WEBVTT 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.