1 00:00:00,278 --> 00:00:02,200 Hi guys, and welcome back. 2 00:00:02,200 --> 00:00:05,030 In this video, we're going to learn about functions. 3 00:00:05,030 --> 00:00:06,630 We've been using some functions 4 00:00:06,630 --> 00:00:08,900 such as a print and line and sum, 5 00:00:08,900 --> 00:00:11,500 but now we're going to be defining our own. 6 00:00:11,500 --> 00:00:14,590 And that means that we can put some bits of code, 7 00:00:14,590 --> 00:00:16,900 and give them a name, so that we can re-use 8 00:00:16,900 --> 00:00:19,130 those bits of code whenever we want. 9 00:00:19,130 --> 00:00:21,030 Let's learn more about them. 10 00:00:21,030 --> 00:00:22,780 I'm going to create my first function, 11 00:00:22,780 --> 00:00:24,800 and I'm going to call it hello. 12 00:00:24,800 --> 00:00:27,700 So you start off by using the def keyword, 13 00:00:27,700 --> 00:00:29,350 and then you type out the name 14 00:00:29,350 --> 00:00:31,740 of the function you want to define. 15 00:00:31,740 --> 00:00:35,040 Then you put a pair of brackets, and a colon, 16 00:00:35,040 --> 00:00:38,120 and that begins the function body. 17 00:00:38,120 --> 00:00:40,930 Here we're going to put print, hello. 18 00:00:40,930 --> 00:00:45,470 And what happens now is that we have a function created. 19 00:00:45,470 --> 00:00:49,170 When Python reaches a def keyword, it then says, okay, 20 00:00:49,170 --> 00:00:52,700 we've got a hello variable that we are creating. 21 00:00:52,700 --> 00:00:57,010 Only this variable is not a normal value variable 22 00:00:57,010 --> 00:01:01,770 like a string or an integer, this is a callable variable. 23 00:01:01,770 --> 00:01:05,740 That means that it's a variable whose value is a function. 24 00:01:05,740 --> 00:01:08,730 And a function you can call or execute. 25 00:01:08,730 --> 00:01:12,230 Calling, running, or executing a function are all synonyms, 26 00:01:12,230 --> 00:01:14,070 so they mean the same thing. 27 00:01:14,070 --> 00:01:16,760 So when you define a function, 28 00:01:16,760 --> 00:01:19,960 you create a callable variable. 29 00:01:19,960 --> 00:01:22,530 Now creating the function or defining the function 30 00:01:22,530 --> 00:01:24,870 does not run the function body. 31 00:01:24,870 --> 00:01:29,130 It just tells Python that a variable called hello exists. 32 00:01:29,130 --> 00:01:32,810 So if you run this code, nothing actually happens. 33 00:01:32,810 --> 00:01:36,060 As you can see down here, nothing is printed out, 34 00:01:36,060 --> 00:01:38,410 because the function has been defined, 35 00:01:38,410 --> 00:01:41,640 but the function has not been called. 36 00:01:41,640 --> 00:01:43,440 So in order to call the function, 37 00:01:43,440 --> 00:01:46,270 you have to type out the variable name, which is hello, 38 00:01:46,270 --> 00:01:48,250 and then the brackets. 39 00:01:48,250 --> 00:01:51,720 And that, these two brackets after a variable name, 40 00:01:51,720 --> 00:01:55,600 is what actually calls the function and executes it. 41 00:01:55,600 --> 00:01:58,520 The order of running these lines in Python is as follows. 42 00:01:58,520 --> 00:02:00,400 First line one runs. 43 00:02:00,400 --> 00:02:03,180 And it tells Python to create this hello variable 44 00:02:03,180 --> 00:02:05,010 that is a callable. 45 00:02:05,010 --> 00:02:07,333 Then, line two does not run. 46 00:02:08,290 --> 00:02:10,020 Line three runs, but it's empty. 47 00:02:10,020 --> 00:02:11,270 And then line four runs, 48 00:02:11,270 --> 00:02:13,490 which tells Python to execute this function. 