WEBVTT 1 00:00:00.620 --> 00:00:02.220 Hi guys, and welcome back. 2 00:00:02.220 --> 00:00:04.400 In this video, we're going to talk about function, 3 00:00:04.400 --> 00:00:06.430 arguments and parameters. 4 00:00:06.430 --> 00:00:09.220 That is, how we can give our functions data 5 00:00:09.220 --> 00:00:10.940 so that they can use it. 6 00:00:10.940 --> 00:00:13.460 For example, the print function makes use of this 7 00:00:13.460 --> 00:00:15.920 so that it can accept a string to print out 8 00:00:15.920 --> 00:00:17.060 into the console. 9 00:00:17.060 --> 00:00:20.880 We're going to learn how to do that in our own functions. 10 00:00:20.880 --> 00:00:23.863 Let me define a function that has two parameters. 11 00:00:27.590 --> 00:00:29.800 What I've done here is I have created a function 12 00:00:29.800 --> 00:00:33.960 called add and inside the brackets I've put the names 13 00:00:33.960 --> 00:00:35.530 of its two parameters. 14 00:00:35.530 --> 00:00:38.210 So that's x and y. 15 00:00:38.210 --> 00:00:41.880 Notice that I have put the word pass inside the function 16 00:00:41.880 --> 00:00:45.680 body, and what that means in Python is do nothing. 17 00:00:45.680 --> 00:00:47.900 It is needed, at least something is needed 18 00:00:47.900 --> 00:00:49.960 inside a function because Python expects 19 00:00:49.960 --> 00:00:51.110 the indented block there. 20 00:00:51.110 --> 00:00:52.900 So that's why I've put that there, but it doesn't 21 00:00:52.900 --> 00:00:54.060 mean anything. 22 00:00:54.060 --> 00:00:57.700 We've got our two parameters here which are x and y. 23 00:00:57.700 --> 00:01:00.380 We have two parameters because they are two variable 24 00:01:00.380 --> 00:01:04.040 names separated by a comma and what happens is that when 25 00:01:04.040 --> 00:01:07.420 Python runs the function, when you call the function itself, 26 00:01:07.420 --> 00:01:11.080 these two variables are created and the function 27 00:01:11.080 --> 00:01:13.790 can use them inside the function body. 28 00:01:13.790 --> 00:01:15.400 When you get to the end of the function body, 29 00:01:15.400 --> 00:01:17.550 the variables disappear. 30 00:01:17.550 --> 00:01:20.550 So what are the values of x and y? 31 00:01:20.550 --> 00:01:23.160 Well, that depends on what you give the function 32 00:01:23.160 --> 00:01:24.010 when you call it. 33 00:01:24.010 --> 00:01:28.360 So for example, if you add and put five comma three, 34 00:01:28.360 --> 00:01:31.740 then these here are all called arguments. 35 00:01:31.740 --> 00:01:33.890 These here are called parameters. 36 00:01:33.890 --> 00:01:38.490 So each argument provides a value to one parameter. 37 00:01:38.490 --> 00:01:43.253 Five is the value of x, and three will be the value of y. 38 00:01:44.410 --> 00:01:47.060 Of course our add function doesn't do anything just now 39 00:01:47.060 --> 00:01:49.050 so we are going to make it do something. 40 00:01:49.050 --> 00:01:51.490 We're going to say result is x plus y, 41 00:01:51.490 --> 00:01:53.650 and then we're going to print result. 42 00:01:53.650 --> 00:01:57.270 So now we have it here our function with two parameters, 43 00:01:57.270 --> 00:02:00.280 and then our function call with two arguments. 44 00:02:00.280 --> 00:02:03.730 When the function call happens, these values get placed 45 00:02:03.730 --> 00:02:06.390 into these variables and the function runs. 46 00:02:06.390 --> 00:02:08.760 So here result will be eight 47 00:02:08.760 --> 00:02:09.940 and then we will print that out. 48 00:02:09.940 --> 00:02:12.880 So let me just run that for you, and you can see that eight 49 00:02:12.880 --> 00:02:14.113 comes out down there. 50 00:02:17.180 --> 00:02:18.420 Let's do another example. 51 00:02:18.420 --> 00:02:21.423 Now a function without any parameters. 52 00:02:22.960 --> 00:02:25.990 So if we have a function such as this say hello function 53 00:02:25.990 --> 00:02:27.440 that doesn't have any parameters, 54 00:02:27.440 --> 00:02:29.840 notice that the brackets are empty. 55 00:02:29.840 --> 00:02:31.460 You cannot give it arguments. 56 00:02:31.460 --> 00:02:34.410 So if you type say hello and you pass in Bob, 57 00:02:34.410 --> 00:02:38.250 this will fail in Python because you are passing an argument 58 00:02:38.250 --> 00:02:40.320 to a function that has no parameters. 