WEBVTT 1 00:00:00.600 --> 00:00:02.060 Hi guys and welcome back. 2 00:00:02.060 --> 00:00:04.700 In this video we're going to learn about type hinting. 3 00:00:04.700 --> 00:00:06.981 Type hinting is a new feature in Python 3.5. 4 00:00:06.981 --> 00:00:09.398 So you must be using that Python version or later 5 00:00:09.398 --> 00:00:12.050 in order to be able to use this. 6 00:00:12.050 --> 00:00:15.120 Here we've got a function called list average. 7 00:00:15.120 --> 00:00:16.570 And that takes in a sequence 8 00:00:16.570 --> 00:00:19.340 and returns the sum of the sequence divided by the length. 9 00:00:19.340 --> 00:00:23.410 You can call this with pretty much anything you want. 10 00:00:23.410 --> 00:00:28.410 Right, you can do list a-v-g with one, two, three. 11 00:00:28.869 --> 00:00:31.350 Obviously this is not a sequence 12 00:00:31.350 --> 00:00:33.480 and it's certainly not a list. 13 00:00:33.480 --> 00:00:35.400 So maybe if you're calling this 14 00:00:35.400 --> 00:00:37.270 you'll realise that it doesn't make much sense. 15 00:00:37.270 --> 00:00:39.010 But you can call it. 16 00:00:39.010 --> 00:00:41.060 It will give you an error if you do that. 17 00:00:42.060 --> 00:00:43.120 As you can see, 18 00:00:43.120 --> 00:00:46.390 it tells you that you can't do sum of an integer 19 00:00:46.390 --> 00:00:48.180 or len of an integer. 20 00:00:48.180 --> 00:00:50.270 But while you're coding, 21 00:00:50.270 --> 00:00:51.840 there's nothing there to tell you 22 00:00:51.840 --> 00:00:53.970 that you might be making a mistake. 23 00:00:53.970 --> 00:00:55.331 So let's fix that. 24 00:00:55.331 --> 00:01:00.331 You can write inside the here, inside the brackets, 25 00:01:00.680 --> 00:01:02.630 colon list. 26 00:01:02.630 --> 00:01:04.110 And what this tells Python is that 27 00:01:04.110 --> 00:01:07.020 this thing should be a list. 28 00:01:07.020 --> 00:01:08.734 After the brackets you can type 29 00:01:08.734 --> 00:01:13.734 dash greater than, making a small arrow there, float. 30 00:01:14.100 --> 00:01:15.550 And what that tells Python is that 31 00:01:15.550 --> 00:01:18.683 this function is going to return a float. 32 00:01:19.900 --> 00:01:24.190 So this is how you do type hinting in Python. 33 00:01:24.190 --> 00:01:28.312 However let me say that using the list object as a hint here 34 00:01:28.312 --> 00:01:29.895 is usually not recommended. 35 00:01:29.895 --> 00:01:33.630 Usually you'll be doing from typing import list. 36 00:01:33.630 --> 00:01:35.688 And I know we haven't looked at importing 37 00:01:35.688 --> 00:01:37.000 in Python just yet. 38 00:01:37.000 --> 00:01:38.750 We're going to learn about that in the next video actually. 39 00:01:38.750 --> 00:01:40.850 But you'll be using that 40 00:01:40.850 --> 00:01:42.290 and then you'll be putting that there. 41 00:01:42.290 --> 00:01:44.810 That's the most common way of doing it. 42 00:01:44.810 --> 00:01:46.670 We're going to learn about import in the next video. 43 00:01:46.670 --> 00:01:50.820 So this tells you that the sequence is a list of elements. 44 00:01:50.820 --> 00:01:52.260 You don't know what elements. 45 00:01:52.260 --> 00:01:54.020 And it returns a float. 46 00:01:54.020 --> 00:01:56.200 Notice though that VSCode, 47 00:01:56.200 --> 00:01:57.440 which is the editor that I'm using now, 48 00:01:57.440 --> 00:01:59.810 doesn't tell me that I could be making a mistake. 49 00:01:59.810 --> 00:02:01.090 If you're using PyCharm 50 00:02:01.090 --> 00:02:04.210 or if you're running a programme called a linter, 51 00:02:04.210 --> 00:02:06.910 then that programme or PyCharm will tell you, 52 00:02:06.910 --> 00:02:08.775 hey you're passing in a number here, 53 00:02:08.775 --> 00:02:11.780 but the expectation is that you should pass a list. 54 00:02:11.780 --> 00:02:14.823 So that is why we do type hinting. 55 00:02:28.080 --> 00:02:29.420 You can type a hint anywhere. 56 00:02:29.420 --> 00:02:33.510 So for example, you can type hint your class book. 57 00:02:33.510 --> 00:02:37.441 You can say that the name should be a s-t-r for a string 58 00:02:37.441 --> 00:02:40.070 and the page count should be an int. 59 00:02:40.070 --> 00:02:41.870 You don't have to specify a return value 60 00:02:41.870 --> 00:02:44.159 because this function doesn't return anything. 61 00:02:44.159 --> 00:02:47.100 Here we've got our from typing import list again. 