1 1 00:00:00,620 --> 00:00:02,220 Hi guys, and welcome back. 2 2 00:00:02,220 --> 00:00:04,400 In this video, we're going to talk about function, 3 3 00:00:04,400 --> 00:00:06,430 arguments and parameters. 4 4 00:00:06,430 --> 00:00:09,220 That is, how we can give our functions data 5 5 00:00:09,220 --> 00:00:10,940 so that they can use it. 6 6 00:00:10,940 --> 00:00:13,460 For example, the print function makes use of this 7 7 00:00:13,460 --> 00:00:15,920 so that it can accept a string to print out 8 8 00:00:15,920 --> 00:00:17,060 into the console. 9 9 00:00:17,060 --> 00:00:20,880 We're going to learn how to do that in our own functions. 10 10 00:00:20,880 --> 00:00:23,863 Let me define a function that has two parameters. 11 11 00:00:27,590 --> 00:00:29,800 What I've done here is I have created a function 12 12 00:00:29,800 --> 00:00:33,960 called add and inside the brackets I've put the names 13 13 00:00:33,960 --> 00:00:35,530 of its two parameters. 14 14 00:00:35,530 --> 00:00:38,210 So that's x and y. 15 15 00:00:38,210 --> 00:00:41,880 Notice that I have put the word pass inside the function 16 16 00:00:41,880 --> 00:00:45,680 body, and what that means in Python is do nothing. 17 17 00:00:45,680 --> 00:00:47,900 It is needed, at least something is needed 18 18 00:00:47,900 --> 00:00:49,960 inside a function because Python expects 19 19 00:00:49,960 --> 00:00:51,110 the indented block there. 20 20 00:00:51,110 --> 00:00:52,900 So that's why I've put that there, but it doesn't 21 21 00:00:52,900 --> 00:00:54,060 mean anything. 22 22 00:00:54,060 --> 00:00:57,700 We've got our two parameters here which are x and y. 23 23 00:00:57,700 --> 00:01:00,380 We have two parameters because they are two variable 24 24 00:01:00,380 --> 00:01:04,040 names separated by a comma and what happens is that when 25 25 00:01:04,040 --> 00:01:07,420 Python runs the function, when you call the function itself, 26 26 00:01:07,420 --> 00:01:11,080 these two variables are created and the function 27 27 00:01:11,080 --> 00:01:13,790 can use them inside the function body. 28 28 00:01:13,790 --> 00:01:15,400 When you get to the end of the function body, 29 29 00:01:15,400 --> 00:01:17,550 the variables disappear. 30 30 00:01:17,550 --> 00:01:20,550 So what are the values of x and y? 31 31 00:01:20,550 --> 00:01:23,160 Well, that depends on what you give the function 32 32 00:01:23,160 --> 00:01:24,010 when you call it. 33 33 00:01:24,010 --> 00:01:28,360 So for example, if you add and put five comma three, 34 34 00:01:28,360 --> 00:01:31,740 then these here are all called arguments. 35 35 00:01:31,740 --> 00:01:33,890 These here are called parameters. 36 36 00:01:33,890 --> 00:01:38,490 So each argument provides a value to one parameter. 37 37 00:01:38,490 --> 00:01:43,253 Five is the value of x, and three will be the value of y. 38 38 00:01:44,410 --> 00:01:47,060 Of course our add function doesn't do anything just now 39 39 00:01:47,060 --> 00:01:49,050 so we are going to make it do something. 40 40 00:01:49,050 --> 00:01:51,490 We're going to say result is x plus y, 41 41 00:01:51,490 --> 00:01:53,650 and then we're going to print result. 42 42 00:01:53,650 --> 00:01:57,270 So now we have it here our function with two parameters, 43 43 00:01:57,270 --> 00:02:00,280 and then our function call with two arguments. 44 44 00:02:00,280 --> 00:02:03,730 When the function call happens, these values get placed 45 45 00:02:03,730 --> 00:02:06,390 into these variables and the function runs. 46 46 00:02:06,390 --> 00:02:08,760 So here result will be eight 47 47 00:02:08,760 --> 00:02:09,940 and then we will print that out. 48 48 00:02:09,940 --> 00:02:12,880 So let me just run that for you, and you can see that eight 49 49 00:02:12,880 --> 00:02:14,113 comes out down there. 50 50 00:02:17,180 --> 00:02:18,420 Let's do another example. 51 51 00:02:18,420 --> 00:02:21,423 Now a function without any parameters. 52 52 00:02:22,960 --> 00:02:25,990 So if we have a function such as this say hello function 53 53 00:02:25,990 --> 00:02:27,440 that doesn't have any parameters, 54 54 00:02:27,440 --> 00:02:29,840 notice that the brackets are empty. 55 55 00:02:29,840 --> 00:02:31,460 You cannot give it arguments. 56 56 00:02:31,460 --> 00:02:34,410 So if you type say hello and you pass in Bob, 57 57 00:02:34,410 --> 00:02:38,250 this will fail in Python because you are passing an argument 58 58 00:02:38,250 --> 00:02:40,320 to a function that has no parameters. 