WEBVTT 1 00:00:00.150 --> 00:00:01.830 Hi guys, and welcome back. 2 00:00:01.830 --> 00:00:03.510 In this video, we're going to learn about 3 00:00:03.510 --> 00:00:05.890 first-class functions. 4 00:00:05.890 --> 00:00:08.700 In first-class function, just means that functions 5 00:00:08.700 --> 00:00:09.920 are just variables. 6 00:00:09.920 --> 00:00:12.660 And you can pass them in as arguments to functions and use 7 00:00:12.660 --> 00:00:15.830 them in the same way you would use any other variable. 8 00:00:15.830 --> 00:00:16.730 Let's get started. 9 00:00:18.170 --> 00:00:21.460 Here we have a divide function that raises a zero division 10 00:00:21.460 --> 00:00:23.540 error if the divisor is zero. 11 00:00:23.540 --> 00:00:24.820 We've seen this before. 12 00:00:24.820 --> 00:00:28.390 And then we have a rather weird looking function. 13 00:00:28.390 --> 00:00:29.530 Calculate. 14 00:00:29.530 --> 00:00:33.480 That takes in any number of arguments and collects them 15 00:00:33.480 --> 00:00:35.370 into this value stopple. 16 00:00:35.370 --> 00:00:37.480 We've seen this before and if you can't remember how this 17 00:00:37.480 --> 00:00:41.640 works, please look at the unpacking arguments lecture. 18 00:00:41.640 --> 00:00:46.310 Then it also takes a mandatory key word argument parameter. 19 00:00:46.310 --> 00:00:48.130 So that is operator. 20 00:00:48.130 --> 00:00:53.130 And then what it does, is it calls the operator function 21 00:00:53.320 --> 00:00:55.680 with the values. 22 00:00:55.680 --> 00:00:57.310 That's kinda weird. 23 00:00:57.310 --> 00:01:00.470 How does that work? How does it know it is a function? 24 00:01:00.470 --> 00:01:02.740 Well, the simple answer is, it doesn't. 25 00:01:02.740 --> 00:01:05.660 However, because we've got these brackets after the 26 00:01:05.660 --> 00:01:09.840 variable name, this here must be a function, 27 00:01:09.840 --> 00:01:12.920 since that is the syntax for calling a function. 28 00:01:12.920 --> 00:01:16.160 No where calling this operator variable as a function 29 00:01:16.160 --> 00:01:20.330 with the values passed in as individual argument. 30 00:01:20.330 --> 00:01:23.430 How would we use this calculate function? 31 00:01:23.430 --> 00:01:27.300 You could do something like result equal calculate 32 00:01:27.300 --> 00:01:28.930 and then notice that calculate takes in 33 00:01:28.930 --> 00:01:30.810 any number of arguments. 34 00:01:30.810 --> 00:01:33.600 So we're gonna type twenty and four 35 00:01:33.600 --> 00:01:36.570 and then were gonna type operator equals 36 00:01:36.570 --> 00:01:38.900 the divide function. 37 00:01:38.900 --> 00:01:43.120 But this will pass in the divide function 38 00:01:43.120 --> 00:01:44.940 as the operator 39 00:01:44.940 --> 00:01:48.910 and then we will only call it when we reach line nine. 40 00:01:48.910 --> 00:01:52.250 Notice that this divide function is not called here, 41 00:01:52.250 --> 00:01:54.470 it is only used as a value. 42 00:01:54.470 --> 00:01:56.360 If you wanted to call this function, 43 00:01:56.360 --> 00:01:58.160 you always have to put the brackets at the end. 44 00:01:58.160 --> 00:02:00.440 That's how you call a function in Python. 45 00:02:00.440 --> 00:02:04.970 So by not putting the brackets, you are passing in the value 46 00:02:04.970 --> 00:02:08.800 to which this name divide refers to, which is the function, 47 00:02:08.800 --> 00:02:11.200 as the value for this parameter. 48 00:02:11.200 --> 00:02:13.820 So the operator parameter now has the same value 49 00:02:13.820 --> 00:02:15.340 as the divide variable. 50 00:02:15.340 --> 00:02:17.740 They are both this function here. 51 00:02:17.740 --> 00:02:20.960 So you can call it as a function, passing in the values 52 00:02:20.960 --> 00:02:24.130 as individual values so that will give you twenty for the 53 00:02:24.130 --> 00:02:27.150 dividend and four for the divisor. 54 00:02:27.150 --> 00:02:30.380 This here is an example of using a first-class function 55 00:02:30.380 --> 00:02:33.160 because you can see that you can pass it in as an argument 56 00:02:33.160 --> 00:02:34.423 to another function. 