1 1 00:00:00,150 --> 00:00:01,830 Hi guys, and welcome back. 2 2 00:00:01,830 --> 00:00:03,510 In this video, we're going to learn about 3 3 00:00:03,510 --> 00:00:05,890 first-class functions. 4 4 00:00:05,890 --> 00:00:08,700 In first-class function, just means that functions 5 5 00:00:08,700 --> 00:00:09,920 are just variables. 6 6 00:00:09,920 --> 00:00:12,660 And you can pass them in as arguments to functions and use 7 7 00:00:12,660 --> 00:00:15,830 them in the same way you would use any other variable. 8 8 00:00:15,830 --> 00:00:16,730 Let's get started. 9 9 00:00:18,170 --> 00:00:21,460 Here we have a divide function that raises a zero division 10 10 00:00:21,460 --> 00:00:23,540 error if the divisor is zero. 11 11 00:00:23,540 --> 00:00:24,820 We've seen this before. 12 12 00:00:24,820 --> 00:00:28,390 And then we have a rather weird looking function. 13 13 00:00:28,390 --> 00:00:29,530 Calculate. 14 14 00:00:29,530 --> 00:00:33,480 That takes in any number of arguments and collects them 15 15 00:00:33,480 --> 00:00:35,370 into this value stopple. 16 16 00:00:35,370 --> 00:00:37,480 We've seen this before and if you can't remember how this 17 17 00:00:37,480 --> 00:00:41,640 works, please look at the unpacking arguments lecture. 18 18 00:00:41,640 --> 00:00:46,310 Then it also takes a mandatory key word argument parameter. 19 19 00:00:46,310 --> 00:00:48,130 So that is operator. 20 20 00:00:48,130 --> 00:00:53,130 And then what it does, is it calls the operator function 21 21 00:00:53,320 --> 00:00:55,680 with the values. 22 22 00:00:55,680 --> 00:00:57,310 That's kinda weird. 23 23 00:00:57,310 --> 00:01:00,470 How does that work? How does it know it is a function? 24 24 00:01:00,470 --> 00:01:02,740 Well, the simple answer is, it doesn't. 25 25 00:01:02,740 --> 00:01:05,660 However, because we've got these brackets after the 26 26 00:01:05,660 --> 00:01:09,840 variable name, this here must be a function, 27 27 00:01:09,840 --> 00:01:12,920 since that is the syntax for calling a function. 28 28 00:01:12,920 --> 00:01:16,160 No where calling this operator variable as a function 29 29 00:01:16,160 --> 00:01:20,330 with the values passed in as individual argument. 30 30 00:01:20,330 --> 00:01:23,430 How would we use this calculate function? 31 31 00:01:23,430 --> 00:01:27,300 You could do something like result equal calculate 32 32 00:01:27,300 --> 00:01:28,930 and then notice that calculate takes in 33 33 00:01:28,930 --> 00:01:30,810 any number of arguments. 34 34 00:01:30,810 --> 00:01:33,600 So we're gonna type twenty and four 35 35 00:01:33,600 --> 00:01:36,570 and then were gonna type operator equals 36 36 00:01:36,570 --> 00:01:38,900 the divide function. 37 37 00:01:38,900 --> 00:01:43,120 But this will pass in the divide function 38 38 00:01:43,120 --> 00:01:44,940 as the operator 39 39 00:01:44,940 --> 00:01:48,910 and then we will only call it when we reach line nine. 40 40 00:01:48,910 --> 00:01:52,250 Notice that this divide function is not called here, 41 41 00:01:52,250 --> 00:01:54,470 it is only used as a value. 42 42 00:01:54,470 --> 00:01:56,360 If you wanted to call this function, 43 43 00:01:56,360 --> 00:01:58,160 you always have to put the brackets at the end. 44 44 00:01:58,160 --> 00:02:00,440 That's how you call a function in Python. 45 45 00:02:00,440 --> 00:02:04,970 So by not putting the brackets, you are passing in the value 46 46 00:02:04,970 --> 00:02:08,800 to which this name divide refers to, which is the function, 47 47 00:02:08,800 --> 00:02:11,200 as the value for this parameter. 48 48 00:02:11,200 --> 00:02:13,820 So the operator parameter now has the same value 49 49 00:02:13,820 --> 00:02:15,340 as the divide variable. 50 50 00:02:15,340 --> 00:02:17,740 They are both this function here. 51 51 00:02:17,740 --> 00:02:20,960 So you can call it as a function, passing in the values 52 52 00:02:20,960 --> 00:02:24,130 as individual values so that will give you twenty for the 53 53 00:02:24,130 --> 00:02:27,150 dividend and four for the divisor. 54 54 00:02:27,150 --> 00:02:30,380 This here is an example of using a first-class function 55 55 00:02:30,380 --> 00:02:33,160 because you can see that you can pass it in as an argument 56 56 00:02:33,160 --> 00:02:34,423 to another function. 