59 59 00:02:40,320 --> 00:02:42,850 So if we run this, you'll see that it says 60 60 00:02:42,850 --> 00:02:45,330 say hello takes zero positional arguments, 61 61 00:02:45,330 --> 00:02:46,790 but one was given. 62 62 00:02:46,790 --> 00:02:49,070 We haven't looked at what positional arguments mean yet, 63 63 00:02:49,070 --> 00:02:51,360 but Bob is a positional argument. 64 64 00:02:51,360 --> 00:02:53,020 More on that later on. 65 65 00:02:53,020 --> 00:02:55,700 So we've given this function one positional argument, 66 66 00:02:55,700 --> 00:02:57,540 but this function doesn't take any. 67 67 00:02:57,540 --> 00:02:59,080 It doesn't have any parameters. 68 68 00:02:59,080 --> 00:03:01,430 So that's why the error happens. 69 69 00:03:01,430 --> 00:03:04,560 Similarly, if we instead decide to pass in a name, 70 70 00:03:04,560 --> 00:03:06,503 and we do something like this, 71 71 00:03:08,210 --> 00:03:11,370 now you can call the function with an argument. 72 72 00:03:11,370 --> 00:03:14,535 So if we run this, you'll see that we get "Hello, Bob" 73 73 00:03:14,535 --> 00:03:16,940 because this function prints the string "Hello" 74 74 00:03:16,940 --> 00:03:20,815 with the name variable inside it, but having a parameter 75 75 00:03:20,815 --> 00:03:23,540 means that you must pass in an argument. 76 76 00:03:23,540 --> 00:03:27,250 So you cannot call the function without an argument. 77 77 00:03:27,250 --> 00:03:31,930 If you do, it says that say hello is missing one required 78 78 00:03:31,930 --> 00:03:34,480 positional argument and then it gives you the name 79 79 00:03:34,480 --> 00:03:35,850 of the positional argument. 80 80 00:03:35,850 --> 00:03:38,570 So it's saying that you need to pass in a value 81 81 00:03:38,570 --> 00:03:41,640 for this name parameter, but you didn't. 82 82 00:03:41,640 --> 00:03:43,810 So you should. 83 83 00:03:43,810 --> 00:03:47,030 So when you put Bob in here, we've said that this is a 84 84 00:03:47,030 --> 00:03:50,170 positional argument, and the reason why it's called 85 85 00:03:50,170 --> 00:03:53,740 a positional argument is because if you have multiple 86 86 00:03:53,740 --> 00:03:58,740 arguments such as these two here, and then you call it 87 87 00:03:58,770 --> 00:04:02,700 like that, the only way Python has of determining 88 88 00:04:02,700 --> 00:04:06,450 which parameter takes which value is by position. 89 89 00:04:06,450 --> 00:04:09,990 So Bob is the first argument or in the first position. 90 90 00:04:09,990 --> 00:04:13,410 So the name parameter will take the value of Bob, 91 91 00:04:13,410 --> 00:04:15,520 and Smith is the second argument. 92 92 00:04:15,520 --> 00:04:19,020 So the second parameter will take the value of that. 93 93 00:04:19,020 --> 00:04:21,070 So these are called positional arguments 94 94 00:04:21,070 --> 00:04:23,070 because the position they're in will affect 95 95 00:04:23,070 --> 00:04:26,450 the value of its relevant parameter. 96 96 00:04:26,450 --> 00:04:30,040 However, Python also has arguments that are not positional. 97 97 00:04:30,040 --> 00:04:33,760 They are called keyword or named arguments. 98 98 00:04:33,760 --> 00:04:38,000 For example, you can do surname equal Bob 99 99 00:04:38,000 --> 00:04:40,285 and name equal Smith. 100 100 00:04:40,285 --> 00:04:43,710 Now Python will be able to tell that you want 101 101 00:04:43,710 --> 00:04:47,210 the surname parameter to take the value of Bob 102 102 00:04:47,210 --> 00:04:51,210 and the named parameter to take the value of Smith. 103 103 00:04:51,210 --> 00:04:55,450 These here are named or keyword arguments. 104 104 00:04:55,450 --> 00:04:58,190 Named or keyword arguments are really helpful 105 105 00:04:58,190 --> 00:05:01,280 because they allow us to very easily determine 106 106 00:05:01,280 --> 00:05:05,260 when we are reading this line here which argument 107 107 00:05:05,260 --> 00:05:07,940 is for which parameter, and indeed if we hadn't 108 108 00:05:07,940 --> 00:05:11,300 come across this function before, it would also tell us 109 109 00:05:11,300 --> 00:05:13,550 what the function does to some extent. 