49 00:02:13,490 --> 00:02:17,640 So Python jumps back up, and then runs line number two. 50 00:02:17,640 --> 00:02:19,430 Notice that Python knows line number two 51 00:02:19,430 --> 00:02:21,140 is part of the function body, 52 00:02:21,140 --> 00:02:23,220 because of course it is indented, 53 00:02:23,220 --> 00:02:24,940 so it's got these four spaces in front. 54 00:02:24,940 --> 00:02:26,770 You can put more lines in a function, 55 00:02:26,770 --> 00:02:29,510 as long as they are also indented, if you want. 56 00:02:29,510 --> 00:02:31,030 Let's define another function. 57 00:02:31,030 --> 00:02:35,090 Here I've created a callable variable, user age in seconds. 58 00:02:35,090 --> 00:02:36,800 And it is a function. 59 00:02:36,800 --> 00:02:39,190 And now, we can put some code inside of it. 60 00:02:39,190 --> 00:02:40,760 So we're going to implement the first app 61 00:02:40,760 --> 00:02:42,490 that we coded earlier on. 62 00:02:42,490 --> 00:02:46,370 So user age is going be int of input, enter your age. 63 00:02:46,370 --> 00:02:48,750 Here we ask the user for their age, 64 00:02:48,750 --> 00:02:50,860 and then we're going to turn it into seconds, 65 00:02:50,860 --> 00:02:52,340 so that we can print it out. 66 00:02:52,340 --> 00:02:54,720 So age in seconds is going to be user age, 67 00:02:54,720 --> 00:02:58,330 times 365, times 24, times 60 times 60. 68 00:02:58,330 --> 00:03:01,290 That is years, multiplied by days, 69 00:03:01,290 --> 00:03:03,860 hours, minutes, and seconds. 70 00:03:03,860 --> 00:03:05,710 And then we're going to print it out. 71 00:03:07,540 --> 00:03:08,780 Just like that. 72 00:03:08,780 --> 00:03:10,300 Of course, just to reiterate, 73 00:03:10,300 --> 00:03:12,960 if we run this, nothing happens. 74 00:03:12,960 --> 00:03:17,820 You do have to call the function, user age in seconds. 75 00:03:17,820 --> 00:03:21,480 And if you do that, you can actually type down here. 76 00:03:21,480 --> 00:03:23,720 Let me repeat at this point that 77 00:03:23,720 --> 00:03:25,290 sometimes I'm pressing the play button, 78 00:03:25,290 --> 00:03:27,730 and other times I'm running the code in a terminal, 79 00:03:27,730 --> 00:03:30,870 because pressing the play button does not allow you to type. 80 00:03:30,870 --> 00:03:32,480 So you have to run this code in a terminal 81 00:03:32,480 --> 00:03:33,840 if you want to type. 82 00:03:33,840 --> 00:03:35,720 Let's enter the age, such as 30, 83 00:03:35,720 --> 00:03:39,050 and then we get your age in seconds is that. 84 00:03:39,050 --> 00:03:41,240 Now one of the benefits of doing this 85 00:03:41,240 --> 00:03:44,550 is that what you have now are two 86 00:03:44,550 --> 00:03:46,630 reasonably separate bits of code. 87 00:03:46,630 --> 00:03:50,670 One, which is what the code does, and the other one, 88 00:03:50,670 --> 00:03:53,460 which is the code that we'll actually run. 89 00:03:53,460 --> 00:03:57,410 Now this is very much a conceptual separation. 90 00:03:57,410 --> 00:03:59,990 So the code is really just a single file, 91 00:03:59,990 --> 00:04:02,280 which runs line by line and so on. 92 00:04:02,280 --> 00:04:04,260 But conceptually, as a person, 93 00:04:04,260 --> 00:04:06,870 you can think of it as running the code, 94 00:04:06,870 --> 00:04:09,090 and defining the code. 95 00:04:09,090 --> 00:04:12,760 So it can help you understand and read through your code 96 00:04:12,760 --> 00:04:16,946 a bit better if you make that conceptual separation. 