59 00:02:40.320 --> 00:02:42.850 So if we run this, you'll see that it says 60 00:02:42.850 --> 00:02:45.330 say hello takes zero positional arguments, 61 00:02:45.330 --> 00:02:46.790 but one was given. 62 00:02:46.790 --> 00:02:49.070 We haven't looked at what positional arguments mean yet, 63 00:02:49.070 --> 00:02:51.360 but Bob is a positional argument. 64 00:02:51.360 --> 00:02:53.020 More on that later on. 65 00:02:53.020 --> 00:02:55.700 So we've given this function one positional argument, 66 00:02:55.700 --> 00:02:57.540 but this function doesn't take any. 67 00:02:57.540 --> 00:02:59.080 It doesn't have any parameters. 68 00:02:59.080 --> 00:03:01.430 So that's why the error happens. 69 00:03:01.430 --> 00:03:04.560 Similarly, if we instead decide to pass in a name, 70 00:03:04.560 --> 00:03:06.503 and we do something like this, 71 00:03:08.210 --> 00:03:11.370 now you can call the function with an argument. 72 00:03:11.370 --> 00:03:14.535 So if we run this, you'll see that we get "Hello, Bob" 73 00:03:14.535 --> 00:03:16.940 because this function prints the string "Hello" 74 00:03:16.940 --> 00:03:20.815 with the name variable inside it, but having a parameter 75 00:03:20.815 --> 00:03:23.540 means that you must pass in an argument. 76 00:03:23.540 --> 00:03:27.250 So you cannot call the function without an argument. 77 00:03:27.250 --> 00:03:31.930 If you do, it says that say hello is missing one required 78 00:03:31.930 --> 00:03:34.480 positional argument and then it gives you the name 79 00:03:34.480 --> 00:03:35.850 of the positional argument. 80 00:03:35.850 --> 00:03:38.570 So it's saying that you need to pass in a value 81 00:03:38.570 --> 00:03:41.640 for this name parameter, but you didn't. 82 00:03:41.640 --> 00:03:43.810 So you should. 83 00:03:43.810 --> 00:03:47.030 So when you put Bob in here, we've said that this is a 84 00:03:47.030 --> 00:03:50.170 positional argument, and the reason why it's called 85 00:03:50.170 --> 00:03:53.740 a positional argument is because if you have multiple 86 00:03:53.740 --> 00:03:58.740 arguments such as these two here, and then you call it 87 00:03:58.770 --> 00:04:02.700 like that, the only way Python has of determining 88 00:04:02.700 --> 00:04:06.450 which parameter takes which value is by position. 89 00:04:06.450 --> 00:04:09.990 So Bob is the first argument or in the first position. 90 00:04:09.990 --> 00:04:13.410 So the name parameter will take the value of Bob, 91 00:04:13.410 --> 00:04:15.520 and Smith is the second argument. 92 00:04:15.520 --> 00:04:19.020 So the second parameter will take the value of that. 93 00:04:19.020 --> 00:04:21.070 So these are called positional arguments 94 00:04:21.070 --> 00:04:23.070 because the position they're in will affect 95 00:04:23.070 --> 00:04:26.450 the value of its relevant parameter. 96 00:04:26.450 --> 00:04:30.040 However, Python also has arguments that are not positional. 97 00:04:30.040 --> 00:04:33.760 They are called keyword or named arguments. 98 00:04:33.760 --> 00:04:38.000 For example, you can do surname equal Bob 99 00:04:38.000 --> 00:04:40.285 and name equal Smith. 100 00:04:40.285 --> 00:04:43.710 Now Python will be able to tell that you want 101 00:04:43.710 --> 00:04:47.210 the surname parameter to take the value of Bob 102 00:04:47.210 --> 00:04:51.210 and the named parameter to take the value of Smith. 103 00:04:51.210 --> 00:04:55.450 These here are named or keyword arguments. 104 00:04:55.450 --> 00:04:58.190 Named or keyword arguments are really helpful 105 00:04:58.190 --> 00:05:01.280 because they allow us to very easily determine 106 00:05:01.280 --> 00:05:05.260 when we are reading this line here which argument 107 00:05:05.260 --> 00:05:07.940 is for which parameter, and indeed if we hadn't 108 00:05:07.940 --> 00:05:11.300 come across this function before, it would also tell us 109 00:05:11.300 --> 00:05:13.550 what the function does to some extent. 