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Hey, in this video, you will learn one of the most important elements of Python, and that is custom

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functions.

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So let's look at our code and see how custom functions.

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Will improve our program because currently we have a problem with the program and the problem is redundancy.

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So we have repetitive codes.

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In our code base.

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These two lines here are repeated in here.

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And here as well.

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And here too.

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So four times we have the same code.

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There certainly should be a better way not to have to write repetitive code.

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And the answer is custom.

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Some functions.

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So now I'll go ahead and create a custom function and you'll see how a custom function becomes beneficial

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in avoiding redundant code.

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Custom functions are usually created on top of your code, so outside of the loop.

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So it costs and function is like a slot where you put some codes and then you just call that custom

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function.

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So the slot must have a name.

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So let's call this get to dos.

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And that's the syntax.

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That's how you start.

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So you start with the def keyword and then you specify a name, whatever you like.

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The rules of naming it are the same as naming variables, so you shouldn't start with a number.

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You shouldn't have spaces in the name.

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You can have underscores.

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That's fine.

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So that's the first line.

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In the next line, then you put the code.

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So in our case, it would be these two lines.

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So I'm going to copy them and paste them in here.

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But we need indentation.

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So here you want to paste.

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That's code.

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So make sure it's for spaces.

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For spaces is the recommendation.

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So I have four spaces here.

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One, two, three, four and eight here.

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One, two, three, four.

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One, two, three, four.

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So this indentation here is because we have this width context manager and this indentation here is

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part of the function.

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Make some space.

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It is recommended to have two spaces between a function and other parts of the code.

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So two break lines.

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So that's the code that is repetitive.

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We put it in a function and then we want to add another line at the end which starts with return.

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So return is a statement just like we have other keywords.

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So this is also a keywords.

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So a return to dos.

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So that there is the function definition.

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Once you have defined the function, then you can remove these lines and replace them with the function

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call.

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So that's like the other functions we have been working on.

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Like you see open input print.

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So this also has the same syntax.

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Now it doesn't get any argument.

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You'll see how to create functions, which also get arguments.

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For this lecture, let's focus on functions which don't get any arguments.

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So this gets to do.

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Function will return something.

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Right.

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That is the list.

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Now we need to store that into a variable.

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The variable is normally this one to dos.

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So we store that list in that variable there and then we append to dos to that variable.

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And we can do the same in the other parts of the code.

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So to do is equal to get to deuce.

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So whatever you have this function in reads mode, write, don't confuse it with write mode.

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We will work on that later.

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That's not the same code, correct?

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We should be clear about that.

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So here is another open function with the read modes to do is equal to get to dos with parentheses.

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Don't forget the parentheses because that is a function called.

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And the last one to do so is equal to get to deuce.

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And that's it.

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So let's run the codes to make sure it's working.

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Let's add say to do

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make soup

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make pizza, enter show and everything seems to be working fine.

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So we tried the show condition where we have this function and everything is working fine.

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So.

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You know, that we had these built in functions in Python, such as open and input.

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So I've been referring to these as functions, but these are actually function calls.

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So here we call the function.

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The function itself is written somewhere else.

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So just like here, this is not the function.

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This is the function call.

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The function itself is written in here.

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So that's the instructions on what this function does.

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Now, where are these functions written?

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Well, if you are on patch or you can right click here and you can go to go to and go to implementations.

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If you are in another ID, then just watch the video.

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You don't have to perform the same.

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So just see what I'll explain here.

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So by going there, this will take you to the function definition for the input function.

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So again, we can try another function such as open right click go to.

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Implementations.

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It takes you again to the definition of the open function.

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If I press this asterisk, it will expand the definition.

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So this is just comments.

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This is not code.

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It's the description of the function, which is actually what you get when you do help open in the command

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line.

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So if you do help open, you actually see the same description

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and that is referred to as a docstring.

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So it's the documentation of the function.

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And then down here you have paths so you don't actually see the code here, you only see this docstring

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and.

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Pass.

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And that is because all these functions so open and input, these were written in C language.

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So C is the language.

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Used to ride the interpreter of Python.

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Remember when we installed the interpreter from Python dot org?

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That is a program which interprets Python language and that program is written in C language.