97 00:04:16,946 --> 00:04:19,120 For example here we're running the code, 98 00:04:19,120 --> 00:04:21,513 so we could actually add some more things. 99 00:04:24,720 --> 00:04:25,970 Such as that. 100 00:04:25,970 --> 00:04:30,660 Now if we run this again, you get a nice message. 101 00:04:30,660 --> 00:04:32,600 And then you get your goodbye there. 102 00:04:32,600 --> 00:04:33,740 But as I said earlier, 103 00:04:33,740 --> 00:04:37,280 these lines here are just running from top to bottom. 104 00:04:37,280 --> 00:04:41,949 So you can move that line to the top if you want. 105 00:04:41,949 --> 00:04:45,550 And you can put this line anywhere you like. 106 00:04:45,550 --> 00:04:49,620 And Python will still print out the exact same stuff. 107 00:04:49,620 --> 00:04:51,070 Welcome to the age in seconds programme 108 00:04:51,070 --> 00:04:53,210 will be printed out first. 109 00:04:53,210 --> 00:04:55,760 Then the user age in seconds will be printed out next, 110 00:04:55,760 --> 00:04:57,510 which runs through this function, 111 00:04:57,510 --> 00:05:00,060 and finally goodbye will be printed out after. 112 00:05:00,060 --> 00:05:05,060 However, by treating these two as slightly separated 113 00:05:05,130 --> 00:05:07,720 parts of code, it can actually be much easier 114 00:05:07,720 --> 00:05:10,110 to reason about, much easier to read, 115 00:05:10,110 --> 00:05:11,870 and much easier to think about. 116 00:05:11,870 --> 00:05:15,063 So that is just a tip there from me to you. 117 00:05:17,640 --> 00:05:19,590 A couple of things that you should not do 118 00:05:19,590 --> 00:05:23,390 when you're working with functions is re-use names. 119 00:05:23,390 --> 00:05:26,060 So if you define a function called print, 120 00:05:26,060 --> 00:05:28,440 and then you try to print hello world, 121 00:05:28,440 --> 00:05:30,530 you're going to get a problem. 122 00:05:30,530 --> 00:05:33,280 Because when you define the print function, 123 00:05:33,280 --> 00:05:36,080 that creates this print variable that you can use, 124 00:05:36,080 --> 00:05:38,110 and then you're actually using it, here. 125 00:05:38,110 --> 00:05:41,650 This is no longer the Python built in print function 126 00:05:41,650 --> 00:05:45,130 that we're calling, this is calling your own function. 127 00:05:45,130 --> 00:05:46,880 And this is almost never what you want, 128 00:05:46,880 --> 00:05:49,023 although it is valid Python code, 129 00:05:49,023 --> 00:05:51,320 it's almost never what you want. 130 00:05:51,320 --> 00:05:54,200 And another reason why you get an error by doing this, 131 00:05:54,200 --> 00:05:57,830 is because you're putting in a string inside the brackets, 132 00:05:57,830 --> 00:06:00,640 but this function cannot take in a string. 133 00:06:00,640 --> 00:06:03,210 We're going to learn how to do that in a future video. 134 00:06:03,210 --> 00:06:06,720 So if you run that, you're going to get an error. 135 00:06:06,720 --> 00:06:09,330 It says, print takes zero positional arguments, 136 00:06:09,330 --> 00:06:10,950 but one was given. 137 00:06:10,950 --> 00:06:12,720 That means that we've put a string in there, 138 00:06:12,720 --> 00:06:15,210 but our function does not accept a string. 139 00:06:15,210 --> 00:06:16,810 Another thing that you may want to avoid, 140 00:06:16,810 --> 00:06:20,350 is imagine you've got a list of your friend names, 141 00:06:20,350 --> 00:06:22,993 and then you have a function that adds a friend. 142 00:06:23,900 --> 00:06:27,253 Then you take in the friend name from the user's input. 