WEBVTT 1 00:00:00.790 --> 00:00:02.931 Hi guys and welcome back. 2 00:00:02.931 --> 00:00:03.930 In this video we're going to learn about 3 00:00:03.930 --> 00:00:05.870 return values and functions. 4 00:00:05.870 --> 00:00:07.340 And that is something really confusing 5 00:00:07.340 --> 00:00:09.165 for new Python developers. 6 00:00:09.165 --> 00:00:11.870 Here we've got a function called add 7 00:00:11.870 --> 00:00:13.990 that prints X and Y. 8 00:00:13.990 --> 00:00:16.310 If we call add with the values five and eight 9 00:00:16.310 --> 00:00:17.760 it should print 13. 10 00:00:17.760 --> 00:00:19.133 So let's run that and you'll see 11 00:00:19.133 --> 00:00:21.250 that 13 gets printed out. 12 00:00:21.250 --> 00:00:24.060 However, something that we have been doing 13 00:00:24.060 --> 00:00:25.950 with some other functions in our code 14 00:00:25.950 --> 00:00:29.450 has been assigning the value of it to a variable. 15 00:00:29.450 --> 00:00:32.460 For example result, equal, add, five and eight. 16 00:00:32.460 --> 00:00:36.400 If we then print result you could potentially 17 00:00:36.400 --> 00:00:39.680 expect 13 to be printed out. 18 00:00:39.680 --> 00:00:42.090 However, here's what's gonna happen. 19 00:00:42.090 --> 00:00:44.840 We're going to run line five which is going to 20 00:00:44.840 --> 00:00:48.410 start running the function and it's going to print 13. 21 00:00:48.410 --> 00:00:50.490 Then we're going to run line six 22 00:00:50.490 --> 00:00:52.790 which is again going to run the function, 23 00:00:52.790 --> 00:00:55.710 because here, notice there is a function call 24 00:00:55.710 --> 00:00:57.420 and that is going to run this line, 25 00:00:57.420 --> 00:00:59.270 which is going to print 13. 26 00:00:59.270 --> 00:01:04.180 Then we're going to assign something to the result variable, 27 00:01:04.180 --> 00:01:06.890 and finally we're going to print the result variable. 28 00:01:06.890 --> 00:01:11.890 So we should see 13, 13, and then the value 29 00:01:12.220 --> 00:01:13.810 of the result variable printed out. 30 00:01:13.810 --> 00:01:15.810 So we're going to print out three lines. 31 00:01:17.460 --> 00:01:22.320 And you can see that we get 13, 13, and then None. 32 00:01:22.320 --> 00:01:25.320 None is a special value in Python that means 33 00:01:25.320 --> 00:01:28.510 no value, or missing value, or undeclared value. 34 00:01:28.510 --> 00:01:29.730 Take your pick. 35 00:01:29.730 --> 00:01:32.920 So that means that this function here, 36 00:01:32.920 --> 00:01:36.360 when it finished running did something 37 00:01:36.360 --> 00:01:40.010 that made our result variable be equal to None, 38 00:01:40.010 --> 00:01:42.610 because when we printed result it printed None, 39 00:01:42.610 --> 00:01:44.450 therefore result must be None. 40 00:01:44.450 --> 00:01:48.250 So that is where return values come in. 41 00:01:48.250 --> 00:01:50.700 Functions return None. 42 00:01:50.700 --> 00:01:53.200 That's what they do by default. 43 00:01:53.200 --> 00:01:55.520 That means that whenever you call a function 44 00:01:55.520 --> 00:01:59.140 you can assign it's return value to a variable 45 00:01:59.140 --> 00:02:01.900 or you can use it in another part of your programme, 46 00:02:01.900 --> 00:02:05.142 such as a mathematical expression or something like that, 47 00:02:05.142 --> 00:02:07.190 and that return value is what you get 48 00:02:07.190 --> 00:02:08.470 when you call a function. 49 00:02:08.470 --> 00:02:10.530 So, functions return None by default 50 00:02:10.530 --> 00:02:13.044 which means that the result variable 51 00:02:13.044 --> 00:02:14.310 is being assigned to None. 52 00:02:14.310 --> 00:02:16.340 If we want to do it differently 53 00:02:16.340 --> 00:02:19.230 you're going to have to return your own value. 54 00:02:19.230 --> 00:02:21.880 So we're gonna say return X plus Y 55 00:02:21.880 --> 00:02:25.380 without the brackets instead of print. 