1 00:00:05,440 --> 00:00:10,800 In the last video, I mentioned that there's a more concise way to write our add_shopping_item 2 00:00:10,800 --> 00:00:16,400 function. So have a look at the code on lines six through nine. We check to see 3 00:00:16,400 --> 00:00:22,800 if the item's in the dictionary, on line 6. If it is, we add amount to the current value. 4 00:00:22,800 --> 00:00:27,120 We've used augmented assignment to do that - augmented assignment works fine with values 5 00:00:27,120 --> 00:00:32,000 in a dictionary. Otherwise, we store amount in the dictionary, creating a new entry 6 00:00:32,000 --> 00:00:39,120 with the key item. That sort of code is so common, that Python's dictionaries provide a way to help. 7 00:00:39,120 --> 00:00:45,929 What I'm going to do is comment out these four lines, and use the setdefault method instead. 8 00:00:46,800 --> 00:00:53,120 So I'm going to type data, left square bracket, item in that left square bracket, equals then data 9 00:00:53,120 --> 00:01:05,000 dot setdefault. Then in parentheses, item comma 0, and outside of the parentheses put plus amount. 10 00:01:05,040 --> 00:01:09,600 The setdefault method returns the value from the dictionary, if the key exists. 11 00:01:09,600 --> 00:01:14,800 If the key doesn't exist, it creates a new entry for the key, assigns the default value to it, 12 00:01:14,800 --> 00:01:20,800 which is zero in this case, and then returns that default value. We add amount to the value returned, 13 00:01:20,800 --> 00:01:25,840 then store the value back to the dictionary. In a moment, we'll look at what it does in more detail. 14 00:01:25,840 --> 00:01:31,977 First though, let's run the program to make sure that it still all works. 15 00:01:31,977 --> 00:01:41,440 4 for Egg sandwich, 2 for Chicken and chips, 5 for Beans on toast, zero to finish. 16 00:01:41,440 --> 00:01:49,074 The output looks correct there; 120 for bread, 10 for butter, and so on. So looking back at the code, 17 00:01:49,074 --> 00:01:56,000 line 10 is doing the same job as lines 6 to 9, but is much more concise. It's also less 18 00:01:56,000 --> 00:02:01,840 obvious what it's doing, but don't let that put you off using it. Using setdefault is a useful tool 19 00:02:01,840 --> 00:02:06,320 in your toolkit. Some more examples will help with understanding what it does. 20 00:02:06,320 --> 00:02:13,390 Let's create a new Python file now. We're going to call this one dict_defaults. 21 00:02:19,200 --> 00:02:23,200 Alright, so we don't want to type in another dictionary here so I'm going to import pantry; 22 00:02:23,200 --> 00:02:29,806 from contents import pantry. 23 00:02:29,920 --> 00:02:35,120 Alright, so let's see how things work differently, when we use setdefault to access a dictionary. 24 00:02:35,120 --> 00:02:38,160 We know we've got chicken in the pantry, so I'm going to start with that. 25 00:02:38,160 --> 00:02:46,480 On line three I'm going to type chicken_quantity is equal to pantry dot 26 00:02:46,480 --> 00:02:54,640 setdefault, and in parentheses, chicken in double quotes, then comma zero. On the next line, 27 00:02:54,640 --> 00:03:02,960 print parentheses, we'll do an f string, and type in the double quotes chicken colon then curly 28 00:03:02,960 --> 00:03:10,240 braces, and print out chicken_quantity. Alright, so let's run the program, 29 00:03:10,240 --> 00:03:14,080 and you can see we get the result 500 - that's correct. 30 00:03:14,080 --> 00:03:21,520 That's how much chicken we have. If we go back and look at our contents.py again, 31 00:03:21,520 --> 00:03:30,160 there's chicken: 500 on line 2. So what happened here? The setdefault method returns the value of a 32 00:03:30,160 --> 00:03:37,600 key, if the key exists in the dictionary. Chicken, as we've said on line three, does exist so we got its 33 00:03:37,600 --> 00:03:43,760 value. So far, set default is behaving the same as get. The difference is in what happens when the key 34 00:03:43,760 --> 00:03:49,440 doesn't exist in the dictionary. Let's type some more code in - we'll do the same thing for beans. 