57 57 00:02:35,500 --> 00:02:38,630 Functions in Python are just variables that happen to be 58 58 00:02:38,630 --> 00:02:42,290 callable. You can call them with the brackets at the end. 59 59 00:02:42,290 --> 00:02:44,170 But they're no different than any other value 60 60 00:02:44,170 --> 00:02:45,043 other than that. 61 61 00:02:46,540 --> 00:02:48,860 So now you can print the result if you like, 62 62 00:02:48,860 --> 00:02:51,410 and you'll see that that has [inaudible] called 63 63 00:02:51,410 --> 00:02:53,433 the divide function. And you get 5.0. 64 64 00:02:54,400 --> 00:02:56,700 If you change this to a zero though, 65 65 00:02:56,700 --> 00:03:00,280 you will end up with trace back because you tried to 66 66 00:03:00,280 --> 00:03:01,720 divide by zero. 67 67 00:03:01,720 --> 00:03:04,210 Something important that will be a little bit of problem in 68 68 00:03:04,210 --> 00:03:06,710 this code is that the calculate function 69 69 00:03:06,710 --> 00:03:08,670 takes in any number of values. 70 70 00:03:08,670 --> 00:03:11,490 But this operator expects two values. 71 71 00:03:11,490 --> 00:03:14,580 So if you put anymore, then you will also get an error. 72 72 00:03:14,580 --> 00:03:16,640 Because now you're gonna pass in too 73 73 00:03:16,640 --> 00:03:17,960 many positional arguments. 74 74 00:03:17,960 --> 00:03:20,290 See divide takes two positional arguments 75 75 00:03:20,290 --> 00:03:21,850 but three were given. 76 76 00:03:21,850 --> 00:03:24,400 So that is a small problem with this code here. 77 77 00:03:24,400 --> 00:03:26,340 But anyway the point of it was to show you 78 78 00:03:26,340 --> 00:03:27,950 how first-class functions work. 79 79 00:03:27,950 --> 00:03:30,060 Let's do another example. 80 80 00:03:30,060 --> 00:03:33,430 I'm going to define a search function that takes in 81 81 00:03:33,430 --> 00:03:35,943 a sequence of things to search through. 82 82 00:03:37,290 --> 00:03:40,880 It will also take what you expect to find 83 83 00:03:40,880 --> 00:03:44,460 and finally it will also take a function that will be used 84 84 00:03:44,460 --> 00:03:47,450 to extract information from each item in the sequence, 85 85 00:03:47,450 --> 00:03:49,400 match it against the expected value, 86 86 00:03:49,400 --> 00:03:51,313 and see if we find what we wanted. 87 87 00:03:52,270 --> 00:03:55,700 So we will iterate over the sequence, 88 88 00:03:55,700 --> 00:04:00,700 then we will run the finder function on the element 89 89 00:04:01,230 --> 00:04:04,210 and see if it is equal to the expected value. 90 90 00:04:04,210 --> 00:04:06,350 And if it is, we will return the element 91 91 00:04:06,350 --> 00:04:08,833 otherwise we will raise a runtime error, 92 92 00:04:10,000 --> 00:04:11,740 saying something like, 93 93 00:04:11,740 --> 00:04:15,383 could not find an element with expected. 94 94 00:04:17,040 --> 00:04:20,040 So this is a little bit of a confusing function there 95 95 00:04:20,040 --> 00:04:22,120 because it uses some parameters together with 96 96 00:04:22,120 --> 00:04:24,460 other parameters to find a third parameter. 97 97 00:04:24,460 --> 00:04:27,890 So a little bit weird but lets say now we have a list of 98 98 00:04:27,890 --> 00:04:32,890 friends and we want to find out which friend is called 99 99 00:04:33,080 --> 00:04:36,170 Bob Smith. Which here doesn't doesn't exist. 100 100 00:04:36,170 --> 00:04:39,890 The first thing we have to do is we have define a function 101 101 00:04:39,890 --> 00:04:42,980 that will run on one of these dictionary's, 102 102 00:04:42,980 --> 00:04:45,110 and give us back the name. 103 103 00:04:45,110 --> 00:04:47,620 So we will say, get friend name, 104 104 00:04:47,620 --> 00:04:50,630 and we will make it take one parameter 105 105 00:04:50,630 --> 00:04:53,690 and we'll say return friend, name. 106 106 00:04:53,690 --> 00:04:57,690 So here we have a function that, if we run it on an element 107 107 00:04:57,690 --> 00:05:01,330 in this sequence, we give us back the name property 108 108 00:05:01,330 --> 00:05:02,370 of that element. 109 109 00:05:02,370 --> 00:05:06,200 Be that what it may, remember this may fail if the element 110 110 00:05:06,200 --> 00:05:08,090 doesn't have a name property. 111 111 00:05:08,090 --> 00:05:11,670 Then we're going to print out the search, 112 112 00:05:11,670 --> 00:05:13,480 which is this function up here. 113 113 00:05:13,480 --> 00:05:16,120 We're gonna pass in the friends as our sequence 114 114 00:05:16,120 --> 00:05:18,790 so this search function is going to iterate 115 115 00:05:18,790 --> 00:05:20,223 over our friends. 116 116 00:05:21,210 --> 00:05:25,130 Then we're going to look for, Bob Smith. 