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Welcome back everyone in this lecture we're going to finish off this section by talking about functions

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and then right after that we'll have some exercises for you.

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Now creating clean repeatable code is a key part of becoming an effective programmer.

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Functions allow us to create blocks of code that can be easily executed many times without needing to

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constantly rewrite the entire block of code over and over again.

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Basically if you have a chunk of code that you intend on reusing more than once it probably makes sense

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to assign it to a function that we just do a simple function call instead of writing all that code over

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and over again.

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So let's see the actual syntax of a Python function.

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We start off with the DPF key word that basically tells Python that what you're about to write is the

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name of the function.

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Then you have the name of your function which you can call whatever you want and you'd start with name

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of function here with underscores.

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Keep in mind by convention names of functions should be lower case than we have open and closed parentheses

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a colon and then a block of code.

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Right below that block of code you could optionally have a documentation string and let's separate out

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by two sets of triple quotes and that just explains the function.

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In case you want someone else to be able to view your documentation string and then you'll see we have

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something like.

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PRINT Hello as the actual code to be executed.

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So in your actual Python script or at the command line if you were to call name a function over here

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at the bottom in purple and then open close princes when you run that you would get back.

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Hello.

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So is the simplest function possible basically just printing something out for you.

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Now often you're going to want your functions to accept parameters or arguments.

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Here we want to accept a name parameter and then we're going to say hello plus that name essentially

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concatenate that name to Hello.

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So what this actually looks like when you execute it is a name a function and you passing in the string

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Jose and the output there is then.

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Hello Jose.

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Just concatenating Hello whatever name was provided and notice here the name variable is something you

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could choose yourself just like you could choose a temporary variable.

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When using for loops.

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Now typically we're not just printing things with functions.

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Instead we're going to be using the return keyword to send back the result of the function instead of

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just printing it out.

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Return allows us to assign the output of the function to a new variable.

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So here we have another really simple function.

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It's called DFA.

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And then we say add underscore function.

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So this functions don't add function and it takes two parameters.

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Number one and number two.

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So it expects these to be numbers and then it's going to return number one plus num to notice a difference

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here.

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It's not printing the results.

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Instead it's returning the results.

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That way you can actually save these results to a variable.

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So I can say the result is equal to add underscore function one comma to and then you can print that

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result and you get back the number three OK let's explore more examples of using functions by heading

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over to the Adam text editor.

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Let's get started by going over the basic syntax of a Python function.

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We start by using the DFA keyword and you notice there is actually autocomplete here.

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So you could hit enter and it will autocomplete this function for you but right now I want to go through

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each of these pieces one by one.

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So you have the DFI keyword and then you can provide the name of your function.

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So this will typically be lowercase and use snake casing.

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For example we'll say something like.

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Reports underscore person then you're going to have open and close parentheses.

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And later I'll show you how you can add arguments that this right now will assume no arguments for this

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function.

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Then you say colon.

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And then here you're going to actually execute a block of code every time this report underscore person

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function is called.

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So here I'm just going to say print reporting person.

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Now let's actually run this function.

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So over here notice in the station I'm already outside of this function and the say report person open

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and close print sees to actually run them.

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So get noticed in the station.

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This is the function that self report person is now outside of the.

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Now I'm going to run this code and as I expand this I get back reporting person as my output.

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Now what happens if I forget these parentheses.

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And I've run this I don't get any output.

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That's because they actually execute the report person function.

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You're going to need those open and close parentheses.

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So keep that in mind as you're working with Python and functions OK.

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So right now our argument or excuse me our function is pretty simple.

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It's not accepting any arguments.

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So if we want to accept arguments we just need to give it an argument name of our choosing such as name

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and then we need to do something with it here.

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So let's say instead of reporting person now we're going to say reporting and I'm going to concatenate

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this with name.

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So what happens if I try running report person again with no additional arguments I'm going to get an

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error and the error says hey report person is missing.