35 00:03:49,440 --> 00:03:53,280 On line six, I'm going to type beans_quantity 36 00:03:53,280 --> 00:03:58,560 equals pantry.setdefault, and in parentheses, we'll type beans in double quotes, 37 00:03:58,560 --> 00:04:04,080 and a comma 0 again, then closing right parenthesis. We'll do the same thing as we did on line 4 - 38 00:04:04,080 --> 00:04:09,680 we'll print it out. So print parentheses, do an f string, then beans in the double quotes colon 39 00:04:09,680 --> 00:04:17,760 then curly braces beans_quantity, and end of the line. Now if we run this, 40 00:04:17,760 --> 00:04:21,519 we get zero for the quantity of beans. 41 00:04:21,519 --> 00:04:28,000 The reason for that; beans is not in the pantry dictionary, as you can see there. 42 00:04:28,000 --> 00:04:32,560 And because beans wasn't in the dictionary, setdefault returned the default value that we 43 00:04:32,560 --> 00:04:38,240 specified on line six. That was zero, and that's the value we get printed out. And once again, this looks 44 00:04:38,240 --> 00:04:45,000 similar to using the get method with a default value. So we can actually check that, so let's try; 45 00:04:45,000 --> 00:04:55,680 ketchup_quantity is equal to pantry.get this time, ketchup in double quotes 46 00:04:55,680 --> 00:05:02,000 comma zero. On the next line, same thing - we'll print it out; print parentheses f-string, 47 00:05:02,000 --> 00:05:07,840 and in the double quotes, ketchup colon and then curly braces ketchup_quantity, 48 00:05:07,840 --> 00:05:15,040 and end the line. And if we run this, you get ketchup zero, and that's consistent with the fact 49 00:05:15,040 --> 00:05:21,200 that in our pantry dictionary, ketchup doesn't exist. So the get method on line nine appears 50 00:05:21,200 --> 00:05:25,920 to be doing the same thing as setdefault on line six. The difference, though, is that set 51 00:05:25,920 --> 00:05:31,360 default doesn't just return the default value - it adds the key to the dictionary as well. We're 52 00:05:31,360 --> 00:05:35,760 going to print out the dictionary keys and values so that we can check what it contains; 53 00:05:35,760 --> 00:05:45,520 line 12, print parentheses. Next line, print parentheses double quotes. I'm going to type pantry, 54 00:05:45,520 --> 00:05:49,594 now single quotes, now contains. 55 00:05:51,040 --> 00:05:59,040 Then on line 15, for key comma value in sorted parentheses that's pantry.items, 56 00:05:59,040 --> 00:06:05,920 parentheses, closing parenthesis and a colon, and we'll print key comma value. 57 00:06:05,920 --> 00:06:11,905 And I've sorted the dictionary items, just to make things easier to check. If we run this - 58 00:06:12,880 --> 00:06:19,840 on the second line of the output, we can see beans there, now with the value zero. That key was added 59 00:06:20,640 --> 00:06:26,720 in line six of our code. But checking carefully, we can see that there's no ketchup entry showing, 60 00:06:26,720 --> 00:06:32,000 which is correct, because the setdefault method adds the key to the dictionary, if it doesn't exist. 61 00:06:32,000 --> 00:06:37,040 When it adds the key, it uses the default value as the value to store in the dictionary. get, on 62 00:06:37,040 --> 00:06:42,080 the other hand, only returns the item with the default value. It doesn't add it to the dictionary, 63 00:06:42,080 --> 00:06:47,040 if it doesn't exist. The setdefault method adds the key to the dictionary, if it doesn't exist. 64 00:06:47,040 --> 00:06:51,920 When it adds the key, it uses the default value as the value to store in the dictionary. 