56 00:02:25.380 --> 00:02:27.280 Now there's an important thing, 57 00:02:27.280 --> 00:02:30.050 which is that if we remove these lines- 58 00:02:30.050 --> 00:02:31.470 I'm just going to put the hash symbol 59 00:02:31.470 --> 00:02:34.610 in front of them which signals that they are a comment 60 00:02:34.610 --> 00:02:37.160 and they won't run when the code runs. 61 00:02:37.160 --> 00:02:39.260 If we press play now you'll see that 62 00:02:39.260 --> 00:02:41.560 nothing gets printed out, because of course 63 00:02:41.560 --> 00:02:44.090 we've executed the add function 64 00:02:44.090 --> 00:02:46.410 but this function does not print anything. 65 00:02:46.410 --> 00:02:48.550 It just returns something. 66 00:02:48.550 --> 00:02:50.720 So, because we're not printing anything 67 00:02:50.720 --> 00:02:52.310 nothing gets printed out. 68 00:02:52.310 --> 00:02:57.290 Now if we do this, we call the add function 69 00:02:57.290 --> 00:02:59.930 which returns the addition of the two arguments, 70 00:02:59.930 --> 00:03:02.360 five and eight, and then make it equal to 71 00:03:02.360 --> 00:03:03.483 the result variable. 72 00:03:04.317 --> 00:03:05.670 That means that result is going to be equal to 73 00:03:05.670 --> 00:03:08.580 the return value of the function which is gonna be 13. 74 00:03:08.580 --> 00:03:10.750 And then we're going to print 13. 75 00:03:10.750 --> 00:03:13.563 so when we run this code you'll see 13. 76 00:03:14.800 --> 00:03:17.380 At this point I'll say that very often 77 00:03:17.380 --> 00:03:19.270 I will see students doing this, 78 00:03:19.270 --> 00:03:24.270 print X and Y, and then they will try to do this. 79 00:03:25.520 --> 00:03:27.953 Print, add, five and eight. 80 00:03:29.200 --> 00:03:32.350 This is a very common thing I see in my course, 81 00:03:32.350 --> 00:03:35.240 and notice that when you run this code, 82 00:03:35.240 --> 00:03:37.940 you're going to get 13 and then None. 83 00:03:37.940 --> 00:03:39.870 Again, that's because the add function 84 00:03:39.870 --> 00:03:43.040 gets called first so that the print function 85 00:03:43.040 --> 00:03:44.490 knows what to print. 86 00:03:44.490 --> 00:03:48.010 So here it prints 13 and then returns None, 87 00:03:48.010 --> 00:03:50.900 because all functions return None by default. 88 00:03:50.900 --> 00:03:54.620 So you get 13 printed out and then you get None printed out. 89 00:03:54.620 --> 00:03:56.790 And this is very common, so if you're seeing this problem 90 00:03:56.790 --> 00:03:58.170 or you've encountered this before, 91 00:03:58.170 --> 00:03:59.473 now you know why that is. 92 00:04:01.260 --> 00:04:03.440 Let's go back to printing the result 93 00:04:03.440 --> 00:04:05.540 of calling the function and we're going 94 00:04:05.540 --> 00:04:08.590 to make it both print and return. 95 00:04:08.590 --> 00:04:10.350 So now when we do this you're going to 96 00:04:10.350 --> 00:04:13.100 see 13 printed out twice. 97 00:04:13.100 --> 00:04:16.480 One, because the function itself prints 13 out, 98 00:04:16.480 --> 00:04:18.070 and the second one because we're actually 99 00:04:18.070 --> 00:04:20.070 printing the return value of the function 100 00:04:20.070 --> 00:04:22.450 that we've assigned to our variable. 101 00:04:22.450 --> 00:04:25.140 Return not only gives a value back to 102 00:04:25.140 --> 00:04:26.780 the caller at the end of the function, 103 00:04:26.780 --> 00:04:29.440 it actually terminates the function entirely. 104 00:04:29.440 --> 00:04:31.810 So if we were to move this return value 105 00:04:31.810 --> 00:04:35.770 to before the print and then press play 106 00:04:35.770 --> 00:04:38.470 you'll see that only one 13 gets printed out, 107 00:04:38.470 --> 00:04:40.420 because as soon as the function reaches 108 00:04:41.864 --> 00:04:44.290 this return statement it returns and exits the function 109 00:04:44.290 --> 00:04:46.370 and gives the value back to the caller. 110 00:04:46.370 --> 00:04:50.230 So this print function never gets called. 111 00:04:50.230 --> 00:04:52.197 Only this one does. 112 00:04:52.197 --> 00:04:55.740 That is another common problem that 113 00:04:55.740 --> 00:04:58.090 some people will put a return value 114 00:04:58.090 --> 00:05:01.170 elsewhere in the function as well as 115 00:05:01.170 --> 00:05:02.640 the one at the end of the function. 