1 00:00:05,360 --> 00:00:09,840 The values method returns a view of the dictionary's values. It's similar 2 00:00:09,840 --> 00:00:15,280 to the keys method, but instead of a list of keys, we get a list of values. 3 00:00:15,280 --> 00:00:18,800 Once again I'll start with an example. What I'm going to do is comment out the code we 4 00:00:18,800 --> 00:00:26,628 used in the previous lecture, and put the new code before it. So basically, all this stuff here from d2, 5 00:00:26,628 --> 00:00:35,926 down to the bottom here. Alright. We'll come up here. We'll put a comment about this. 6 00:00:44,800 --> 00:00:55,200 Alright. We'll start by typing v equals d dot values parentheses, then we'll print v. I don't recommend 7 00:00:55,200 --> 00:00:59,360 using single letter names for your variables, by the way. It's okay to do that for small 8 00:00:59,360 --> 00:01:06,162 snippets like these, but use sensible variable names in your programs. Okay, so let's run this, 9 00:01:06,880 --> 00:01:14,774 and we should get a dict_values object printed out, and we can see it over here. 10 00:01:15,440 --> 00:01:20,000 And I might move that down to the bottom, 11 00:01:22,400 --> 00:01:27,360 just leaving a bit of space there for subtitles. Alright, so a dict_values object 12 00:01:27,360 --> 00:01:31,760 looks a lot like a list, looking at that output, and supports some of the operations that we can 13 00:01:31,760 --> 00:01:37,520 perform on lists. Just like the dict_keys object that we saw earlier, we can't add 14 00:01:37,520 --> 00:01:43,040 new items to a dict_values object. It'll automatically be updated if we add or remove keys, 15 00:01:43,040 --> 00:01:49,200 in the underlying dictionary. So as an example of that, we add some code here; 16 00:01:49,200 --> 00:02:01,810 d 10 in square brackets, equals ten in double quotes, print parentheses v. So if we run that, 17 00:02:02,640 --> 00:02:08,281 the string value 10 appears at the end of the dict_values when we add it to the dictionary. 18 00:02:08,281 --> 00:02:12,824 Now the documentation mentions this, so let's switch back to my browser. 19 00:02:13,760 --> 00:02:18,000 Scroll down a bit more, until we get to our values which we're talking about, 20 00:02:18,000 --> 00:02:23,120 and click on this documentation of view objects link. So it says here, 21 00:02:23,120 --> 00:02:29,694 that view objects provide a dynamic view on the dictionary's entries. If the dictionary changes, 22 00:02:29,694 --> 00:02:36,320 the changes will be seen in the keys, values and items views. As you can see, these view objects 23 00:02:36,320 --> 00:02:42,560 support a limited subset of the operations that we can perform with lists. One thing we can't do 24 00:02:42,560 --> 00:02:46,481 is index into them. That's a shame because there are times when it would be useful, 25 00:02:46,481 --> 00:02:53,200 but we can't do it. There is a reason for that, though. I compared the keys and values objects to lists, 26 00:02:53,200 --> 00:02:58,080 but they work more like sets. We'll be looking at sets soon, and you'll understand why we can't 27 00:02:58,080 --> 00:03:03,760 index into these objects, once you've learnt about sets. Alright, so back to our code. Values can be 28 00:03:03,760 --> 00:03:10,984 very useful if you want to check if a value exists in a dictionary. Let's look at some code for that. 29 00:03:11,520 --> 00:03:19,360 Line 22, print parentheses four in double quotes, in v. And we'll do another one there. 30 00:03:19,360 --> 00:03:27,093 Next line, print parentheses double quotes 11 in double quotes, in v - 31 00:03:27,600 --> 00:03:35,550 obviously with a space there. Let's run this, and we can see that line 22 prints True, 32 00:03:35,550 --> 00:03:40,720 because four is a value in the dictionary. The value eleven isn't in the dictionary so we get False, 33 00:03:40,720 --> 00:03:46,640 printed by line 23. Sometimes you might have to check if a value exists in your dictionary. 