62 00:02:47.100 --> 00:02:49.913 You can also import Tuple, Set and there's a bunch more. 63 00:02:49.913 --> 00:02:52.650 And you can use it like that. 64 00:02:52.650 --> 00:02:55.952 If you wanted to define that the books parameter 65 00:02:55.952 --> 00:02:59.530 should be a list of book objects. 66 00:02:59.530 --> 00:03:03.700 Given that the class book exists of course. 67 00:03:03.700 --> 00:03:05.330 So this is how you can do that. 68 00:03:05.330 --> 00:03:08.040 Also notice that I have signalled a string being returned 69 00:03:08.040 --> 00:03:09.520 from the s-t-r function 70 00:03:09.520 --> 00:03:11.540 and again that's that being used there. 71 00:03:11.540 --> 00:03:13.700 So the key benefit of doing this sort of stuff 72 00:03:13.700 --> 00:03:17.120 is that you'll get told if you pass in the wrong thing. 73 00:03:17.120 --> 00:03:18.856 As we learn more about Python, 74 00:03:18.856 --> 00:03:22.270 then we'll be using these type hints more and more. 75 00:03:22.270 --> 00:03:24.100 So you'll see me using them every now and then 76 00:03:24.100 --> 00:03:25.900 in the next few videos. 77 00:03:25.900 --> 00:03:27.860 One more example, that we've already seen, 78 00:03:27.860 --> 00:03:31.350 our book that has the class methods for hardcover 79 00:03:31.350 --> 00:03:32.230 and paperback. 80 00:03:32.230 --> 00:03:33.800 Now we can type hint it. 81 00:03:33.800 --> 00:03:36.060 And make sure that when we are using this code 82 00:03:36.060 --> 00:03:38.961 we get told whether the name, book type and weight 83 00:03:38.961 --> 00:03:41.780 arguments that we're using to create a book 84 00:03:41.780 --> 00:03:43.230 are correct or not. 85 00:03:43.230 --> 00:03:45.450 So here we're saying that the name should be a string. 86 00:03:45.450 --> 00:03:46.882 Name, colon, string. 87 00:03:46.882 --> 00:03:48.940 Book type should be a string. 88 00:03:48.940 --> 00:03:50.746 And the weight should be an integer. 89 00:03:50.746 --> 00:03:53.105 The repr function returns a string. 90 00:03:53.105 --> 00:03:56.930 And the hardcover and paperback methods take in a string 91 00:03:56.930 --> 00:04:01.090 and an integer and return a book object. 92 00:04:01.090 --> 00:04:04.170 This is how you signal that these methods 93 00:04:04.170 --> 00:04:08.377 return an object of the same type that you're currently in. 94 00:04:08.377 --> 00:04:12.890 You can't do this because it will give you an error 95 00:04:12.890 --> 00:04:15.210 because this method here is created 96 00:04:15.210 --> 00:04:17.660 before the class has finished processing. 97 00:04:17.660 --> 00:04:20.075 So this is created while the class is being processed. 98 00:04:20.075 --> 00:04:22.470 So that's why you can't use that there. 99 00:04:22.470 --> 00:04:24.650 So in Python we put them inside quotes 100 00:04:24.650 --> 00:04:27.786 to signal that this method returns a book object 101 00:04:27.786 --> 00:04:30.930 which is the same type that we're currently in. 102 00:04:30.930 --> 00:04:33.470 If this returned a different class, 103 00:04:33.470 --> 00:04:36.827 then you would do that, say if it returned a bookshelf. 104 00:04:36.827 --> 00:04:38.840 Then you can totally do that. 105 00:04:38.840 --> 00:04:41.171 But if it's the same type that you are currently in. 106 00:04:41.171 --> 00:04:43.800 Then you have to signal it like that. 107 00:04:43.800 --> 00:04:45.420 All right, just a few examples of 108 00:04:45.420 --> 00:04:47.858 how to use type hinting in Python. 109 00:04:47.858 --> 00:04:49.910 Again this is a refresher course, 110 00:04:49.910 --> 00:04:53.040 so we're not going into detail of how everything is working. 111 00:04:53.040 --> 00:04:54.467 But we will be using these type hints 112 00:04:54.467 --> 00:04:56.020 over the next few videos 113 00:04:56.020 --> 00:04:57.330 and you'll get more familiar with them. 114 00:04:57.330 --> 00:04:58.280 They're really straight forward. 115 00:04:58.280 --> 00:05:00.100 It's just there to help you as a developer. 116 00:05:00.100 --> 00:05:02.060 They don't stop your code from running. 117 00:05:02.060 --> 00:05:04.160 They don't raise any errors or anything like that. 118 00:05:04.160 --> 00:05:05.080 It's just to help you. 119 00:05:05.080 --> 00:05:06.100 Thanks for joining me in this video. 120 00:05:06.100 --> 00:05:07.927 And I'll see you in the next one.