1 00:00:05,280 --> 00:00:10,080 In this video, we'll look at a couple of ways to remove items from a dictionary, and discuss when 2 00:00:10,080 --> 00:00:15,440 you might use each one. That'll give us the basic operations we can perform with the dictionary, and 3 00:00:15,440 --> 00:00:20,240 we can move on to using them for more interesting things. We've seen how to delete an item from a 4 00:00:20,240 --> 00:00:26,800 list. using the del keyword. We first used that in the lecture Replacing a slice, back in the lists 5 00:00:26,800 --> 00:00:31,280 and tuples section. So review those lectures if you've forgotten how to delete from a list. 6 00:00:32,400 --> 00:00:38,053 Deleting from a dictionary is very similar, but we use the key rather than an index position. 7 00:00:38,053 --> 00:00:40,720 So to see that in operation, what I'm going to do is get rid 8 00:00:40,720 --> 00:00:46,756 of my starfighter. Who needs two jets after all? So on line 19, I'm going to type del 9 00:00:46,756 --> 00:00:55,772 vehicles square brackets double quotes and starfighter, and if I run this now, 10 00:00:56,160 --> 00:01:00,000 you can see that the starfighter has now been removed from the dictionary. 11 00:01:00,880 --> 00:01:07,105 So that's easy and obvious, so why does Python provide another way? Well if you've read 12 00:01:07,105 --> 00:01:13,595 The Zen of Python, you know that there should be one - and preferably only one - obvious way to do it. 13 00:01:13,595 --> 00:01:19,760 If you haven't read The Zen of Python, it's a tongue-in-cheek summary of Python's guiding principles. 14 00:01:19,760 --> 00:01:27,499 You could google for it, or create a new Python file, so I'm going to call mine zen.py. 15 00:01:31,840 --> 00:01:34,720 You only need one line in the file: import this. 16 00:01:37,680 --> 00:01:39,840 If we run it, 17 00:01:43,760 --> 00:01:50,080 and there it is - that's The Zen of Python printed out. Now as I said, it's slightly humorous, 18 00:01:50,080 --> 00:01:53,920 but there are some important principles in there. You'll hear Python programmers 19 00:01:53,920 --> 00:01:58,947 referring to it, so this is a good time to check it out. Pause the video now while you read it, 20 00:01:58,947 --> 00:02:03,564 then come back to dict_intro.py when you're ready to continue. 21 00:02:04,960 --> 00:02:12,640 Alright. So let's see why we need another way to remove items from the dictionary, or one 22 00:02:12,640 --> 00:02:19,120 reason anyway. What if we try to remove something that doesn't exist? So I'll start out on line 20 23 00:02:19,120 --> 00:02:25,545 by removing my formula one racing car, which I think has the key f1. 24 00:02:25,545 --> 00:02:30,199 Same deal there for double quotes and the square brackets. If I run this now, 25 00:02:31,520 --> 00:02:36,800 we can see that the program crashes. Accessing a dictionary with a key that doesn't exist 26 00:02:36,800 --> 00:02:42,937 will crash your program. So that's one reason why we've got a get method, 27 00:02:44,800 --> 00:02:50,000 and that's to retrieve values that might not exist. Now we could wrap all our deletions in a test, 28 00:02:50,000 --> 00:02:54,320 to make sure that the key exists, but Python provides a more efficient way. 29 00:02:54,320 --> 00:02:59,879 Lists and dictionaries have a pop method. So what I'm going to do is comment out line 20, 30 00:02:59,879 --> 00:03:07,525 and instead, on the next line type vehicles dot pop parentheses double quotes f1. 31 00:03:08,596 --> 00:03:14,480 Okay, we'll run the program, and you can see that it's crashed in exactly the same way. 32 00:03:14,480 --> 00:03:19,280 That's deliberate - it means you can use pop in cases where you've already checked that 33 00:03:19,280 --> 00:03:24,625 the key exists. You can also use it if you want to get an error. Deleting that Justin Bieber song 34 00:03:24,625 --> 00:03:29,920 that somehow found its way onto your playlist, is a very important operation. Now if you spelled 35 00:03:29,920 --> 00:03:35,120 Bieber incorrectly, it's better that you get an error so that you can correct the mistake. 