Create Reusable Components with Functions in Python

Man: Functions start with the keyword def followed by the function name. Then there's a set of parentheses, and you'll put any parameters inside the parentheses followed by a colon. The next line gets indented, and you write the code for your function here. We can say parent Heisenberg, and then we can execute that function just by calling the name.

To run that, I'll give it the Python command followed by the name of the file containing my code, and it prints out the Heisenberg.

You can use the return keyword to return the result of a function. In Python, functions always return something even if you don't. Let me show you what that means.

In our current function, we're not returning anything explicitly, so I can say fu is equal to say name, which will execute our function. Then parent fu will print the return result. If I run that again, it executes the function, runs the parent statement and then a return value is actually none. A Python function will return none if you don't tell it what to do otherwise.

Let's explicitly return something in our function. We'll start with the including parameter that we're going to call name, and then we'll just change our function to return name. When we call that, we need to include the parameter name that the function requires, so I'll include my name. When we execute that, the value of fu now is no longer none, but it's the value that we passed into the function.

One other thing that's considered good practice in Python functions is to include a doc string. A doc string is just a string that tells what the function does, so this returns the name passed in as a parameter. What that does is it builds into the Python help system documentation for your function.

You can see whenever I hover over this in visual code, it grabs that doc string, return and show the documentation I created for that function. That would work even if the function was in a different file. Or if you were using the Python console, you could get that using the Python DIR command followed by the name of the function.

The parameters that we have here, in this case, the one called name, are local to the scope of the function. Let me show you what that means. We'll do that by creating a new function. We'll create one called add, and we'll give it two parameters, num one, num two. We'll just return the sum of those. Now I'm going to create another instance or another variable called num one, and it's going to be equal to the return value of our functions.

Let's try that out and see what that looks like. It's probably going to help if we print that out to the console. So try that again. It returns three. You can see that the variable named num one inside the scope of the function doesn't conflict with the variable named num one outside the scope of the function.

It's good to know that that's not really recommended best practices because if you're reading this code, you can get confused very easily as to which num one's actually being referred to. But I just wanted to point out that the scope here keeps that from becoming a problem.

We can also have default arguments in here. We can set num one equal to a default of one and num two equal to a default of two. That allows us to call the function without having any parameters supplied at all.

One of the reasons you may want to do this is if you have a function that has many parameters, then you can specify the default. That's going to allow you to call those functions with calling fewer parameters especially if some of those are commonly the same thing.

We can use keywords for our argument names as well. I'm going to create another function here called Madlibs. It's going to have a required function called name, and then two optional parameters. The first is a noun that we're giving it a default value of shoes and then adjective that we're going to give a default value of red. We'll just concatenate all of that into a string that will return.

We'll provide the name parameter as the first argument to our format method, our adjective will come next, followed by the noun. We can create a variable called Madlib which is equal to our Madlibs function and will provide as the required argument my name and then print that out so you can see what that looks like real quick. The end result is it says, "Will has red shoes."

To specify that as a keyword argument, we have our name specified, and then I can say our noun and set that equal to a different value, and then I can specify also the adjective as well. Now it returns Will has black boots.

Here's why you would want to do keywords. I can take this, and I can supply those out of order or in any order that I want, and it still works.

Let me show you another way that this works because they are still positional arguments. I can supply the word boots and black. Now we haven't specified any keyword arguments, but it still works.

Let me show you what you can't do. I can't say a keyword argument and then return to positional arguments. So that's not going to work.

Once you start using the keyword arguments, you have to finish out the function with keyword arguments. We can fix that by specifying our keyword argument for the rest of the function. That works. Then just for completeness, I can show you that it works whenever we use them as all keyword arguments as well.