57 00:02:35.500 --> 00:02:38.630 Functions in Python are just variables that happen to be 58 00:02:38.630 --> 00:02:42.290 callable. You can call them with the brackets at the end. 59 00:02:42.290 --> 00:02:44.170 But they're no different than any other value 60 00:02:44.170 --> 00:02:45.043 other than that. 61 00:02:46.540 --> 00:02:48.860 So now you can print the result if you like, 62 00:02:48.860 --> 00:02:51.410 and you'll see that that has [inaudible] called 63 00:02:51.410 --> 00:02:53.433 the divide function. And you get 5.0. 64 00:02:54.400 --> 00:02:56.700 If you change this to a zero though, 65 00:02:56.700 --> 00:03:00.280 you will end up with trace back because you tried to 66 00:03:00.280 --> 00:03:01.720 divide by zero. 67 00:03:01.720 --> 00:03:04.210 Something important that will be a little bit of problem in 68 00:03:04.210 --> 00:03:06.710 this code is that the calculate function 69 00:03:06.710 --> 00:03:08.670 takes in any number of values. 70 00:03:08.670 --> 00:03:11.490 But this operator expects two values. 71 00:03:11.490 --> 00:03:14.580 So if you put anymore, then you will also get an error. 72 00:03:14.580 --> 00:03:16.640 Because now you're gonna pass in too 73 00:03:16.640 --> 00:03:17.960 many positional arguments. 74 00:03:17.960 --> 00:03:20.290 See divide takes two positional arguments 75 00:03:20.290 --> 00:03:21.850 but three were given. 76 00:03:21.850 --> 00:03:24.400 So that is a small problem with this code here. 77 00:03:24.400 --> 00:03:26.340 But anyway the point of it was to show you 78 00:03:26.340 --> 00:03:27.950 how first-class functions work. 79 00:03:27.950 --> 00:03:30.060 Let's do another example. 80 00:03:30.060 --> 00:03:33.430 I'm going to define a search function that takes in 81 00:03:33.430 --> 00:03:35.943 a sequence of things to search through. 82 00:03:37.290 --> 00:03:40.880 It will also take what you expect to find 83 00:03:40.880 --> 00:03:44.460 and finally it will also take a function that will be used 84 00:03:44.460 --> 00:03:47.450 to extract information from each item in the sequence, 85 00:03:47.450 --> 00:03:49.400 match it against the expected value, 86 00:03:49.400 --> 00:03:51.313 and see if we find what we wanted. 87 00:03:52.270 --> 00:03:55.700 So we will iterate over the sequence, 88 00:03:55.700 --> 00:04:00.700 then we will run the finder function on the element 89 00:04:01.230 --> 00:04:04.210 and see if it is equal to the expected value. 90 00:04:04.210 --> 00:04:06.350 And if it is, we will return the element 91 00:04:06.350 --> 00:04:08.833 otherwise we will raise a runtime error, 92 00:04:10.000 --> 00:04:11.740 saying something like, 93 00:04:11.740 --> 00:04:15.383 could not find an element with expected. 94 00:04:17.040 --> 00:04:20.040 So this is a little bit of a confusing function there 95 00:04:20.040 --> 00:04:22.120 because it uses some parameters together with 96 00:04:22.120 --> 00:04:24.460 other parameters to find a third parameter. 97 00:04:24.460 --> 00:04:27.890 So a little bit weird but lets say now we have a list of 98 00:04:27.890 --> 00:04:32.890 friends and we want to find out which friend is called 99 00:04:33.080 --> 00:04:36.170 Bob Smith. Which here doesn't doesn't exist. 100 00:04:36.170 --> 00:04:39.890 The first thing we have to do is we have define a function 101 00:04:39.890 --> 00:04:42.980 that will run on one of these dictionary's, 102 00:04:42.980 --> 00:04:45.110 and give us back the name. 103 00:04:45.110 --> 00:04:47.620 So we will say, get friend name, 104 00:04:47.620 --> 00:04:50.630 and we will make it take one parameter 105 00:04:50.630 --> 00:04:53.690 and we'll say return friend, name. 106 00:04:53.690 --> 00:04:57.690 So here we have a function that, if we run it on an element 107 00:04:57.690 --> 00:05:01.330 in this sequence, we give us back the name property 108 00:05:01.330 --> 00:05:02.370 of that element. 109 00:05:02.370 --> 00:05:06.200 Be that what it may, remember this may fail if the element 110 00:05:06.200 --> 00:05:08.090 doesn't have a name property. 111 00:05:08.090 --> 00:05:11.670 Then we're going to print out the search, 112 00:05:11.670 --> 00:05:13.480 which is this function up here. 113 00:05:13.480 --> 00:05:16.120 We're gonna pass in the friends as our sequence 114 00:05:16.120 --> 00:05:18.790 so this search function is going to iterate 115 00:05:18.790 --> 00:05:20.223 over our friends. 116 00:05:21.210 --> 00:05:25.130 Then we're going to look for, Bob Smith. 