1 00:00:05,280 --> 00:00:10,400 in this video we'll take a look at the remaining dict methods - the ones we haven't used yet. We're 2 00:00:10,400 --> 00:00:14,320 not going to write complete programs that use them. We'll use some simple examples, 3 00:00:14,320 --> 00:00:18,320 to demonstrate their behavior. The methods can be very useful when you need them, 4 00:00:18,320 --> 00:00:22,560 but aren't as commonly used as the methods we've already covered. I'm going to create a 5 00:00:22,560 --> 00:00:28,422 new python file for these examples, I'm going to call this one dict_methods.py 6 00:00:34,480 --> 00:00:37,040 Alright, so we're going to need a dictionary to work with, and a simple 7 00:00:37,040 --> 00:00:41,120 one will best demonstrate the effects of these methods. I'm going to paste in a dictionary, 8 00:00:41,120 --> 00:00:46,400 and a list. You can copy these definitions from the file d.text, in the resources for this video, 9 00:00:47,840 --> 00:00:52,480 and you can see them on screen there. Alright, I'm going to start with the fromkeys class method, 10 00:00:53,040 --> 00:00:56,560 in the documentation. Now the Python dictionary class is called dict, 11 00:00:56,560 --> 00:01:02,080 and up until now we've referred to it as a type. When the documentation talks about a class method, 12 00:01:02,080 --> 00:01:06,000 it means you can use the class name to call the method. So let's see how that works: 13 00:01:06,800 --> 00:01:16,058 so we're going to type new_dict is equal to dict.fromkeys and, in parentheses, pantry_items, 14 00:01:16,640 --> 00:01:23,840 and we'll print out the new dictionary. On line 16 we use dict.fromkeys to call the method. 15 00:01:23,840 --> 00:01:29,360 The dictionary class is called dict, and we're using that class to call the fromkeys method. 16 00:01:29,360 --> 00:01:34,320 We're passing an iterable to the method. I've used the pantry_items list from line 14. 17 00:01:35,200 --> 00:01:39,520 Alright, so let's run the program, and check the output. So we've got a dictionary with five keys, 18 00:01:39,520 --> 00:01:45,680 all having the value None. The keys came from the iterable that we passed to the from_keys method. 19 00:01:45,680 --> 00:01:51,360 Each key is one of the items in the pantry_items list. Now, we can change that default value, 20 00:01:51,360 --> 00:01:57,200 which is currently None, by passing another value to the method. Come up here, put a comma zero. 21 00:01:58,880 --> 00:02:06,720 Run that, this time all the values are zero. so this fromkeys method is a useful way to create 22 00:02:06,720 --> 00:02:12,480 an initial dictionary, which your code will then change. Each key starts off with the default value 23 00:02:12,480 --> 00:02:17,520 0, in this case. If you were creating an empty pantry, for example, this could be a good way to 24 00:02:17,520 --> 00:02:22,000 do it. The code would then change the amounts, when the cook entered how much they had of each 25 00:02:22,000 --> 00:02:27,120 item. Right, so I'm going to comment out those two lines, and we'll move on now to the keys method. 26 00:02:28,880 --> 00:02:33,120 I'm going to demonstrate what it does, but I can't give many useful examples of when you'd use it. 27 00:02:33,680 --> 00:02:38,560 It's really a legacy from Python 2, where it was needed for iterating over a dictionary's keys. 28 00:02:39,440 --> 00:02:44,320 We'll type keys equals d dot keys parentheses, then we'll print out keys on the next line. 29 00:02:46,400 --> 00:02:53,920 If we run this, the output looks a bit like a list. When the keys method was added to Python 2.2 30 00:02:53,920 --> 00:02:59,360 it did return a list - a list containing all the dictionary keys. That's not very efficient, 31 00:02:59,360 --> 00:03:03,920 because it involved copying the keys, which could be a problem with very large dictionaries. 32 00:03:03,920 --> 00:03:09,200 In Python 3, the keys method produces a view object. It's similar to a list, 33 00:03:09,200 --> 00:03:14,400 but you can't add or remove items from it. If you add or remove keys to the underlying dictionary, 34 00:03:14,400 --> 00:03:20,000 the value will be updated automatically. There are very few situations where you have to use this 35 00:03:20,000 --> 00:03:25,600 keys method, in Python 3. Some programmers use it because it makes the meaning of the code explicit; 36 00:03:26,800 --> 00:03:28,124 let's look at an example 37 00:03:30,800 --> 00:03:36,480 So on line 22 I'm going to type for item in d colon then print item in parentheses. 38 00:03:38,080 --> 00:03:42,640 So it's not very obvious what's happening here. We can see that we're iterating over something, 39 00:03:42,640 --> 00:03:46,560 but there's no indication of what that something is. If we run the program, 40 00:03:48,160 --> 00:03:52,560 we can see the dictionary keys printed out. Now, some programmers would actually change that 41 00:03:53,120 --> 00:04:02,400 to be d dot keys and parentheses. If I run that again, we get exactly the same results, 42 00:04:03,440 --> 00:04:08,800 but it's now clearer what we're iterating over, on line 22. When you see dot keys, you know you 43 00:04:08,800 --> 00:04:13,520 must be dealing with a dictionary. So that could be a good way to make the code easier to read 44 00:04:13,520 --> 00:04:18,320 in a large program. As I said, though, it's rare that you must use the keys method. 45 00:04:18,320 --> 00:04:22,480 The only situation I can think of, where you'd have to use it, is if you want to perform certain 46 00:04:23,036 --> 00:04:28,396 set operations on the keys of a dictionary. We'll come back to that, when we've learnt about sets. 47 00:04:28,880 --> 00:04:32,880 Alright, so that leaves the update and values methods. We'll look at those next, 48 00:04:32,880 --> 00:04:37,600 then I'll discuss the copy method that I've skipped over. See you in the next video.