143 00:06:30,210 --> 00:06:31,360 And then here's something 144 00:06:31,360 --> 00:06:33,640 that you almost definitely want to avoid, 145 00:06:33,640 --> 00:06:38,560 and that is to say friends equal friends plus friend name. 146 00:06:38,560 --> 00:06:39,480 What's happening here 147 00:06:39,480 --> 00:06:43,990 is you're re-using the friends variable 148 00:06:43,990 --> 00:06:45,930 in two different places. 149 00:06:45,930 --> 00:06:48,962 And because you are inside a function, 150 00:06:48,962 --> 00:06:52,080 Python actually creates a new space for variables 151 00:06:52,080 --> 00:06:54,410 that is separate from the global space, 152 00:06:54,410 --> 00:06:57,450 so functions have their own little space for your variables. 153 00:06:57,450 --> 00:06:59,920 And when you do friends equal, 154 00:06:59,920 --> 00:07:02,693 Python is creating a new variable called friends 155 00:07:02,693 --> 00:07:06,430 that only exists inside the function. 156 00:07:06,430 --> 00:07:08,620 The same thing with friend name. 157 00:07:08,620 --> 00:07:10,750 So, you now have two variables. 158 00:07:10,750 --> 00:07:13,690 Friends, in the global scope of the file, 159 00:07:13,690 --> 00:07:15,880 and friends in the function scope 160 00:07:15,880 --> 00:07:17,980 of this add friend function. 161 00:07:17,980 --> 00:07:20,510 This is almost never something you want to do. 162 00:07:20,510 --> 00:07:25,095 Because even though you are using a variable called friends, 163 00:07:25,095 --> 00:07:27,880 you are also defining a new variable. 164 00:07:27,880 --> 00:07:31,820 So let me run this here. 165 00:07:31,820 --> 00:07:35,493 So let me run that, and you can see that if I enter Rolf, 166 00:07:36,540 --> 00:07:38,210 you get your error here. 167 00:07:38,210 --> 00:07:41,787 And what's happening is that this variable exists. 168 00:07:41,787 --> 00:07:45,640 And if you call this something like F, 169 00:07:45,640 --> 00:07:47,180 then this code is fine. 170 00:07:47,180 --> 00:07:49,770 Because Python will see the friends variable. 171 00:07:49,770 --> 00:07:51,950 It will check whether it exists inside the function, 172 00:07:51,950 --> 00:07:55,500 but it doesn't, so it uses the one outside the function. 173 00:07:55,500 --> 00:07:56,620 So that's fine. 174 00:07:56,620 --> 00:07:57,453 But as soon as you do 175 00:07:57,453 --> 00:07:59,637 friends equal friends plus friend name, 176 00:07:59,637 --> 00:08:03,068 Python is going to look at this friends variable, 177 00:08:03,068 --> 00:08:07,510 and notice that it's being defined inside the function. 178 00:08:07,510 --> 00:08:11,280 And you can't do that, because you can't use a variable 179 00:08:11,280 --> 00:08:13,070 in the same line where you're defining it, 180 00:08:13,070 --> 00:08:14,690 it doesn't makes sense. 181 00:08:14,690 --> 00:08:16,460 So it will give you this error. 182 00:08:16,460 --> 00:08:19,630 It's akin to doing X equal X plus five, 183 00:08:19,630 --> 00:08:21,340 but X doesn't exist. 184 00:08:21,340 --> 00:08:23,940 So Python will also be very confused if you do that. 185 00:08:24,890 --> 00:08:27,510 Alright, so almost never will you want 186 00:08:27,510 --> 00:08:31,170 to shadow a variable name from an outer scope. 187 00:08:31,170 --> 00:08:33,670 That is a technical definition for what we're doing here. 188 00:08:33,670 --> 00:08:36,600 We are shadowing the friends variable 189 00:08:36,600 --> 00:08:39,790 by redefining it inside a function 190 00:08:39,790 --> 00:08:42,880 where it already exists on the outside. 