117 117 00:05:25,130 --> 00:05:28,300 And finally how we're going to extract that value 118 118 00:05:28,300 --> 00:05:31,080 from each element is going to be by the 119 119 00:05:31,080 --> 00:05:33,663 get friend name function. 120 120 00:05:34,620 --> 00:05:37,390 So just to recap, get friend name 121 121 00:05:37,390 --> 00:05:39,260 will be the finder function. 122 122 00:05:39,260 --> 00:05:42,100 So that will run on each element. 123 123 00:05:42,100 --> 00:05:44,440 For example, for the first one, it will run on this 124 124 00:05:44,440 --> 00:05:47,290 dictionary here, and it will give you back the name 125 125 00:05:47,290 --> 00:05:51,123 property of that value. So you'll get back Rolf Smith. 126 126 00:05:52,010 --> 00:05:54,920 The Rolf Smith will be here, and you'll compare it with 127 127 00:05:54,920 --> 00:05:57,820 the expected value, which is Bob Smith. 128 128 00:05:57,820 --> 00:06:00,170 If they match, you will return that element, 129 129 00:06:00,170 --> 00:06:01,550 which is this dictionary. 130 130 00:06:01,550 --> 00:06:03,670 The same thing will happen for all the other ones. 131 131 00:06:03,670 --> 00:06:06,180 At this point, we don't have our friend called Bob Smith. 132 132 00:06:06,180 --> 00:06:08,830 So by running this we will get the run-time error. 133 133 00:06:08,830 --> 00:06:11,340 Could not find an element with expected. 134 134 00:06:11,340 --> 00:06:13,883 However, if we change this to Rolf Smith, 135 135 00:06:15,240 --> 00:06:17,090 then you will get it to work. 136 136 00:06:17,090 --> 00:06:20,770 The name is Rolf Smith and the age is twenty-four. 137 137 00:06:20,770 --> 00:06:23,400 This is quite a confusing bit of code but is a prime 138 138 00:06:23,400 --> 00:06:27,320 example of how first-class functions can be useful. 139 139 00:06:27,320 --> 00:06:30,050 So I would recommend giving it a go, playing around with it, 140 140 00:06:30,050 --> 00:06:32,490 see if it makes sense. And if it doesn't please ask a 141 141 00:06:32,490 --> 00:06:33,950 question in the course Q and A. 142 142 00:06:33,950 --> 00:06:36,120 Of course we're always happy to help. 143 143 00:06:36,120 --> 00:06:38,600 This is also a great place to use a lambda function. 144 144 00:06:38,600 --> 00:06:41,990 So instead of using that function, you can just define 145 145 00:06:41,990 --> 00:06:43,173 something like this, 146 146 00:06:44,190 --> 00:06:46,720 and just created it there where it is used. 147 147 00:06:46,720 --> 00:06:49,740 This is an alternative. No one is better than the other. 148 148 00:06:49,740 --> 00:06:51,820 You can do what you prefer. The other one is slightly longer 149 149 00:06:51,820 --> 00:06:53,660 but it does have a nicer name. 150 150 00:06:53,660 --> 00:06:55,080 Because you get the function name and it tells 151 151 00:06:55,080 --> 00:06:55,970 you what it does. 152 152 00:06:55,970 --> 00:06:57,520 So that's a good thing as well. 153 153 00:06:59,700 --> 00:07:01,500 Finally, you can use neither of these, 154 154 00:07:01,500 --> 00:07:04,640 do you can not define your own function or use a 155 155 00:07:04,640 --> 00:07:05,473 lambda function. 156 156 00:07:05,473 --> 00:07:09,130 You can just use a built-in from operator, 157 157 00:07:09,130 --> 00:07:11,030 import item getter. 158 158 00:07:11,030 --> 00:07:14,510 And here instead of this lambda function that is used to get 159 159 00:07:14,510 --> 00:07:17,395 a property of a object, 160 160 00:07:17,395 --> 00:07:19,950 you can use item getter 161 161 00:07:19,950 --> 00:07:20,980 with name. 162 162 00:07:20,980 --> 00:07:22,360 And that does exactly the same thing. 163 163 00:07:22,360 --> 00:07:26,220 This here, is a function that creates a function and 164 164 00:07:26,220 --> 00:07:27,570 lets you use it afterwards. 165 165 00:07:27,570 --> 00:07:29,730 So this is a little bit more advanced. We're going to learn 166 166 00:07:29,730 --> 00:07:32,923 more about functions that create other functions later on. 167 167 00:07:34,242 --> 00:07:36,673 So there you have it, that is the same thing. 168 168 00:07:39,010 --> 00:07:41,390 I set aside the operator built-in module, 169 169 00:07:41,390 --> 00:07:42,360 in fact quite a few of 170 170 00:07:42,360 --> 00:07:45,683 the Python built-in modules are amazing and really useful. 171 171 00:07:46,770 --> 00:07:48,450 All right, that's everything for this video. 172 172 00:07:48,450 --> 00:07:50,500 I hope you've enjoyed it, I hope you've learned something, 173 173 00:07:50,500 --> 00:07:52,150 and I'll see you in the next one.