117 00:05:25.130 --> 00:05:28.300 And finally how we're going to extract that value 118 00:05:28.300 --> 00:05:31.080 from each element is going to be by the 119 00:05:31.080 --> 00:05:33.663 get friend name function. 120 00:05:34.620 --> 00:05:37.390 So just to recap, get friend name 121 00:05:37.390 --> 00:05:39.260 will be the finder function. 122 00:05:39.260 --> 00:05:42.100 So that will run on each element. 123 00:05:42.100 --> 00:05:44.440 For example, for the first one, it will run on this 124 00:05:44.440 --> 00:05:47.290 dictionary here, and it will give you back the name 125 00:05:47.290 --> 00:05:51.123 property of that value. So you'll get back Rolf Smith. 126 00:05:52.010 --> 00:05:54.920 The Rolf Smith will be here, and you'll compare it with 127 00:05:54.920 --> 00:05:57.820 the expected value, which is Bob Smith. 128 00:05:57.820 --> 00:06:00.170 If they match, you will return that element, 129 00:06:00.170 --> 00:06:01.550 which is this dictionary. 130 00:06:01.550 --> 00:06:03.670 The same thing will happen for all the other ones. 131 00:06:03.670 --> 00:06:06.180 At this point, we don't have our friend called Bob Smith. 132 00:06:06.180 --> 00:06:08.830 So by running this we will get the run-time error. 133 00:06:08.830 --> 00:06:11.340 Could not find an element with expected. 134 00:06:11.340 --> 00:06:13.883 However, if we change this to Rolf Smith, 135 00:06:15.240 --> 00:06:17.090 then you will get it to work. 136 00:06:17.090 --> 00:06:20.770 The name is Rolf Smith and the age is twenty-four. 137 00:06:20.770 --> 00:06:23.400 This is quite a confusing bit of code but is a prime 138 00:06:23.400 --> 00:06:27.320 example of how first-class functions can be useful. 139 00:06:27.320 --> 00:06:30.050 So I would recommend giving it a go, playing around with it, 140 00:06:30.050 --> 00:06:32.490 see if it makes sense. And if it doesn't please ask a 141 00:06:32.490 --> 00:06:33.950 question in the course Q and A. 142 00:06:33.950 --> 00:06:36.120 Of course we're always happy to help. 143 00:06:36.120 --> 00:06:38.600 This is also a great place to use a lambda function. 144 00:06:38.600 --> 00:06:41.990 So instead of using that function, you can just define 145 00:06:41.990 --> 00:06:43.173 something like this, 146 00:06:44.190 --> 00:06:46.720 and just created it there where it is used. 147 00:06:46.720 --> 00:06:49.740 This is an alternative. No one is better than the other. 148 00:06:49.740 --> 00:06:51.820 You can do what you prefer. The other one is slightly longer 149 00:06:51.820 --> 00:06:53.660 but it does have a nicer name. 150 00:06:53.660 --> 00:06:55.080 Because you get the function name and it tells 151 00:06:55.080 --> 00:06:55.970 you what it does. 152 00:06:55.970 --> 00:06:57.520 So that's a good thing as well. 153 00:06:59.700 --> 00:07:01.500 Finally, you can use neither of these, 154 00:07:01.500 --> 00:07:04.640 do you can not define your own function or use a 155 00:07:04.640 --> 00:07:05.473 lambda function. 156 00:07:05.473 --> 00:07:09.130 You can just use a built-in from operator, 157 00:07:09.130 --> 00:07:11.030 import item getter. 158 00:07:11.030 --> 00:07:14.510 And here instead of this lambda function that is used to get 159 00:07:14.510 --> 00:07:17.395 a property of a object, 160 00:07:17.395 --> 00:07:19.950 you can use item getter 161 00:07:19.950 --> 00:07:20.980 with name. 162 00:07:20.980 --> 00:07:22.360 And that does exactly the same thing. 163 00:07:22.360 --> 00:07:26.220 This here, is a function that creates a function and 164 00:07:26.220 --> 00:07:27.570 lets you use it afterwards. 165 00:07:27.570 --> 00:07:29.730 So this is a little bit more advanced. We're going to learn 166 00:07:29.730 --> 00:07:32.923 more about functions that create other functions later on. 167 00:07:34.242 --> 00:07:36.673 So there you have it, that is the same thing. 168 00:07:39.010 --> 00:07:41.390 I set aside the operator built-in module, 169 00:07:41.390 --> 00:07:42.360 in fact quite a few of 170 00:07:42.360 --> 00:07:45.683 the Python built-in modules are amazing and really useful. 171 00:07:46.770 --> 00:07:48.450 All right, that's everything for this video. 172 00:07:48.450 --> 00:07:50.500 I hope you've enjoyed it, I hope you've learned something, 173 00:07:50.500 --> 00:07:52.150 and I'll see you in the next one.