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Scope is a fancy way of saying we're going to use this variable,

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or we're going to use this function.

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And I think the best way to learn about scope is to really

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just experience it, because I find trying to describe it

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in words doesn't actually make sense.

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And when I was first learning scope, someone tried describing it

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to me, and I was like, "I don't get it.

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Like I can use a variable in one place, but not another place",

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which is effectively, yeah, that's what it is,

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but there are rules behind it.

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So let's go ahead and create an example here.

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We've got 'language = "Python", and then if I do 'def

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change_language(): language = "JavaScript"'.

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And let's return that 'language' in this function, and we 'print(

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language)'. This is a really good example of scope.

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So we've originally set it to be "Python",

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we've got this function in here called 'change_language()', and

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it's reassigning the exact same variable here to be called "JavaScript".

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So when I run this cell, do you think this is

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going to be "Python", or do you think this is going to be "JavaScript"?

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[no audio]

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Let's go ahead and run this cell, and we're going to see

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that it's "Python".

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Now, the reason for this is because this 'language = "Python"',

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'language' variable is in a higher scope, and the "JavaScript"

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variable is in a function.

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So essentially, the "JavaScript" variable isn't allowed to

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be accessed outside of this function at all.

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This is called scope, where we have sure, we have a variable

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called 'language' here and here.

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But inside of this function, Python is saying, "Hey, there

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is another variable in here called 'language', and I'm going

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to use this one in here", and it's going to be completely

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unaware of this other one for the time being.

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Now what happens if I actually run this function?

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So I'm going to run 'change_language', and I try to figure out

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what 'language' is again.

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So I've run this language.

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It has now said, okay, 'language = "JavaScript", and

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it returned that 'language', as we can see here.

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So now that it's been overwritten, should 'language' be "JavaScript",

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or should it be "Python",

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because we executed some code that overwrote that variable.

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Well, if we run the cell, we're still going to see that it's still "Python".

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So Python uses this thing called the LEGB rule.

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'L' stands for 'Local'.

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So names assigned in any way with a function like 'def' or

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'lambda'. So it's going to look for a variable in here, and then

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it's going to look for, enclosed function locals, that's the

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'E', and that's names in local scope and any enclosing functions

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or functions that are outside of functions,

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because in Python, we can nest functions inside of other

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functions. We'll touch on that a little bit later.

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Then the 'G' is 'Global'.

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So the global module that's outside of here. And then the

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built-in Python library, that's the 'B'.

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And that means if there is a variable that Python is already

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using, then it's going to look for that.

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It's one of those keywords, such as the word 'for', or 'while',

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or 'True' or 'False', those are all keywords,

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and so Python is going to try to reserve those.

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You can overwrite them, but please don't, because you're

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going to cause a lot of problems if you overwrite them.

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So that's why we try not to overwrite keyword variables that

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Python already gives us, or keywords that Python gives us.

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So a good way to look at this

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is really Python is going to say, "Okay, there's language

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= "Python"."

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There's a function in here, but it doesn't actually execute

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that function right away.

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It just says, because this code is basically touching this

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line here that is going to register it, but it isn't actually

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executed. But when you do execute, this function, it's going

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to look from the inside to the outside.

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So it's going to look for a 'language' in here called "JavaScript",

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and if that does not exist, it's going to then use the 'language'

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up here called "Python".

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So let's go ahead, comment this line out, and let's rerun

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this. Okay, it still says, "Python", but when we run

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'change_language()', it's going to 'return language'.

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Now, this used to say "JavaScript", but this now says "Python".

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'language', here for all intents of purposes, does not exist.

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And so because it couldn't find a variable in here called 'language',

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this just simply didn't exist,

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it looked outside of its own scope.

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And so it looked up here for the word 'language', or the variable

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'language', and used that one.

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Now, let's create a little more complicated example, and

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I don't expect you to totally know what's going on here,

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but we are going to take a look at nested functions.

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So if I have 'name = "Kalob"', this is a global variable.

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If I then create a function called 'greeting', and I put the

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'name' in here as "Zephyr", this is an 'enclosing variable'.

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And if I put another function in here 'def say_hello', and

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this 'name = "Henry", this one is local.

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And so what we can say, because this is a function inside of a function,

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we can run this function inside of the 'greeting' function,

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and then we can run 'greeting' on its own.

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So let's go ahead and run 'greeting'.

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And before I do that, let's take a look at what we think

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is going to happen here.

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So I'm going to run 'greeting'.

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'name' is already defined outside of it.

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Then we're defining 'name' inside of this to be 'name = "Zephyr"'.

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And inside of here,

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the 'name = "Henry"'.

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And actually, what we should do is print this out.

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'print("Name is ", name)'.

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There we go. That's actually going to work for us.

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And so remember, it's going to go from the inside to the outside.

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And so we're running 'greeting',

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but inside of 'greeting' it's running 'say_hello',

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so it's running another function in here, and that 'name' is

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currently "Henry".

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And so if we have this correct, this should say 'Name is Henry",

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as we expect.

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But let's comment this out,

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so that 'name' effectively no longer exists inside of the

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'say_hello' function.

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We run this again, and it's going to say "Zephyr", because

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I can't find it in here, but it's going to look just outside of its function.

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Now, let's say 'name' doesn't exist inside of 'greeting'.

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Well, because it's looking for a 'name' here, it doesn't exist;

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it's looking for a 'name' in here, also doesn't exist;

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it's now going to look outside of all of the functions in

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the global scope.

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Let's run this cell and run that once more, and we're going

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to see the "Name is Kalob".

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Now, what this means, is you can create a function in here

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with absolutely no variables,

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and as long as there is a variable that it's using that's

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outside of it, you can use it.

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Now, that sounds a little crazy.

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But let's say the 'website = "pythonforverybody.com",

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and we have a new function in here called 'def get_website'.

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We can 'return website'.

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And when we run 'get_website', this will return 'pythonforeverybody.com',

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despite the fact that there is no variable

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in here, and that's because it exists just outside of the

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function scope.

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But we can now say 'website = 'nothing because Kalob

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ruined this''.

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[no audio]

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It now exists in here, so it exists in two places, but the

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function is going to look inside of itself

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first. Says, "Oh, yep.

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Okay, there is actually a variable called website.

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So let's use that one".

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But again, if that variable didn't exist in here, it would

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look in the global scope.

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Now, that's actually sort of a really bad way of using a

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function, and that can get very, very confusing for other

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people who are looking at your code.

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So typically we avoid that, and we just put the 'website'

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in here. So the 'website' could be "pythonforeverybody.com"

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as a default, and that would be fine.

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We can get rid of this, or we could overwrite this with, "anything

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else", assuming I could spell. There we go.

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"anything else".

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And so this is a much better way than leveraging the Python

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scope. Now, if you come from JavaScript, it's not uncommon

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for JavaScript to leverage its own scope like that.

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But in Python, we try really hard not to do that.

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We try to explicitly pass in data into our functions just

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so we know exactly what's going on in here.

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And the reason for that is if I just gave you this piece

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of code here, that's a 'def

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get_website', and 'return website',

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but you didn't know where 'website' came from.

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Maybe you're looking at a file that has 2,000 lines of code,

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and you can't seem to find this variable, that's going to

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be very confusing because you're going to look at this, and say,

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"Yep, this is a function, and it's returning a variable called website.

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But where is website?"

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And if you have to stop and ask yourself, where is that,

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then that's probably a good case for using a function argument

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or a parameter.

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[no audio]