34 00:03:46,640 --> 00:03:50,800 Generally though, dictionaries are intended to be accessed via their keys. 35 00:03:50,800 --> 00:03:55,040 If you find that your code is performing a lot of checks on the values, then you should think about 36 00:03:55,040 --> 00:04:01,120 rearranging your code. A dictionary isn't efficient for searching for values. We know that the string 37 00:04:01,120 --> 00:04:07,440 value 4 exists in our dictionary, because we got True printed by line 22, but we've got no idea 38 00:04:07,440 --> 00:04:13,280 what its key is. There are ways around that. One approach that you'll find online, is to convert 39 00:04:13,280 --> 00:04:20,000 both the keys and the values to lists. And the code for that would be something like this; 40 00:04:22,400 --> 00:04:29,920 so on line 25, I start by typing keys equals list parentheses d dot keys. 41 00:04:29,920 --> 00:04:41,840 Then on the next line, values equals list v in parentheses, and I'll make a comment here. 42 00:04:45,520 --> 00:04:55,920 Alright then, if, and on the next line, four in double quotes, is in values colon, and the next line, index is equal 43 00:04:55,920 --> 00:05:08,320 to values.index four in double quotes, in the parentheses. And key equals keys and square bracket, 44 00:05:08,320 --> 00:05:12,000 index and then right square bracket. And then we'll print 45 00:05:12,000 --> 00:05:20,080 parentheses and it's an f string double quotes curly braces d square bracket key, 46 00:05:20,080 --> 00:05:28,720 another square bracket to close it, then our right curly brace, was found with the key and then key in 47 00:05:28,720 --> 00:05:35,360 curly braces. Alright, so let's run the program to check that that actually works. So we can see 48 00:05:35,360 --> 00:05:40,800 that that does work, but it's not very efficient. We have to duplicate all the keys and values 49 00:05:40,800 --> 00:05:47,120 of the dictionary, for one thing. Lines 25 and 26 create new lists containing the keys and values 50 00:05:47,120 --> 00:05:52,880 from the dictionary. With very large dictionaries, this will use a lot of memory. As an alternative, 51 00:05:52,880 --> 00:06:00,000 you could do the same thing by looping through the dictionary items. Let's have a look at that. 52 00:06:01,120 --> 00:06:09,280 So put a print parenthesis on its own, on line 32, to make a bit of space. And continuing on, for key comma value 53 00:06:09,280 --> 00:06:13,749 in d.items parentheses colon. 54 00:06:14,400 --> 00:06:22,160 Next line, if the value is equal to four in double quotes colon print - and actually, 55 00:06:22,160 --> 00:06:27,797 we'll just take a copy of that line because it's going to be exactly the same, save a bit of typing. 56 00:06:30,320 --> 00:06:36,080 So let's run this and see if it works, and you can see that it has worked. It's given us the same result, 57 00:06:36,080 --> 00:06:42,320 and this time we haven't had to copy the keys and values. It also works if the same value appears 58 00:06:42,320 --> 00:06:47,360 more than once in the dictionary. So as an example of that, let's go back up to our dictionary. 59 00:06:47,360 --> 00:06:54,917 Up here, we're going to add another entry. I'm going to add four, using roman numerals as the key, 60 00:06:59,040 --> 00:07:07,200 like so. Back down to our code again, and we'll run this. As you can see, we get both of the keys 61 00:07:07,200 --> 00:07:13,200 for the value four, with that second block of code there. The code on lines 26 to 31 62 00:07:13,200 --> 00:07:17,520 only finds the first occurrence. We'd have to do a bit more work to find all the keys for the value four, 63 00:07:17,520 --> 00:07:22,320 using that first approach. Alright, so that's the values method. It does have its uses, but again, if 64 00:07:22,320 --> 00:07:26,313 you find yourself using it a lot, then you should consider if a dictionary is the most suitable 65 00:07:26,313 --> 00:07:32,514 data structure for your particular program. We'll be looking at how dictionaries store their data soon. 66 00:07:32,514 --> 00:07:36,960 When you've seen how dictionaries work, you'll understand why retrieving a key is significantly 67 00:07:36,960 --> 00:07:43,840 faster than getting a value. Okay, so I promised to talk about the copy method. Let's go back. 68 00:07:46,960 --> 00:07:50,480 So specifically, I'm going to explain about that shallow copy 69 00:07:50,480 --> 00:07:55,990 that the documentation mentions. That will also explain the comment in the documentation of fromkeys, 70 00:07:55,990 --> 00:07:59,920 where it says that it generally doesn't make sense for value to be a mutable object, 71 00:07:59,920 --> 00:08:06,480 such as an empty list. So we'll learn all about that in the next few videos.