WEBVTT 0 00:06.410 --> 00:11.840 The first thing I want to talk about is a concept known in programming as Scope. 1 00:11.840 --> 00:16.880 And this scope is something that we've actually come across already in our programming, 2 00:16.880 --> 00:20.120 but I wanted to leave it until we've seen functions, 3 00:20.120 --> 00:24.290 we've seen if and while loops, different types and blocks of code, 4 00:24.290 --> 00:28.280 before we start talking about this really important concept. 5 00:28.430 --> 00:35.390 Now, if you want to visualize this concept, imagine you have a house with a fence around it and you 6 00:35.390 --> 00:38.900 plant an apple tree inside your garden. 7 00:39.020 --> 00:43.910 Think about who can access those apples, just you and your family, right? 8 00:44.480 --> 00:50.720 But what if instead you had an apple tree in the neighborhood by the sidewalk, for example? 9 00:50.720 --> 00:55.880 Well, this is kind of free for all Anybody can go and access those apples. 10 00:56.030 --> 00:59.810 This is a starting point for how to understand scope. 11 01:00.440 --> 01:06.290 Take a look at the code that's in the starting file and try to understand what it's doing. 12 01:06.320 --> 01:10.670 First I create a variable called enemies and I set it to equal 1. 13 01:10.700 --> 01:17.000 Then I create a function called increase_enemies() and I try to set that variable to 2. 14 01:17.140 --> 01:22.420 Finally, I call my function triggering all the lines of code inside. 15 01:22.420 --> 01:29.320 And now I want you to predict what will this print statement print in the console and what will this 16 01:29.320 --> 01:30.910 print statement print. 17 01:31.240 --> 01:34.480 And it might not be what you expect it to be. 18 01:34.630 --> 01:42.610 So I would expect that here I've got a variable which is equal to 1 when this function is called this 19 01:42.610 --> 01:45.760 variable enemies should now be equal to 2. 20 01:45.790 --> 01:51.820 So when it's printed down here from outside my function this should be equal to 2, 21 01:51.820 --> 01:54.760 and this should also be equal to 2. 22 01:54.790 --> 02:01.630 But look at what happens instead, I get enemies from inside the function equal to 2, 23 02:01.660 --> 02:05.290 but from the outside the function it's equal to 1. 24 02:05.290 --> 02:07.180 So what's going on here? 25 02:07.300 --> 02:11.800 To really understand this, we have to understand the concept of scope. 26 02:11.800 --> 02:15.490 And scope is a really, really important thing in programming. 27 02:15.490 --> 02:20.620 You see it in every programming language, and although it's a little bit different between the programming 28 02:20.620 --> 02:27.640 languages, it all goes back to that concept of the apple tree being within a fence or outside a fence. 29 02:27.640 --> 02:32.650 So let's take a look at some other examples before we come back to solve this problem. 30 02:33.610 --> 02:37.780 The first thing to think about is a concept called Local Scope. 31 02:37.810 --> 02:41.200 Local scope exists within functions. 32 02:41.200 --> 02:48.430 So for example, if I was creating a game and I had a function called drink_potion(), which is somehow 33 02:48.430 --> 02:56.320 going to strengthen my player, well, maybe it would have a variable that's created inside the function. 34 02:56.320 --> 02:59.560 So it could be something like potion_strength, 35 03:00.280 --> 03:04.210 and I could set that to start off with a value of 2. 36 03:04.510 --> 03:11.590 Now in this case, if I was to go and print this variable potion_strength from within the function, 37 03:11.590 --> 03:13.420 notice I'm indented here. 38 03:13.420 --> 03:17.590 And of course I need to call this function for that to happen. 39 03:18.630 --> 03:21.180 Notice what happens when I run the code. 40 03:21.750 --> 03:23.400 It's equal to 2. 41 03:23.430 --> 03:25.410 That's what we would expect, right? 42 03:25.440 --> 03:34.230 But notice what happens if I try to print the potion_strength from outside the function here. 43 03:35.040 --> 03:41.910 This firstly gives me an error right when I'm writing it inside the editor, and when I try to run it, 44 03:41.940 --> 03:44.580 it gives me an error inside the console. 45 03:44.670 --> 03:47.130 And the error is a NameError. 