1 1 00:00:00,790 --> 00:00:02,931 Hi guys and welcome back. 2 2 00:00:02,931 --> 00:00:03,930 In this video we're going to learn about 3 3 00:00:03,930 --> 00:00:05,870 return values and functions. 4 4 00:00:05,870 --> 00:00:07,340 And that is something really confusing 5 5 00:00:07,340 --> 00:00:09,165 for new Python developers. 6 6 00:00:09,165 --> 00:00:11,870 Here we've got a function called add 7 7 00:00:11,870 --> 00:00:13,990 that prints X and Y. 8 8 00:00:13,990 --> 00:00:16,310 If we call add with the values five and eight 9 9 00:00:16,310 --> 00:00:17,760 it should print 13. 10 10 00:00:17,760 --> 00:00:19,133 So let's run that and you'll see 11 11 00:00:19,133 --> 00:00:21,250 that 13 gets printed out. 12 12 00:00:21,250 --> 00:00:24,060 However, something that we have been doing 13 13 00:00:24,060 --> 00:00:25,950 with some other functions in our code 14 14 00:00:25,950 --> 00:00:29,450 has been assigning the value of it to a variable. 15 15 00:00:29,450 --> 00:00:32,460 For example result, equal, add, five and eight. 16 16 00:00:32,460 --> 00:00:36,400 If we then print result you could potentially 17 17 00:00:36,400 --> 00:00:39,680 expect 13 to be printed out. 18 18 00:00:39,680 --> 00:00:42,090 However, here's what's gonna happen. 19 19 00:00:42,090 --> 00:00:44,840 We're going to run line five which is going to 20 20 00:00:44,840 --> 00:00:48,410 start running the function and it's going to print 13. 21 21 00:00:48,410 --> 00:00:50,490 Then we're going to run line six 22 22 00:00:50,490 --> 00:00:52,790 which is again going to run the function, 23 23 00:00:52,790 --> 00:00:55,710 because here, notice there is a function call 24 24 00:00:55,710 --> 00:00:57,420 and that is going to run this line, 25 25 00:00:57,420 --> 00:00:59,270 which is going to print 13. 26 26 00:00:59,270 --> 00:01:04,180 Then we're going to assign something to the result variable, 27 27 00:01:04,180 --> 00:01:06,890 and finally we're going to print the result variable. 28 28 00:01:06,890 --> 00:01:11,890 So we should see 13, 13, and then the value 29 29 00:01:12,220 --> 00:01:13,810 of the result variable printed out. 30 30 00:01:13,810 --> 00:01:15,810 So we're going to print out three lines. 31 31 00:01:17,460 --> 00:01:22,320 And you can see that we get 13, 13, and then None. 32 32 00:01:22,320 --> 00:01:25,320 None is a special value in Python that means 33 33 00:01:25,320 --> 00:01:28,510 no value, or missing value, or undeclared value. 34 34 00:01:28,510 --> 00:01:29,730 Take your pick. 35 35 00:01:29,730 --> 00:01:32,920 So that means that this function here, 36 36 00:01:32,920 --> 00:01:36,360 when it finished running did something 37 37 00:01:36,360 --> 00:01:40,010 that made our result variable be equal to None, 38 38 00:01:40,010 --> 00:01:42,610 because when we printed result it printed None, 39 39 00:01:42,610 --> 00:01:44,450 therefore result must be None. 40 40 00:01:44,450 --> 00:01:48,250 So that is where return values come in. 41 41 00:01:48,250 --> 00:01:50,700 Functions return None. 42 42 00:01:50,700 --> 00:01:53,200 That's what they do by default. 43 43 00:01:53,200 --> 00:01:55,520 That means that whenever you call a function 44 44 00:01:55,520 --> 00:01:59,140 you can assign it's return value to a variable 45 45 00:01:59,140 --> 00:02:01,900 or you can use it in another part of your programme, 46 46 00:02:01,900 --> 00:02:05,142 such as a mathematical expression or something like that, 47 47 00:02:05,142 --> 00:02:07,190 and that return value is what you get 48 48 00:02:07,190 --> 00:02:08,470 when you call a function. 49 49 00:02:08,470 --> 00:02:10,530 So, functions return None by default 50 50 00:02:10,530 --> 00:02:13,044 which means that the result variable 51 51 00:02:13,044 --> 00:02:14,310 is being assigned to None. 52 52 00:02:14,310 --> 00:02:16,340 If we want to do it differently 53 53 00:02:16,340 --> 00:02:19,230 you're going to have to return your own value. 54 54 00:02:19,230 --> 00:02:21,880 So we're gonna say return X plus Y 55 55 00:02:21,880 --> 00:02:25,380 without the brackets instead of print. 