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So all these functions are actually written in C language and we cannot see the code.

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You'd ask, Well, what is this then?

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Well, this is just a fake representation of the function and this is done by Python, so other IDs

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may not offer this feature.

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So this is not so much useful.

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Just wanted to show you that these are actually written somewhere.

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In this case, it's in C language.

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You'll see that other functions which will use later in the course are written in Python actually,

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but not these main functions.

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So let's close the console.

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And next, I want to talk about variable scope, to be exact.

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So if I was to open a new python file here.

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Just give it a name, whatever you like, and create a new function.

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Such as?

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Greet.

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This function has a variable which has a value.

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Let's say hello.

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And then maybe another variable you message equal to message thought.

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Capitalize and return new message.

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Meanwhile, I would like to right click here and keep our program open.

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Here on the right.

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So maybe you want to see that, too.

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To compare the functions.

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So both functions have a def keyword and return statement as well.

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The syntax is the same.

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They both have a variable declaration.

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In the body.

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This one has two variables here, here and here.

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So now I can call this function.

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Let's say greeting is equal to greet and then print out greeting.

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And we need to make one more break line there for being consistent with recommendations of style and

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run the code.

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Yeah, run test.

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And that's the output.

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Hello with Capital H.

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So what's happened is that when the function is called, then this is returned.

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And what is the value of that?

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Well.

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So here message gets this halo string and then the value of new message will be Halo with H capitalized.

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And so the function returns Halo capitalized.

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Therefore the value of this is going to be Halo.

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And that value is stored in the variable greeting here.

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And then we print out that value.

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So we get.

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Hello.

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So far, so good.

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What happened to though, if we print new message and this is where we talk about variable scope.

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So if we run this program, we're going to get an error.

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It says Name, error, name.

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New message is not defined.

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How comes since we define it here?

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Well, that's because this variable.

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Is only valid inside the grid function, so its scope is local.

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That's why you cannot access that variable from here.

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You'd ask, Why is that?

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Well, let's take an example.

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Let's say you go to a cafe and you go to the bartender and you order a coffee.

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You just say, I want a coffee.

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So these words are the function call.

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You just call the function.

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The function itself is the process of making the coffee.

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So the codes of the function, the elements of the function definition are maybe coffee beans.

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It's the machine, the water, the interactions between all these.

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And it's not a good idea for you as a client to have access to these elements which make up the function,

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the coffee process.

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So the function is the coffee making process.

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So all you get as a client is the coffee.

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That's what we get also from this return.

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Statement.

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So the written statement makes available only one variable, the output variable that you want the function

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to produce in our coffee example, that would be the actual coffee cup with the coffee inside.

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So that is the output.

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Likewise, you cannot access the message variable.

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So every variable in here is.

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Not accessible.

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So you can only access what is returned and you can access it by calling the function, not by referring

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to the variable name itself.

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That's an internal name inside the process.

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Now back to our app.

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We see I have this line here and this is saying that this name shadows to dos from alter scope.

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Alter scope is.

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The variables which are declared outside functions.

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So in this program here, that would be, for example, greeting.

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This is a variable in the outer scope of the program.

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So the problem here in our app, which is not a problem, actually it's a recommendation, but we can

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fix it.

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The thing is that you we have this variable with this name here, and it's not a good idea to have this

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variable with the same name.

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So what we can do is we can change the name to this variable, let's say to this local.

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And then of course.

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Yea, be careful.

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You have to return that same variable.

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Right?

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And so now what we have is we create the local variable in here, which will hold this list from to

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dos.

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And then we return the value of that.

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Therefore, now this other variable, which is.

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Not a local variable.

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It's a global variable.

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So the opposite of local is global.

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So this variable now will be equal to the value that this variable produces in the function here.

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We have the same problem with file.

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So this is also complaining that we have another variable, a global variable with name file, so we

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can convert this to local as well and also reflect that here.

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So be careful, file locally or file local there.

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So that's the final code.

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Let's fix this complaint as well.

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I can reformat the file and that will add a new break line.

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So as I said, it's good to have two break lines between functions and other parts of the code.

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So thanks a lot for following this and I hope it makes sense.

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We will be using functions over and over again throughout the course, so let's keep going.