46 03:47.130 --> 03:51.840 And it tells me that the "name 'potion_strength' is not defined." 47 03:52.080 --> 03:57.660 Clearly, I've defined it here, but why can't I access it outside the function? 48 03:57.690 --> 04:01.050 Well, this goes back to the concept of local scope. 49 04:01.050 --> 04:11.220 When you create a new variable or indeed a new function inside another function, it's only accessible, 50 04:11.220 --> 04:18.090 you can only use it when you're inside that function because it has local scope. 51 04:18.090 --> 04:23.670 It's only valid within the walls of this drink potion function. 52 04:25.320 --> 04:29.400 What if we wanted it to be accessible outside the function? 53 04:29.430 --> 04:33.660 Well, now we need to think about something called Global Scope. 54 04:33.870 --> 04:40.800 The only difference between global scope and local scope is where you define or where you create your 55 04:40.800 --> 04:42.540 variables or your functions. 56 04:42.660 --> 04:44.970 So let's say we have a different variable. 57 04:44.970 --> 04:48.300 Let's say that I created a variable called player_health, 58 04:48.420 --> 04:51.090 and it starts off being equal to 10. 59 04:51.540 --> 04:59.930 Now let's say later on I have the same drink_potion() function and this potion has strength of 2. 60 04:59.960 --> 05:09.650 So now inside my function, drink_potion(), if I wanted to print my player health even though it wasn't 61 05:09.650 --> 05:13.100 defined within the function, this is perfectly possible. 62 05:13.100 --> 05:22.310 And if I call my function drink_potion() and then run the code as it is, you'll see 10 being printed. 63 05:22.310 --> 05:25.430 So this has a global scope, 64 05:25.430 --> 05:31.550 it's available anywhere within our file because it was defined at the top level of the file. 65 05:31.550 --> 05:36.350 Now when I say top level, I don't mean physically at the very top of the file. 66 05:36.350 --> 05:44.690 I mean that it's not within another function like potion_strength() here, because this is defined indented 67 05:44.690 --> 05:46.310 inside a function. 68 05:46.310 --> 05:51.080 So this is a local variable, whereas this is a global variable. 69 05:51.080 --> 05:57.860 And global variables are available within functions no matter how deep it gets nested. 70 05:57.860 --> 06:00.770 And it's also available outside of functions. 71 06:02.120 --> 06:07.520 This concept of global and local scope doesn't just apply to variables. 72 06:07.520 --> 06:13.850 As I alluded to before, it also applies to functions and basically anything else you name. 73 06:13.880 --> 06:17.060 This is a concept called the Namespace. 74 06:17.180 --> 06:19.970 So we defined this variable player_health, 75 06:19.970 --> 06:22.250 we define this function drink_potion, 76 06:22.250 --> 06:26.330 anything that you give a name to has a namespace. 77 06:26.330 --> 06:30.800 And that namespace is valid in certain scopes. 78 06:31.580 --> 06:35.870 This concept of scope applies to basically anything you name. 79 06:36.230 --> 06:42.500 If I was to nest this function drink_potion() inside another function, let's call it game(), 80 06:42.620 --> 06:49.010 well, this drink_potion() now has local scope within the function of game(). 81 06:49.010 --> 06:57.310 So this is why this line is now erroring out because it can't actually see inside this function. 82 06:57.340 --> 07:04.510 So now in order to call this drink_potion(), I have to be within the four walls of the game() function. 83 07:05.140 --> 07:12.040 So whenever you give name to anything, a function or a variable, you have to be aware of where you 84 07:12.040 --> 07:13.000 created it. 85 07:13.030 --> 07:16.540 Now it's easy to notice where you've created functions. 86 07:16.540 --> 07:18.340 We have the def keyword right? 87 07:18.340 --> 07:24.550 But when you create a variable, that's the first time that you give it a name and you set it equal 88 07:24.550 --> 07:25.450 to something. 89 07:25.450 --> 07:31.300 And where you write that line of code defines the scope of that particular variable. 90 07:31.690 --> 07:38.530 Here it's outside of every other function, whereas here it's inside a function actually nested two 91 07:38.530 --> 07:39.430 levels deep, 92 07:39.430 --> 07:43.240 so it has local scope to this function.