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Instructor: It's time to talk about functions.

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This is where things get really interesting.

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Because up until now we've seen functions, right?

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Functions where things like print, like list, Boolean,

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we even saw the input function to get the input

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of whatever the user types.

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And those allowed us to perform actions on our data types.

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But the true power comes when we can start

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creating our own functions.

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That's right.

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We're not limited to whatever Python gives us.

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We're able to create our own functions

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and use them in our programs.

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So let's learn how to do that.

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The way we create a function in Python.

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And functions, by the way,

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exist in all programming languages.

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They're very, very important.

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If we do def,

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that lets the Python interpreter know

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that we're about to define a function.

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Def is short for define.

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So the interpreter is gonna say,

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all right, they're about to define a function.

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What's the function going to be?

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Well, we can create whatever we want.

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We use the same naming case as we do with variables

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to define our functions.

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So let's create a function.

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Again, we'll call it, say_hello.

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Just a variable that I created,

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but this time it's a function

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because we use the def keyword.

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Now in here, in say_hello,

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we also add the brackets to let the interpreter know

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that this is something that we're gonna take action on,

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or this is going to perform an action on a data type.

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We use the colon and then within this block of code

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we can say, print('hellllooooo').

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And that is a function.

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If I click run, hmm, nothing happened.

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Why is that?

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Well, we've created the say_hello function.

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We've defined it, and now it's living somewhere

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in memory on our machine.

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However, in order to use a function, remember,

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just like we use the print function,

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we have to call it with the brackets.

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So after we define a function, we say, say_hello.

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And did you notice how my repple, as soon as I said, say,

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actually gives me this, say_hello command,

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because I've created it and you see this purple box,

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which shows that it's a function,

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well, we now have it available for us to use

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just like we had the print function available for us to use.

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So if I do say_hello and run it with the brackets,

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and I click run, I get, hellllooooo, how cool is that?

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By the way, what happens if I run it without the brackets

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and I click run?

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Nothing happens.

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Because remember, in order for us to take an action,

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we have to let the interpreter know,

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hey, I want to run say_hello.

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Now the reason functions are so powerful

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is because of the principle that we've talked about, right?

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The idea of DRY, which stands for Do not Repeat Yourself.

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Functions are really, really useful

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when you have things that you wanna do over and over.

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For example, the print function, we've used it a lot.

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Imagine if we had to code that ourselves every single time

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and say what print function does.

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Luckily, Python gives us print

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because it's such a useful tool,

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but if in our program, we wanna say hello multiple times,

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and you can imagine this actually being a lot

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more complicated, maybe 10 lines of code.

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Instead of writing those 10 lines of code over and over,

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I can just define it as a function

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and use it anywhere I want in my program.

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For example, remember this,

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where we had a picture and we printed a Christmas tree?

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Imagine if we wanted to run this multiple times.

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Well, in order to do that,

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I would copy and paste this code and then add it again.

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And if I click run, I now have two Christmas trees.

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But that's, look at that 26 lines of code.

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I just copy and pasted the same thing over and over.

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With a function, we can do something like this.

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I can say, def show_tree():

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and we now have a function.

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But remember the indentation in Python, right?

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Indentation is important.

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We have the semi-colon.

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We're defining a function,

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so we have to create that code block inside saying,

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hey whatever's indented here, that's part of this function.

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So now that we have the show_tree, check this out.

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I can say show_tree, run it again.

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Maybe let's run it three times.

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If I click run, how cool is that?

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I'm able to do the same thing over and over

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by just calling this function.

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And that's the power of functions.

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Functions allow us to keep our code DRY.

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We don't repeat ourselves and reuse things

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that our machines can do over and over.

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And the beauty is that this stays in memory for us.

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Show_tree now means something to this program

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because we've created it.

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So we have our own custom action that we can take.

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Now, what happens if I move show_tree here to the top?

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Well, if I run this, I'll get an error, name error.

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Name show_tree is not defined.

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Why is that?

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Well, because our interpreter goes line by line.

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It first says, all right, picture equals to this value.

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And then it goes to line 11 and says, run show_tree.

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But we haven't defined show_tree yet.

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So the Python interpreter is going to error out and say,

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hey, I have no idea what show_tree is,

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what are you talking about?

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Instead, with a function,

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we need to make sure that we define the functions

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at the beginning.

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So that Python interpreter says,

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all right, show_tree now means this.

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I'm not going to run it.

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I'm not going to use it yet.

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I'm just gonna keep it in memory.

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And when I finally come across show_tree,

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I'll know that it means something.

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I'm going to grab it from memory using show_tree

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and then I'm going to run it using the brackets.

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For example, if I do print(show_tree) without anything

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without the brackets,

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if I click run,

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you see that I get function show_tree at this location.

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This is just the location in memory.

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This is the bookshelf where we store

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that show_tree function.

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Very, very cool.

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And functions are an important powerful concept

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in programming.

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And in the next video,

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we're gonna extend this and explore this a little bit more.

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I'll see you in the next one.

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Bye, bye.