110 00:05:13.550 --> 00:05:15.540 If you read this line here, you can see that okay, 111 00:05:15.540 --> 00:05:17.680 you're going to say hello and you're taking a name 112 00:05:17.680 --> 00:05:19.720 and a surname, so you're probably going to print that out. 113 00:05:19.720 --> 00:05:22.250 That allows you to make some inferences about what this 114 00:05:22.250 --> 00:05:23.660 code is going to do. 115 00:05:23.660 --> 00:05:26.910 So this can be really helpful and it's very widely 116 00:05:26.910 --> 00:05:27.943 used in Python. 117 00:05:29.540 --> 00:05:32.630 For example, let's say that you've got a divide function 118 00:05:32.630 --> 00:05:36.030 where if the divisor is not zero, then you're going to print 119 00:05:36.030 --> 00:05:37.030 the dividend by the divisor. 120 00:05:37.030 --> 00:05:39.130 Otherwise, you're going to print something 121 00:05:39.130 --> 00:05:42.890 like "You fool," you can't divide something by zero. 122 00:05:42.890 --> 00:05:44.420 So we've got a function here. 123 00:05:44.420 --> 00:05:47.930 Then you can call this function divide with something 124 00:05:47.930 --> 00:05:50.230 like 15 and zero. 125 00:05:50.230 --> 00:05:52.130 So this may be okay. 126 00:05:52.130 --> 00:05:56.140 You can press play and you get back "you fool." 127 00:05:56.140 --> 00:06:00.730 However, it can be clearer if you tell the reader 128 00:06:00.730 --> 00:06:03.860 of this line which value is which. 129 00:06:03.860 --> 00:06:06.010 Now with something like division it may be obvious 130 00:06:06.010 --> 00:06:09.270 that 15 comma zero is going to divide 15 by zero. 131 00:06:09.270 --> 00:06:12.120 That seems reasonable to a reader I think, 132 00:06:12.120 --> 00:06:15.870 but, again, it can be cleared if you say dividend is 15, 133 00:06:15.870 --> 00:06:17.710 and divisor is zero. 134 00:06:17.710 --> 00:06:20.550 Notice that usually in Python we do not put spaces 135 00:06:20.550 --> 00:06:23.460 around this equal sign here when it's being used 136 00:06:23.460 --> 00:06:28.390 to separate a value from its keyword argument. 137 00:06:28.390 --> 00:06:30.610 As well as passing dividend and divisor like this, 138 00:06:30.610 --> 00:06:32.657 you could reverse them; put the divisor here 139 00:06:32.657 --> 00:06:34.300 and the dividend there and that would be fine, 140 00:06:34.300 --> 00:06:36.480 although maybe a little more bit confusing, 141 00:06:36.480 --> 00:06:40.420 but also not all arguments have to be keyword arguments. 142 00:06:40.420 --> 00:06:42.440 You can have some positional arguments 143 00:06:42.440 --> 00:06:46.230 and then some keyword arguments, but that's exactly it. 144 00:06:46.230 --> 00:06:48.480 The positional arguments have to go first 145 00:06:48.480 --> 00:06:50.770 and the keyword arguments have to go later. 146 00:06:50.770 --> 00:06:54.680 If you do dividend equal 15 and then you skip the keyword 147 00:06:54.680 --> 00:06:57.010 argument there, you're going to get an error and Python 148 00:06:57.010 --> 00:06:59.460 is going to tell you that a positional argument follows 149 00:06:59.460 --> 00:07:02.540 a keyword argument and that is illegal in Python. 150 00:07:02.540 --> 00:07:04.650 So the positional arguments have to go first, 151 00:07:04.650 --> 00:07:06.133 keyword arguments go later. 152 00:07:07.330 --> 00:07:10.410 I recommend that you use keyword arguments 153 00:07:10.410 --> 00:07:12.540 whenever possible just because you're going to make 154 00:07:12.540 --> 00:07:14.630 your code a little bit easier to read, 155 00:07:14.630 --> 00:07:17.300 unless it's really obvious what's going on. 156 00:07:17.300 --> 00:07:19.925 So for example, in a divide function, maybe you don't need 157 00:07:19.925 --> 00:07:21.780 the keyword arguments. 158 00:07:21.780 --> 00:07:24.220 Similarly, in a say hello function that just takes 159 00:07:24.220 --> 00:07:26.700 a person's name, you probably don't need keyword arguments 160 00:07:26.700 --> 00:07:29.860 either, but as soon as you start doing more complicated 161 00:07:29.860 --> 00:07:32.710 things or functions that are maybe not as obvious, 162 00:07:32.710 --> 00:07:34.490 using keyword arguments is really going to help. 163 00:07:34.490 --> 00:07:36.630 So I recommend that you use them. 164 00:07:36.630 --> 00:07:37.640 That's it for this video though. 165 00:07:37.640 --> 00:07:39.040 Thank you for joining me in this one, 166 00:07:39.040 --> 00:07:40.390 and I'll see you next time.