> 00:03:52,180 meant removing it, though. But we can tidy that up by cutting line 18 out, and 50 00:03:52,180 --> 00:03:59,190 I'll exit out of the program first. So what we want to do there, is cut out line 18, 51 00:03:59,190 --> 00:04:06,069 entirely, and we want to put it inside the else statements are down here, 52 00:04:06,069 --> 00:04:10,180 because this is the code that's executed when we add an item. But let's also take 53 00:04:10,180 --> 00:04:16,180 the opportunity here as well, paste it in here as well, and instead of Adding, 54 00:04:16,180 --> 00:04:19,459 we're gonna change that to Removing. 55 00:04:21,500 --> 00:04:26,550 Alright so let's run the program again - run it exactly the same as we did before: one, 56 00:04:26,550 --> 00:04:32,100 computer, two, monitor and you get Adding each time. We'll add, this time we'll remove 57 00:04:32,100 --> 00:04:37,919 the monitor again, two, says Removing two, our list now only contains computer, 58 00:04:37,919 --> 00:04:42,449 and we'll exit out of it. Alright. So that's how you can append items to a 59 00:04:42,449 --> 00:04:47,010 list and remove items from it. The append method is used to add new items. If you 60 00:04:47,010 --> 00:04:50,910 add the same item again then the list will contain it twice. Lists don't check that 61 00:04:50,910 --> 00:04:55,500 the item that they contain are unique. The remove method removes items from a 62 00:04:55,500 --> 00:04:59,550 list, and if the item you want to remove doesn't exist, you'll get an error. So I'm 63 00:04:59,550 --> 00:05:03,150 going to leave the other operations that you can perform on mutable sequences for 64 00:05:03,150 --> 00:05:07,540 now. In the next video, we'll turn our attention on how to sort a list. 65 00:05:07,540 --> 00:05:10,660 See you in the next video.