36 00:03:35,120 --> 00:03:40,480 If your code fails silently, your playlist might remain embarrassing. If you can't be 37 00:03:40,480 --> 00:03:45,200 sure that the key exists, and don't want to get an error, you can pass the default value that 38 00:03:45,200 --> 00:03:55,200 pop will return, instead of crashing. So we can come up here on line 21 and put a comma None. 39 00:03:55,200 --> 00:04:00,381 Run the program again, and this time it won't crash. So we can see that this time it doesn't crash - 40 00:04:00,381 --> 00:04:05,200 we've just got our regular output. So the pop method removes an item from the dictionary, 41 00:04:05,200 --> 00:04:12,000 and returns the value. If the key doesn't exist, it returns whatever you pass for the default instead. 42 00:04:12,000 --> 00:04:17,600 Now we haven't used the return value but we could. So let's change that. What I'll do instead, 43 00:04:17,600 --> 00:04:25,566 is online 21, result is equal to vehicles.pop, so the rest of that line. Then on the next line, print, 44 00:04:26,400 --> 00:04:33,459 result in parentheses, and we'll just do another print there to make some space. Run the program now, 45 00:04:34,880 --> 00:04:38,480 and we can see at the top there, we've got None printed out as the first line of the output. 46 00:04:39,360 --> 00:04:43,760 Now often you'll pass None as the default, as we've done here, but you can pass something 47 00:04:43,760 --> 00:04:49,908 more appropriate to your application. As an example, I'll provide a string instead. 48 00:04:59,154 --> 00:05:02,345 Okay, now if we run this, 49 00:05:04,320 --> 00:05:08,160 this time you can see we get that message printed out instead of None. 50 00:05:08,160 --> 00:05:13,920 So let's see what happens when we use pop with keys that do exist. I really want a racing car, 51 00:05:13,920 --> 00:05:18,480 so I'm going to get rid of our Learjet. So what I'm going to do now is, after line 22, 52 00:05:19,520 --> 00:05:27,769 I'm going to type plane is equal to vehicles dot pop parentheses learjet in lowercase, 53 00:05:28,720 --> 00:05:35,600 print on the next line, plane, parentheses plane. Then on line 26, bike is equal to vehicles 54 00:05:35,600 --> 00:05:43,680 dot pop parentheses double quotes tenere comma double quotes not present. 55 00:05:45,440 --> 00:05:48,000 Let's also print out bike. 56 00:05:49,840 --> 00:05:54,081 Run the program and we'll check the first three lines of output, at the top there. 57 00:05:54,081 --> 00:05:58,320 So the code on line 21 attempts to pop a key that doesn't exist. We get the default 58 00:05:58,320 --> 00:06:03,120 value printed out - the string that we passed to the pop method. On line 23, 59 00:06:03,920 --> 00:06:08,800 we're popping the learjet from the dictionary, so pop returns the value and we can see that printed 60 00:06:08,800 --> 00:06:16,720 in the second line of the output. I included line 26 to emphasize how the default value could be used. 61 00:06:16,720 --> 00:06:21,520 We passed a default value of not present, but because the tenere is present, we get the value 62 00:06:21,520 --> 00:06:26,640 for the tenere printed out - that's the third line of the output. The default value is only returned 63 00:06:26,640 --> 00:06:32,480 when the key doesn't exist in the dictionary. Okay, so that's two different ways to delete items from 64 00:06:32,480 --> 00:06:38,000 a dictionary. del performs slightly better than pop, but if you want to use one or other form of pop 65 00:06:38,000 --> 00:06:42,560 and ignore del, that's not a bad strategy. You'd need a very large dictionary, and be performing 66 00:06:42,560 --> 00:06:48,480 lots of deletions, for the performance differences to be noticeable. I'll focus on using pop in the 67 00:06:48,480 --> 00:06:53,200 examples, partly because del describes what it does so well. del is short for delete, which is 68 00:06:53,200 --> 00:06:58,514 easy to remember. You're unlikely to have problems working it out when you come across it. So at this point, 69 00:06:58,514 --> 00:07:02,960 we've now covered the basic operations you can perform with a dictionary. In the next 70 00:07:02,960 --> 00:07:08,560 few videos, we'll see some more realistic examples of using dictionaries. See you in the next video.