65 00:06:51,920 --> 00:06:59,200 It then returns the value that the key has. If the key exists, such as chicken on line 3, then you get 66 00:06:59,200 --> 00:07:04,960 the value of the key - 500, in this case - the value showing there for pantry, but also as the output, 67 00:07:04,960 --> 00:07:12,080 at the top of the screen there. That was the output from the print on line four. If the key had to be 68 00:07:12,080 --> 00:07:16,640 added, you'll get the default value that you pass to setdefault, and that's zero in the case of 69 00:07:16,640 --> 00:07:20,960 beans, and again, we can see that showing in the output. The default value doesn't have to be a 70 00:07:20,960 --> 00:07:25,520 number - you can use anything that can be stored in the dictionary. Let's have a look at that. What 71 00:07:25,520 --> 00:07:33,840 I'm going to do, is after ketchup, so here on line 12, I'm going to type z_quantity 72 00:07:33,840 --> 00:07:42,480 is equal to pantry.setdefault double quotes zucchini, 73 00:07:42,480 --> 00:07:46,400 zucchini in those double quotes, comma. Then we're going to put eight in double quotes 74 00:07:46,400 --> 00:07:51,600 this time, instead of a number, and we'll print it out; print parentheses an f string, 75 00:07:51,600 --> 00:07:57,360 in the double quotes zucchini colon, then z_quantity in our curly 76 00:07:57,360 --> 00:08:01,920 braces, and close off the line. I've used zucchini because it'll sort to the end of 77 00:08:01,920 --> 00:08:09,200 the output, and be easy to spot. Zucchini is another name for courgette. Let's run this, 78 00:08:09,200 --> 00:08:13,040 and there's zucchini at the end of the output with a string value 8. 79 00:08:13,040 --> 00:08:20,240 zucchini didn't exist in the dictionary, but was added by the call to setdefault on line 12. Okay. 80 00:08:20,240 --> 00:08:25,680 So now I want to talk about ketchup again. On line 9 of our code, we attempt to retrieve the value 81 00:08:25,680 --> 00:08:31,046 for ketchup. We used the get method to do that, and we got the default value of zero returned. 82 00:08:31,046 --> 00:08:36,000 You can see that in the third line of the output, if you scroll up here. It's important to note that 83 00:08:36,000 --> 00:08:41,120 ketchup wasn't added to the dictionary. When you use get, a non-existent key isn't added 84 00:08:41,120 --> 00:08:48,638 to the dictionary. When you use setdefault, a non-existent key is added to the dictionary. 85 00:08:48,720 --> 00:08:55,200 If you google for python setdefault, you'll find lots of posts talking about a defaultdict. 86 00:08:55,200 --> 00:09:00,160 We're not going to look at defaultdict just yet. It's one of the objects in the collections module, and 87 00:09:00,160 --> 00:09:03,920 it works very much like the dictionaries we've been looking at. The difference is that it can 88 00:09:03,920 --> 00:09:10,570 automatically return a default value, and add a key to the dictionary, if a key doesn't exist. 89 00:09:10,640 --> 00:09:15,680 Often, using a defaultdict can be a good alternative to calling set default on a normal 90 00:09:15,680 --> 00:09:21,360 dictionary. The documentation for defaultdict talks about subclassing and overriding methods. 91 00:09:21,360 --> 00:09:26,640 Those terms won't make sense until after the Object-Oriented Programming section of this course. 92 00:09:26,640 --> 00:09:33,367 We'll look at the collections defaultdict class later, once you've learned about classes in Python. 93 00:09:34,000 --> 00:09:37,120 Alright, so there's still a few dictionary operations that we haven't looked at, 94 00:09:37,120 --> 00:09:41,840 but we've covered the most common things that you'll want to do with dictionaries. We'll check 95 00:09:41,840 --> 00:09:46,400 out the documentation and look at the remaining dictionary methods, after the next video. Before we 96 00:09:46,400 --> 00:09:50,560 do that, though, I'll demonstrate why our shopping list is more useful than just printing out the 97 00:09:50,560 --> 00:09:56,720 values. If you remember, we modified the program to create a shopping list dictionary, rather than just 98 00:09:56,720 --> 00:10:03,840 printing it. So in the next video, we'll see one way that can be useful. See you in the next video.