110 110 00:05:13,550 --> 00:05:15,540 If you read this line here, you can see that okay, 111 111 00:05:15,540 --> 00:05:17,680 you're going to say hello and you're taking a name 112 112 00:05:17,680 --> 00:05:19,720 and a surname, so you're probably going to print that out. 113 113 00:05:19,720 --> 00:05:22,250 That allows you to make some inferences about what this 114 114 00:05:22,250 --> 00:05:23,660 code is going to do. 115 115 00:05:23,660 --> 00:05:26,910 So this can be really helpful and it's very widely 116 116 00:05:26,910 --> 00:05:27,943 used in Python. 117 117 00:05:29,540 --> 00:05:32,630 For example, let's say that you've got a divide function 118 118 00:05:32,630 --> 00:05:36,030 where if the divisor is not zero, then you're going to print 119 119 00:05:36,030 --> 00:05:37,030 the dividend by the divisor. 120 120 00:05:37,030 --> 00:05:39,130 Otherwise, you're going to print something 121 121 00:05:39,130 --> 00:05:42,890 like "You fool," you can't divide something by zero. 122 122 00:05:42,890 --> 00:05:44,420 So we've got a function here. 123 123 00:05:44,420 --> 00:05:47,930 Then you can call this function divide with something 124 124 00:05:47,930 --> 00:05:50,230 like 15 and zero. 125 125 00:05:50,230 --> 00:05:52,130 So this may be okay. 126 126 00:05:52,130 --> 00:05:56,140 You can press play and you get back "you fool." 127 127 00:05:56,140 --> 00:06:00,730 However, it can be clearer if you tell the reader 128 128 00:06:00,730 --> 00:06:03,860 of this line which value is which. 129 129 00:06:03,860 --> 00:06:06,010 Now with something like division it may be obvious 130 130 00:06:06,010 --> 00:06:09,270 that 15 comma zero is going to divide 15 by zero. 131 131 00:06:09,270 --> 00:06:12,120 That seems reasonable to a reader I think, 132 132 00:06:12,120 --> 00:06:15,870 but, again, it can be cleared if you say dividend is 15, 133 133 00:06:15,870 --> 00:06:17,710 and divisor is zero. 134 134 00:06:17,710 --> 00:06:20,550 Notice that usually in Python we do not put spaces 135 135 00:06:20,550 --> 00:06:23,460 around this equal sign here when it's being used 136 136 00:06:23,460 --> 00:06:28,390 to separate a value from its keyword argument. 137 137 00:06:28,390 --> 00:06:30,610 As well as passing dividend and divisor like this, 138 138 00:06:30,610 --> 00:06:32,657 you could reverse them; put the divisor here 139 139 00:06:32,657 --> 00:06:34,300 and the dividend there and that would be fine, 140 140 00:06:34,300 --> 00:06:36,480 although maybe a little more bit confusing, 141 141 00:06:36,480 --> 00:06:40,420 but also not all arguments have to be keyword arguments. 142 142 00:06:40,420 --> 00:06:42,440 You can have some positional arguments 143 143 00:06:42,440 --> 00:06:46,230 and then some keyword arguments, but that's exactly it. 144 144 00:06:46,230 --> 00:06:48,480 The positional arguments have to go first 145 145 00:06:48,480 --> 00:06:50,770 and the keyword arguments have to go later. 146 146 00:06:50,770 --> 00:06:54,680 If you do dividend equal 15 and then you skip the keyword 147 147 00:06:54,680 --> 00:06:57,010 argument there, you're going to get an error and Python 148 148 00:06:57,010 --> 00:06:59,460 is going to tell you that a positional argument follows 149 149 00:06:59,460 --> 00:07:02,540 a keyword argument and that is illegal in Python. 150 150 00:07:02,540 --> 00:07:04,650 So the positional arguments have to go first, 151 151 00:07:04,650 --> 00:07:06,133 keyword arguments go later. 152 152 00:07:07,330 --> 00:07:10,410 I recommend that you use keyword arguments 153 153 00:07:10,410 --> 00:07:12,540 whenever possible just because you're going to make 154 154 00:07:12,540 --> 00:07:14,630 your code a little bit easier to read, 155 155 00:07:14,630 --> 00:07:17,300 unless it's really obvious what's going on. 156 156 00:07:17,300 --> 00:07:19,925 So for example, in a divide function, maybe you don't need 157 157 00:07:19,925 --> 00:07:21,780 the keyword arguments. 158 158 00:07:21,780 --> 00:07:24,220 Similarly, in a say hello function that just takes 159 159 00:07:24,220 --> 00:07:26,700 a person's name, you probably don't need keyword arguments 160 160 00:07:26,700 --> 00:07:29,860 either, but as soon as you start doing more complicated 161 161 00:07:29,860 --> 00:07:32,710 things or functions that are maybe not as obvious, 162 162 00:07:32,710 --> 00:07:34,490 using keyword arguments is really going to help. 163 163 00:07:34,490 --> 00:07:36,630 So I recommend that you use them. 164 164 00:07:36,630 --> 00:07:37,640 That's it for this video though. 165 165 00:07:37,640 --> 00:07:39,040 Thank you for joining me in this one, 166 166 00:07:39,040 --> 00:07:40,390 and I'll see you next time.