56 56 00:02:25,380 --> 00:02:27,280 Now there's an important thing, 57 57 00:02:27,280 --> 00:02:30,050 which is that if we remove these lines- 58 58 00:02:30,050 --> 00:02:31,470 I'm just going to put the hash symbol 59 59 00:02:31,470 --> 00:02:34,610 in front of them which signals that they are a comment 60 60 00:02:34,610 --> 00:02:37,160 and they won't run when the code runs. 61 61 00:02:37,160 --> 00:02:39,260 If we press play now you'll see that 62 62 00:02:39,260 --> 00:02:41,560 nothing gets printed out, because of course 63 63 00:02:41,560 --> 00:02:44,090 we've executed the add function 64 64 00:02:44,090 --> 00:02:46,410 but this function does not print anything. 65 65 00:02:46,410 --> 00:02:48,550 It just returns something. 66 66 00:02:48,550 --> 00:02:50,720 So, because we're not printing anything 67 67 00:02:50,720 --> 00:02:52,310 nothing gets printed out. 68 68 00:02:52,310 --> 00:02:57,290 Now if we do this, we call the add function 69 69 00:02:57,290 --> 00:02:59,930 which returns the addition of the two arguments, 70 70 00:02:59,930 --> 00:03:02,360 five and eight, and then make it equal to 71 71 00:03:02,360 --> 00:03:03,483 the result variable. 72 72 00:03:04,317 --> 00:03:05,670 That means that result is going to be equal to 73 73 00:03:05,670 --> 00:03:08,580 the return value of the function which is gonna be 13. 74 74 00:03:08,580 --> 00:03:10,750 And then we're going to print 13. 75 75 00:03:10,750 --> 00:03:13,563 so when we run this code you'll see 13. 76 76 00:03:14,800 --> 00:03:17,380 At this point I'll say that very often 77 77 00:03:17,380 --> 00:03:19,270 I will see students doing this, 78 78 00:03:19,270 --> 00:03:24,270 print X and Y, and then they will try to do this. 79 79 00:03:25,520 --> 00:03:27,953 Print, add, five and eight. 80 80 00:03:29,200 --> 00:03:32,350 This is a very common thing I see in my course, 81 81 00:03:32,350 --> 00:03:35,240 and notice that when you run this code, 82 82 00:03:35,240 --> 00:03:37,940 you're going to get 13 and then None. 83 83 00:03:37,940 --> 00:03:39,870 Again, that's because the add function 84 84 00:03:39,870 --> 00:03:43,040 gets called first so that the print function 85 85 00:03:43,040 --> 00:03:44,490 knows what to print. 86 86 00:03:44,490 --> 00:03:48,010 So here it prints 13 and then returns None, 87 87 00:03:48,010 --> 00:03:50,900 because all functions return None by default. 88 88 00:03:50,900 --> 00:03:54,620 So you get 13 printed out and then you get None printed out. 89 89 00:03:54,620 --> 00:03:56,790 And this is very common, so if you're seeing this problem 90 90 00:03:56,790 --> 00:03:58,170 or you've encountered this before, 91 91 00:03:58,170 --> 00:03:59,473 now you know why that is. 92 92 00:04:01,260 --> 00:04:03,440 Let's go back to printing the result 93 93 00:04:03,440 --> 00:04:05,540 of calling the function and we're going 94 94 00:04:05,540 --> 00:04:08,590 to make it both print and return. 95 95 00:04:08,590 --> 00:04:10,350 So now when we do this you're going to 96 96 00:04:10,350 --> 00:04:13,100 see 13 printed out twice. 97 97 00:04:13,100 --> 00:04:16,480 One, because the function itself prints 13 out, 98 98 00:04:16,480 --> 00:04:18,070 and the second one because we're actually 99 99 00:04:18,070 --> 00:04:20,070 printing the return value of the function 100 100 00:04:20,070 --> 00:04:22,450 that we've assigned to our variable. 101 101 00:04:22,450 --> 00:04:25,140 Return not only gives a value back to 102 102 00:04:25,140 --> 00:04:26,780 the caller at the end of the function, 103 103 00:04:26,780 --> 00:04:29,440 it actually terminates the function entirely. 104 104 00:04:29,440 --> 00:04:31,810 So if we were to move this return value 105 105 00:04:31,810 --> 00:04:35,770 to before the print and then press play 106 106 00:04:35,770 --> 00:04:38,470 you'll see that only one 13 gets printed out, 107 107 00:04:38,470 --> 00:04:40,420 because as soon as the function reaches 108 108 00:04:41,864 --> 00:04:44,290 this return statement it returns and exits the function 109 109 00:04:44,290 --> 00:04:46,370 and gives the value back to the caller. 110 110 00:04:46,370 --> 00:04:50,230 So this print function never gets called. 111 111 00:04:50,230 --> 00:04:52,197 Only this one does. 112 112 00:04:52,197 --> 00:04:55,740 That is another common problem that 113 113 00:04:55,740 --> 00:04:58,090 some people will put a return value 114 114 00:04:58,090 --> 00:05:01,170 elsewhere in the function as well as 115 115 00:05:01,170 --> 00:05:02,640 the one at the end of the function. 