All language subtitles for 1. Creating Your Own Functions

Earlier in the course we were looking for a mean or an average function that Python might have to allow
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us to make the average of the items of the numbers of a list.
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And we found out that Python has some more fundamental functions such as len to get the number of items
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of a list and sum to make the sum of the numbers of the list but it doesn't have a mean function.
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So we ended up doing this.
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So we had this list we calculated the sum using the sum function calculated the length.
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So that number of items using the lend function and then did the division between sum and len to get them
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mean.
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However the fact that Python doesn't have a mean function doesn't mean we cannot create our own mean
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function.
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So what we are we going to do now is we're going to create our own mean function. To create functions
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in Python you use the def keywords and then pick a name for your function.
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The name has to follow the same naming rules as with variables so it cannot start with the number,
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it cannot start with a math operator.
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So let's say you mean, then you use round parentheses and inside those and the first line ends with
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colon and inside the parentheses optionally you can input some parameters of your function.
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Parameters in other words is the input that your function will get.
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And that will process that's input and that will produce an an output, the input
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in our case it would be a list.
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However here you have to give a name following
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again the same rules as with variables.
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Let's say mylist.
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Okay, then you want to press enter there and you see that's automatically Visual Studio code,
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so the editor indented the next line with four spaces.
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One,, two, three, four.
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So that's what the syntax requires that after the first line.
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So after you have defined what name you want for your function and what parameters you want, you
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have to indent the next two lines.
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So either your editor will do it automatically,
but if it doesn't do it one, two, three, four.
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Just indents four spaces in there.
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You can also make it with one space, so just one space would also work plus it's recommended to use
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four spaces, everyone uses for spaces because it's more visible the indented code.
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So it will make your code more readable and then you can start to calculate the mean, let's say the_mean.
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I'm going to call it like that so I don't confuse it with the function name, that would be the sum of my
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list divided by the length of my list and next.
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What we want to do we want to use the return keyword
and that is followed by what you want to return.
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Well we want to return the mean.
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Now save that, and let me open a terminal I want to execute these scripts now.
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So let me show you what will happen.
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Nothing will actually happen.
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Nothing happens because what Python does is it just reads this definition.
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This is like a blueprint. You have declared what your functions should do.
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But you didn't tell Python to execute your function.
To execute your function
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you want to call the function,
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pass an input value for that parameter that you defined, and also you want to print out, print open parentheses
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there, close parentheses there, save don't forget, and execute.
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That's the mean of those three numbers, so like that
I can easily try this out with many input values just
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like that I don't have to rewrite anything in here.
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That's the beauty of a function.
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So you can see the similarity here.
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Just like we use the sum function, we are using the mean function in the same way.
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So sum gets this input value, mean gets to this input value.
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This happens to be a variable.
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This happens to be a direct value, but it doesn't matter.
If you check the type of mean, type mean, and
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type of sum what you're going to see is that mean is a function.
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So that was printed out, this was printed out by this,
so mean is a function and sum is again a function,
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but it's a built in function so it's built in in the Python interpreter, It was designed by the python
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core development team and this function was designed by me, the programmer.
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So that is how you create and call a function in Python.