116 00:05:02.640 --> 00:05:04.400 This is totally fine, you can do that. 117 00:05:04.400 --> 00:05:06.400 But just make sure that it is what you want. 118 00:05:06.400 --> 00:05:09.453 Because when you return without a value 119 00:05:09.453 --> 00:05:12.440 you're actually going to return None by default. 120 00:05:12.440 --> 00:05:14.590 So by putting a return here in line two 121 00:05:14.590 --> 00:05:16.870 it means that none of these lines are going to run. 122 00:05:16.870 --> 00:05:18.640 Because the function will terminate when you 123 00:05:18.640 --> 00:05:22.930 encounter a return and this here will become None 124 00:05:22.930 --> 00:05:25.570 because there is a return with no value 125 00:05:25.570 --> 00:05:27.330 which means return None. 126 00:05:27.330 --> 00:05:28.520 So our result will be None and 127 00:05:28.520 --> 00:05:30.090 you'll end up printing None out. 128 00:05:30.090 --> 00:05:32.853 If we press play you'll see that none comes out. 129 00:05:35.540 --> 00:05:38.310 Let's go back to our divide function from the last video, 130 00:05:38.310 --> 00:05:41.790 but now instead of printing out, we're going to return. 131 00:05:41.790 --> 00:05:43.170 So here we've got our divide function 132 00:05:43.170 --> 00:05:44.923 with a dividend and a divisor. 133 00:05:44.923 --> 00:05:46.350 If the divisor is not zero, 134 00:05:46.350 --> 00:05:47.920 we're going to return the division, 135 00:05:47.920 --> 00:05:50.660 otherwise we're going to return the string, you fool. 136 00:05:50.660 --> 00:05:53.920 So if we divide 15 by three and we print the result out 137 00:05:53.920 --> 00:05:55.800 we should see a number coming out 138 00:05:55.800 --> 00:05:57.910 and we do get 5.0. 139 00:05:57.910 --> 00:06:01.010 But if you decide to go for 15 and zero 140 00:06:01.010 --> 00:06:02.650 and then you press play, 141 00:06:02.650 --> 00:06:04.833 you'll see that you fool gets printed out. 142 00:06:05.680 --> 00:06:08.060 This is fine, you can totally do two returns 143 00:06:08.060 --> 00:06:10.920 in a single function as long as only one of them runs, 144 00:06:10.920 --> 00:06:12.610 which is what happens in this if statement. 145 00:06:12.610 --> 00:06:15.640 You can still access both returns depending 146 00:06:15.640 --> 00:06:17.510 on these conditions here. 147 00:06:17.510 --> 00:06:20.870 So again a return will terminate the function 148 00:06:20.870 --> 00:06:23.130 when the line is ran and give the value 149 00:06:23.130 --> 00:06:24.870 returned back to the caller. 150 00:06:24.870 --> 00:06:27.498 So in this case, this function call here 151 00:06:27.498 --> 00:06:29.560 will be replaced, essentially, 152 00:06:29.560 --> 00:06:32.900 by whatever the function returns when it runs. 153 00:06:32.900 --> 00:06:34.740 As an aside it's usually not recommended 154 00:06:34.740 --> 00:06:36.530 to have functions returning multiple 155 00:06:36.530 --> 00:06:38.240 different types of data. 156 00:06:38.240 --> 00:06:40.920 For example this function can return a number or a string. 157 00:06:40.920 --> 00:06:43.375 Just because if you wanted to perform mathematics 158 00:06:43.375 --> 00:06:45.870 on the return value of this function 159 00:06:45.870 --> 00:06:50.170 you will encounter problems if you do return a string. 160 00:06:50.170 --> 00:06:53.690 For example if you wanted to print out the result 161 00:06:53.690 --> 00:06:56.680 of dividing this multiplied by five, 162 00:06:56.680 --> 00:06:58.870 then of course you're going to get 163 00:06:58.870 --> 00:07:00.900 you fool, you fool, you fool, you fool. 164 00:07:00.900 --> 00:07:02.540 However if you change this number 165 00:07:02.540 --> 00:07:05.760 then you'll actually get a valid mathematical number. 166 00:07:05.760 --> 00:07:08.200 So, usually not recommended to return 167 00:07:08.200 --> 00:07:09.840 different types of data from a function. 168 00:07:09.840 --> 00:07:12.580 Although as I said, you can do it, 169 00:07:12.580 --> 00:07:15.280 it's just not advised. 170 00:07:15.280 --> 00:07:16.510 That's everything for this video. 171 00:07:16.510 --> 00:07:17.850 Thank you for joining me in this one, 172 00:07:17.850 --> 00:07:19.500 and I'll see you in the next one.