116 116 00:05:02,640 --> 00:05:04,400 This is totally fine, you can do that. 117 117 00:05:04,400 --> 00:05:06,400 But just make sure that it is what you want. 118 118 00:05:06,400 --> 00:05:09,453 Because when you return without a value 119 119 00:05:09,453 --> 00:05:12,440 you're actually going to return None by default. 120 120 00:05:12,440 --> 00:05:14,590 So by putting a return here in line two 121 121 00:05:14,590 --> 00:05:16,870 it means that none of these lines are going to run. 122 122 00:05:16,870 --> 00:05:18,640 Because the function will terminate when you 123 123 00:05:18,640 --> 00:05:22,930 encounter a return and this here will become None 124 124 00:05:22,930 --> 00:05:25,570 because there is a return with no value 125 125 00:05:25,570 --> 00:05:27,330 which means return None. 126 126 00:05:27,330 --> 00:05:28,520 So our result will be None and 127 127 00:05:28,520 --> 00:05:30,090 you'll end up printing None out. 128 128 00:05:30,090 --> 00:05:32,853 If we press play you'll see that none comes out. 129 129 00:05:35,540 --> 00:05:38,310 Let's go back to our divide function from the last video, 130 130 00:05:38,310 --> 00:05:41,790 but now instead of printing out, we're going to return. 131 131 00:05:41,790 --> 00:05:43,170 So here we've got our divide function 132 132 00:05:43,170 --> 00:05:44,923 with a dividend and a divisor. 133 133 00:05:44,923 --> 00:05:46,350 If the divisor is not zero, 134 134 00:05:46,350 --> 00:05:47,920 we're going to return the division, 135 135 00:05:47,920 --> 00:05:50,660 otherwise we're going to return the string, you fool. 136 136 00:05:50,660 --> 00:05:53,920 So if we divide 15 by three and we print the result out 137 137 00:05:53,920 --> 00:05:55,800 we should see a number coming out 138 138 00:05:55,800 --> 00:05:57,910 and we do get 5.0. 139 139 00:05:57,910 --> 00:06:01,010 But if you decide to go for 15 and zero 140 140 00:06:01,010 --> 00:06:02,650 and then you press play, 141 141 00:06:02,650 --> 00:06:04,833 you'll see that you fool gets printed out. 142 142 00:06:05,680 --> 00:06:08,060 This is fine, you can totally do two returns 143 143 00:06:08,060 --> 00:06:10,920 in a single function as long as only one of them runs, 144 144 00:06:10,920 --> 00:06:12,610 which is what happens in this if statement. 145 145 00:06:12,610 --> 00:06:15,640 You can still access both returns depending 146 146 00:06:15,640 --> 00:06:17,510 on these conditions here. 147 147 00:06:17,510 --> 00:06:20,870 So again a return will terminate the function 148 148 00:06:20,870 --> 00:06:23,130 when the line is ran and give the value 149 149 00:06:23,130 --> 00:06:24,870 returned back to the caller. 150 150 00:06:24,870 --> 00:06:27,498 So in this case, this function call here 151 151 00:06:27,498 --> 00:06:29,560 will be replaced, essentially, 152 152 00:06:29,560 --> 00:06:32,900 by whatever the function returns when it runs. 153 153 00:06:32,900 --> 00:06:34,740 As an aside it's usually not recommended 154 154 00:06:34,740 --> 00:06:36,530 to have functions returning multiple 155 155 00:06:36,530 --> 00:06:38,240 different types of data. 156 156 00:06:38,240 --> 00:06:40,920 For example this function can return a number or a string. 157 157 00:06:40,920 --> 00:06:43,375 Just because if you wanted to perform mathematics 158 158 00:06:43,375 --> 00:06:45,870 on the return value of this function 159 159 00:06:45,870 --> 00:06:50,170 you will encounter problems if you do return a string. 160 160 00:06:50,170 --> 00:06:53,690 For example if you wanted to print out the result 161 161 00:06:53,690 --> 00:06:56,680 of dividing this multiplied by five, 162 162 00:06:56,680 --> 00:06:58,870 then of course you're going to get 163 163 00:06:58,870 --> 00:07:00,900 you fool, you fool, you fool, you fool. 164 164 00:07:00,900 --> 00:07:02,540 However if you change this number 165 165 00:07:02,540 --> 00:07:05,760 then you'll actually get a valid mathematical number. 166 166 00:07:05,760 --> 00:07:08,200 So, usually not recommended to return 167 167 00:07:08,200 --> 00:07:09,840 different types of data from a function. 168 168 00:07:09,840 --> 00:07:12,580 Although as I said, you can do it, 169 169 00:07:12,580 --> 00:07:15,280 it's just not advised. 170 170 00:07:15,280 --> 00:07:16,510 That's everything for this video. 171 171 00:07:16,510 --> 00:07:17,850 Thank you for joining me in this one, 172 172 00:07:17,850 --> 00:07:19,500 and I'll see you in the next one.