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The first thing I want to talk about is a concept known in programming as Scope.

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And this scope is something that we've actually come across already in our

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programming, but I wanted to leave it until we've seen functions,

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we've seen if and while loops, different types and blocks of code,

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before we start talking about this really important concept. Now,

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if you want to visualize this concept,

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imagine you have a house with a fence around it and you plant an apple tree

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inside your garden. Think about who can access those apples.

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Just you and your family right? But what if instead,

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you had an apple tree in the neighborhood by the sidewalk, for example.

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Well, this is kind of free for all. Anybody can go and access those apples.

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This is a starting point for how to understand scope.

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Head over to the App Brewery Day 12 start Repl.it and go ahead and fork your own

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copy of it.

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Take a look at the code that's in the starting file in the main.py and try to

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understand what it's doing. First

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I create a variable called enemies and I set it to equal 1.

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Then I create a function called increase_enemies,

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and I try to set that variable to 2. Finally,

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I call my function triggering all the lines of code inside,

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and now I want you to predict what will this print statement print in the

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console and what will this print statement print?

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And it might not be what you expect it to be.

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So I would expect that here I've got a variable which is equal to one.

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When this function is called,

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this variable enemies should now be equal to two.

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So when it's printed down here from outside my function,

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this should be equal to two, and this should also be equal to two.

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But look at what happens instead.

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I get enemies from inside the function equal to 2,

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but from outside the function, it's equal to 1.

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So what's going on here? To really understand this

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we have to understand the concept of scope. And scope is a really,

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really important thing in programing.

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You see it in every programming language and although its a little bit different

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between the programming languages,

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it all goes back to that concept of the apple tree being within a fence or

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outside a fence.

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So let's take a look at some other examples before we come back to solve this

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problem.

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The first thing to think about is a concept called local scope.

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Local scope exists within functions. So for example,

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if I was creating a game and I had a function called drink_potion

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which is somehow going to strengthen my player,

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well, maybe it would have a variable that's created inside the function.

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So it could be something like potion_strength

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and I could set that to start off where the value of two.

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Now in this case

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if I was to go and print this variable potion strength from within the function,

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notice I'm indented here,

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and of course I need to call this function for that to happen.

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Notice what happens when I run the code. It's equal to two.

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That's what we would expect, right?

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But notice what happens if I try to print the potion_strength from outside the

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function here. This firstly gives me an error

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right when I'm writing it inside the editor. And when I tried to run it,

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it gives me an error inside the console. And the error is a name error

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and it tells me that the name potion strength is not defined. Clearly I've

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defined it here, but why can't I access it outside the function?

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Well, this goes back to the concept of local scope.

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When you create a new variable or indeed a new function inside another function,

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it's only accessible,

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you can only use it when you're inside that function because it has local scope.

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It's only valid within the walls of this drink_

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potion function.

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What if we wanted it to be accessible outside the function? Well,

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now we need to think about something called global scope.

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The only difference between global scope and local scope is where you define or

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where you create your variables or your functions.

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So let's say we have a different variable.

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Let's say that I created a variable called a player_health and it starts off

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being equal to 10. Now let's say later on I have the same drink_

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potion function, and this potion has strength of 2.

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So now inside my function drink_potion,

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if I wanted to print my player_health

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even though it wasn't defined within the function, this is perfectly possible.

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And if I call my function

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drink_potion and then run the code as it is,

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you will see 10 being printed.

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So this has a global scope. It's available anywhere within our file because it

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was defined at the top level of the file. Now, when I say top-level,

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I don't mean physically at the very top of the file.

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I mean that it's not within another function, like potion strength here because

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this is defined indented inside a function.

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So this is a local variable

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whereas this is a global variable. And global variables are available within

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functions, no matter how deep it gets nested

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and it's also available outside of functions.

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This concept of global and local scope doesn't just apply to variables.

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As I alluded to before, it also applies to functions and basically anything else

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you name. This is a concept called the Namespace.

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So we defined this variable player_health, we define this function

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drink_potion.

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Anything that you give a name to has a namespace and that namespace is valid in

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certain scopes.

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This concept of scope applies to basically anything you name.

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If I was to nest this function drink_potion inside another function,

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let's call it game. Well,

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this drink_potion now has local scope within the function game.

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So this is why this line is now erroring out because it can't actually see inside

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this function. So now in order to call this drink_potion,

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I have to be within the four walls of the game function.

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So whenever you give name to anything, a function or a variable,

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you have to be aware of where you created it.

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Now it's easy to notice where you've created functions, we have the def keyword,

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right? But when you create a variable

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that's the first time that you give it a name and you set it equal to something.

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And where you write that line of code defines the scope of that particular

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variable. Here it's outside of every other function,

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whereas here it's inside a function actually nested two levels deep.

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So it has a local scope to this function.