1 00:00:05,520 --> 00:00:08,400 Okay, let's have a look at some code that we might use, 2 00:00:08,400 --> 00:00:11,520 to delete restricted items from a packing list. 3 00:00:11,520 --> 00:00:15,520 I've created a new Python file called packing_list.py. 4 00:00:15,520 --> 00:00:21,141 There's a lot of data to type, and typing lists of items is boring, so download packing_list.txt 5 00:00:21,141 --> 00:00:26,160 from the resources for this video, and copy and paste the code into your Python file. 6 00:00:26,160 --> 00:00:30,080 Pause the video while you do that. We'll let the user decide if they're going 7 00:00:30,080 --> 00:00:36,211 to travel by car, or by plane. I've put those options into a dictionary, on line 1 of the code. 8 00:00:36,560 --> 00:00:40,400 Next, we've got a set of items that we'll need, while camping. 9 00:00:40,400 --> 00:00:44,720 We're not wild camping – we'll be eating in the restaurant on the campsite – so please don't ask 10 00:00:44,720 --> 00:00:49,680 where the pans, plates and cutlery are . Below that, and I'll scroll line 29 to 11 00:00:49,680 --> 00:00:51,976 the top of the screen, 12 00:00:52,800 --> 00:00:56,000 we've got some of the items that you can't take on a plane. 13 00:00:56,000 --> 00:01:01,256 Not in your hand luggage, anyway. You can take some of these items in your checked baggage. 14 00:01:01,360 --> 00:01:03,120 But we're travelling light. 15 00:01:03,120 --> 00:01:08,936 That's the data – the code starts on line 39 – and I'll scroll up so we can see it. 16 00:01:09,680 --> 00:01:14,611 Lines 39 to 43 print the menu, using our travel_mode dictionary. 17 00:01:14,800 --> 00:01:19,120 That's code you've seen a few times now, and doesn't need any explanation. 18 00:01:19,120 --> 00:01:24,110 I've included the alternative line, for students using Python 3.5 or earlier. 19 00:01:24,240 --> 00:01:28,972 f-strings aren't available in those versions, and you'll have to use .format instead. 20 00:01:29,040 --> 00:01:34,560 But you'll be used to that by now. Lines 45 to 47 get the user's choice. 21 00:01:34,560 --> 00:01:37,353 That's all simple stuff, so far. 22 00:01:37,680 --> 00:01:43,440 At the end of the code, that's on lines 55 to 58, we print out the set of items to pack. 23 00:01:43,440 --> 00:01:47,840 I've sorted the set, so technically we're printing a list there. There's no real need 24 00:01:47,840 --> 00:01:52,160 to sort the set, but it'll make it easier to check that items have been removed. 25 00:01:52,160 --> 00:01:57,360 Okay, what we're really interested in, is what happens when the user chooses to go by plane. 26 00:01:57,360 --> 00:02:01,440 In that case, we need to remove any restricted items from the set. 27 00:02:01,440 --> 00:02:05,104 We'll add the code to do that, shortly. At the moment, 28 00:02:05,104 --> 00:02:07,680 we've got a dummy pass inside the if block. 29 00:02:07,680 --> 00:02:13,289 Before we look at the code to remove items, let's run the program, to see what it does. 30 00:02:14,240 --> 00:02:18,000 We get the options printed, and the program's waiting for input. 31 00:02:18,000 --> 00:02:21,840 When I enter 1, the program prints the items we should pack. 32 00:02:24,240 --> 00:02:26,597 Okay, run it again: 33 00:02:27,760 --> 00:02:32,640 and choose option 2. At the moment, we get the same set of items. 34 00:02:32,640 --> 00:02:38,320 My stove uses liquid fuel — a bit like petrol — and that's going to cause problems at the airport. 35 00:02:38,320 --> 00:02:42,880 The knife will also raise a few eyebrows, as will the razor blades and scissors. 36 00:02:42,880 --> 00:02:46,560 We need to remove any restricted items when travelling by plane. 37 00:02:46,560 --> 00:02:51,056 Close the Run pane, and we'll see the code to do that. 38 00:02:51,443 --> 00:02:55,540 I'll replace pass with the code to iterate over the restricted items: 39 00:03:00,000 --> 00:03:04,654 Next, we'll use discard to remove the item from our packing list: 40 00:03:11,360 --> 00:03:15,680 Alright, this is an example of when you might want to use discard. We don't care whether 41 00:03:15,680 --> 00:03:20,323 restricted items are in our packing list or not. We just want them gone. 42 00:03:20,323 --> 00:03:25,360 Line 52 will remove restricted items if they're in the set, and do nothing otherwise. 43 00:03:25,360 --> 00:03:31,840 So does it work ? Run the program, and choose option 2. 44 00:03:33,680 --> 00:03:38,560 We sorted the set before printing the items it contains, so it's easy to see that the fuel, 45 00:03:38,560 --> 00:03:43,440 the knife, and the other restricted items, are no longer in our packing list. 46 00:03:43,440 --> 00:03:47,040 Alright, that's how you'd use discard. Note that this won't work 47 00:03:47,040 --> 00:03:51,600 if we use remove instead. Let's see the problem. I'll change discard 48 00:03:51,600 --> 00:03:56,370 to remove, on line 52. I'll add a comment as well: 49 00:04:05,788 --> 00:04:10,664 Run the program, and choose option 2 again. 50 00:04:10,995 --> 00:04:15,692 We get an error, when we try to remove the first restricted item that we haven't packed. 51 00:04:15,760 --> 00:04:19,878 That happened when the code tried to use remove to remove something that didn't exist. 52 00:04:20,000 --> 00:04:24,281 Because sets aren't ordered, you may well get a different item causing the crash. 53 00:04:24,281 --> 00:04:28,480 When we iterate over a set, we don't know the order that the items will appear. 54 00:04:28,480 --> 00:04:32,370 Sets have no ordering, remember! So our code crashed, 55 00:04:32,370 --> 00:04:37,520 because it tried to do something that didn't need to be done. In this case, it tried to remove a 56 00:04:37,520 --> 00:04:42,080 restricted item that we hadn't packed. That's not good, and demonstrates why 57 00:04:42,080 --> 00:04:48,320 discard is more appropriate than remove, here. Ok, I'll finish this video with an observation. 58 00:04:48,320 --> 00:04:51,600 We've used this example to demonstrate using discard. 59 00:04:51,600 --> 00:04:55,680 In this case, where we're discarding all items from another set, there's another approach 60 00:04:55,680 --> 00:04:59,280 that you should seriously consider. So this is an example about how to 61 00:04:59,280 --> 00:05:04,080 use discard. It demonstrates deleting items from a set, in a situation where 62 00:05:04,080 --> 00:05:09,200 you don't care if the item is present or not. We could also have used the set difference. 63 00:05:09,200 --> 00:05:14,014 We'll come back to this example, when we look at set difference, in a later video. 64 00:05:14,014 --> 00:05:18,560 In the next video, we'll see a situation where remove might be more appropriate than discard, 65 00:05:18,560 --> 00:05:23,840 for what we need to do. See you in the next video.