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One required positional argument called name which is basically Python trying to tell you Hey you forgot

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to provide this argument that this function is expecting.

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So let's provide it one will say reporting Cindy will save that.

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Run this again and now it says reporting Cindy.

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So the function did is it took in your argument for name and then it went ahead and said print's reporting

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and it concatenated that name that you provided with reporting.

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Now let's imagine that sometimes you want your user to have some sort of default argument value.

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That's actually quite easy.

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You can just say name is equal to and then whatever default argument you want.

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So maybe you want blank to be the default.

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So I'll save that.

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And now notice what happens if I provide Sindy the nothing changes.

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I still see reporting Sindee but for some reason I don't provide that argument.

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Now I no longer get an error.

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Instead I go to the default which is reporting blank.

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So if you ever want to have a default value for your argument you can just define that default right

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here in the actual function definition.

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You'll say report function the name of your argument as equal to and then whatever the default value

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launches.

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And keep in mind this parameter or argument name it can really be up to you.

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So just to make that really clear I could say Jeli is equal to blank.

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And instead of saying plus name I can just say.

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Plus Jeli and Soilwork the exact same way.

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Reporting blank.

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So these parameter names are totally up to you just like when they were learning about four loops and

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you could choose any parameter name there.

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Now something I really want to make clear is so far we've just been printing out stuff when we call

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our functions.

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And typically that's not what we're going to be doing.

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Instead we're going to be returning variables back.

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So I going to delete all of this and create a new function we're going to say call this function add

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some and I'm going to take someone to and then right now I'm going to print one plus two.

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So that means if I were to call add on two and four I should see six pop out at my command line and

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there it is.

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That's adding two plus four.

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OK.

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So let's say I wanted to save this result for later and use it somewhere or some code for example.

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I want to do the following.

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I would say result is equal to add.

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And then later on I wanted to print results plus 10.

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So this is a mistake and something Beginners often do they just use prints inside of the functions and

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then try to save the result of a print.

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But that's actually not going to work.

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So let's run this and see what happens.

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So notice we get a traceback.

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First off I was able to get that initial 6 out.

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So I did print out two plus four online for one actually called add underscore what's not working is

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me saving this result because this result is actually a nun because print isn't returning anything it's

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just shooting something out to the command line.

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So you can see it.

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So when I scroll down here I may view my whole traceback which is just the error.

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It says Hey on line 6 on print's result plus 10.

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So right here.

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Print result plus then there's an unsupported operation plus for none type an integer because results

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right here.

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That's actually none.

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In fact we can prove it to ourselves by deleting plus 10 saving that run this again and I see none.

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So notice again what's happening here.

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Print isn't returning anything.

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You can't actually save the result of some sort of Printz function.

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So what do we do if we actually want to use this result later on in our code which is typically the

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way your Python programs are going to work.

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In that case we need to use the return keyword so we use the return keyword.

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And this is going to allow us to actually save whatever this function does.

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As a result.

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So now instead of printing out the numbers Ondoy to return that.

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So going to say add 2 and 4.

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Save that as a result and then print that result.

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But let's see what happens if I don't print the result first.

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So I run this and notice I don't get back any sort of result.

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However there were no errors.

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It's only when I print the results that actually get to see it.

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Now I get the see.

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So again the return key word what it's doing is it's adding someone and I'm to here.

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So two and four.

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And then saving it to this result where they can do stuff with it so that it can print results and if

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it wanted to I could then print result plus 10.

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So now when I run this I get back 16 again.

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That's the key difference between using something like print and something like Return Return is going

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to allow you to actually save your results of your function to variables to use later on in your code.

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So typically you're pretty much always using a return with functions instead of print.

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So keep that in mind this is a really common mistake for beginners to make.

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OK we're going to stop here for part one.

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Part two of our functions discussion will focus on solving problems with functions.

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Also cover some useful operators.

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So we'll see you in part two and the very next lecture.