191 00:08:42,880 --> 00:08:45,420 Again this is valid Python code. 192 00:08:45,420 --> 00:08:46,330 Of course this is not, 193 00:08:46,330 --> 00:08:47,620 because you're using the variable 194 00:08:47,620 --> 00:08:48,980 in the same line you're defining it. 195 00:08:48,980 --> 00:08:52,233 But generally, shadowing a variable is valid Python code. 196 00:08:52,233 --> 00:08:55,283 It's just something that you usually don't want to do. 197 00:08:56,860 --> 00:08:58,510 Something else you can't do with functions 198 00:08:58,510 --> 00:09:00,840 is use them before they get defined. 199 00:09:00,840 --> 00:09:03,850 Maybe this is obvious, but here we're calling the function, 200 00:09:03,850 --> 00:09:05,430 say hello, and then we're defining it 201 00:09:05,430 --> 00:09:06,670 down here in line four. 202 00:09:06,670 --> 00:09:09,200 Python won't let you do this, if you try to do that, 203 00:09:09,200 --> 00:09:10,600 you get an error saying 204 00:09:10,600 --> 00:09:13,263 that the say hello name is not defined. 205 00:09:14,470 --> 00:09:16,230 Here we've got a final example 206 00:09:16,230 --> 00:09:18,250 where we have an add friend function 207 00:09:18,250 --> 00:09:21,330 that takes in a global variable, friends, 208 00:09:21,330 --> 00:09:24,290 and appends Rolf to it. 209 00:09:24,290 --> 00:09:27,940 Then we define the friends variable in line five 210 00:09:27,940 --> 00:09:29,640 after the function's been created, 211 00:09:29,640 --> 00:09:31,640 then we call the function add friend, 212 00:09:31,640 --> 00:09:33,490 and finally we print friends. 213 00:09:33,490 --> 00:09:37,790 So this may look like an error, because here in line two, 214 00:09:37,790 --> 00:09:40,780 the friends variable has not yet been defined. 215 00:09:40,780 --> 00:09:44,010 But actually remember that this line of code only runs 216 00:09:44,010 --> 00:09:45,640 when line six runs. 217 00:09:45,640 --> 00:09:46,690 So this is fine. 218 00:09:46,690 --> 00:09:48,090 Although a bit confusing. 219 00:09:48,090 --> 00:09:50,250 The friends variable that this uses 220 00:09:50,250 --> 00:09:53,280 will be defined by the time this line is ran. 221 00:09:53,280 --> 00:09:55,570 So that's why this will work. 222 00:09:55,570 --> 00:09:58,690 Although it will be quite confusing as I said earlier. 223 00:09:58,690 --> 00:10:02,250 Usually you'll want to put your global variables at the top, 224 00:10:02,250 --> 00:10:04,750 so that it's a bit clearer what's going on. 225 00:10:04,750 --> 00:10:09,750 Also this line here is modifying the friends list. 226 00:10:09,980 --> 00:10:13,190 So if you call this function multiple times, 227 00:10:13,190 --> 00:10:16,540 it's going to append Rolf to this list multiple times. 228 00:10:16,540 --> 00:10:19,490 So that's something interesting about this function here, 229 00:10:19,490 --> 00:10:22,230 that you are appending to this list, 230 00:10:22,230 --> 00:10:24,210 so you'll end up with multiple elements, 231 00:10:24,210 --> 00:10:26,370 if you run the function multiple times. 232 00:10:26,370 --> 00:10:28,020 Alright that's everything for this video, 233 00:10:28,020 --> 00:10:29,910 I just wanted to show you 234 00:10:29,910 --> 00:10:32,760 what functions look like in Python, how you define them, 235 00:10:32,760 --> 00:10:35,607 how you use them, and some things that you don't want to do 236 00:10:35,607 --> 00:10:37,950 when you're working with functions. 237 00:10:37,950 --> 00:10:39,300 Thanks for joining me in this one, 238 00:10:39,300 --> 00:10:40,950 and I'll see you in the next one.