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In the last lesson,

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we had a primer on functions so that we can understand the next step

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which is the Python decorator.

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So we've seen that functions can have functionality, inputs,

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and outputs, functions are first-class objects and can be passed around as arguments,

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functions can be nested in other functions,

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and finally, functions can be returned as the output from another function

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without needing to trigger it. With these four things in mind,

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we're finally ready to tackle the Python decorator.

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First of all,

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I'm actually going to create a Python decorator for you so that we can talk

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about how it works.

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Here's what a Python decorator function looks like.

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We started off by creating just a normal function.

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Let's say I call it my decorator_function.

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This function is going to take another function as an input.

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Now, inside the decorator_functionn, I'm going to nest a wrapper_function.

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And this wrapper_function is then going to trigger the actual function that was

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passed in to the decorator function.

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And at the end of calling all the lines in my decorator_function,

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I'm going to return my wrapper_function. But remember

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without the parentheses. Now,

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basically what we've done is we've created a decorator_function

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which can do some stuff and it could control the calling

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of the function that was passed in.

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Remember that a decorator_function is just a function that wraps another

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function and gives that function some additional functionality.

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That's quite a mouthful, but let's take a look at an example.

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Let's say that I wanted to create a simple function

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which I'll call say_hello.

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Now this function is going to be the simplest form of function.

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It has no inputs and it has no outputs

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and all it does is just going to print hello. Now,

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what if I don't want to run this immediately

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the moment when I hit run, what if I wanted to add a delay to this function?

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Well, one way of doing that would be importing the time module

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and then before we print hello,

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we can go ahead and run time.sleep

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and we can sleep for a prespecified number of seconds.

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So now when I call this method, say_hello,

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and when I run it, you'll see that immediately nothing happens.

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And then after a two second delay, you can see the print

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hello actually gets triggered. Now,

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what if I wanted to create a whole bunch of functions

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like say_bye and say_greeting.

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And I wanted to add a delay onto each of these. Well,

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then I would have to copy and paste this code into all three places.

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Now, this is a very simple example,

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but frequently we'll want to add some additional piece of functionality to a

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number of our functions. So this is where the decorator comes in handy.

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What we could do is before we trigger the function that's passed in to

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the decorator_function, we can add the delay.

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So now whenever we decorate a function with this particular decorator_function,

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then it's going to delay for two seconds before it runs the function.

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So let's give this decorator function a little bit more of a descriptive name.

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Let's call it a delay_decorator. Well,

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now what we can do is we can simply use an @ sign

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and we can call the delay_decorator in front of the methods

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which we want to delay. So we can add it to all three of them,

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or we can only add it to some of them.

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But now when I actually run my function, say_hello

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for example, you can see that this one

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because it's got the delay decorator

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it's going to wait for two seconds before it prints hello.

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But if I call say_greeting

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which is the one that doesn't have the decorator above it,

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then you can see it gets triggered immediately.

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So this is a really nice way of adding something that you could do before you

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run the function,

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Right?

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or adding some functionality which you can do after the function. Or

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alternatively, you can modify the function so maybe you wanna run it twice.

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So now when I say hello,

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it's going to not only delay by two seconds,

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but it's also going to run it twice.

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This syntax is what we saw when we were creating our Flask

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app. It's got the @ sign and it basically decorates the function below with a

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decorator function.

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So putting this function through a machine that could modify the function or add

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some functionality before or after. Coming back to our definition,

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a decorator function is simply a function which wraps another function and gives

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it some additional functionality or modifies the functionality.

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Now, coming back to this syntax, this @ sign, it's

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what's known as syntactic sugar.

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It's some syntax that you can write to make it easier to write an alternative

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line of code.

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So let's say we wanted to add the decorator to this say_greeting. Well,

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if instead of using this syntactic sugar with the @ sign

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and then the name of the function,

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we could also just call the name of the decorator and then we could pass in the

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name of our function, say_greeting,

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and then we would end up with the decorated function as the output from this

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method.

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And we can now call the decorated function with the parentheses.

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So now we've triggered this say_greeting function

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and we've passed it through the delay decorator.

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But it's much easier to see at a glance what's going on by using this @ sign

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because you can see now you can just trigger this method by its own name

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and because the @delay_decorator is on top of it,

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then it's going to go through that filter.

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I recommend spending a few minutes looking at this code

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which you can access at this URL which is in the course resources,

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and then trying to recreate this functionality from scratch based on your

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understanding of what we just talked about. And then once you're done,

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when we head back to our Flask application

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you can now see that pretty much all of the code is easily understood.

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We know what this __name__ refers to,,

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we know that this is a decorator

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which is going to make sure that we only trigger this function

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if the user's trying to access the URL

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that is the homepage/.

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If we wanted to create another function, let's call it say_bye.

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This function could also get a decorator

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and this decorator will check to see well, if somebody goes to the URL 

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/bye, well

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then we're going to be triggering this method. Effectively

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the Flask framework is the one that determines which of these methods to call

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and these methods are only called when the decorator says that it's appropriate to do

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so. Now, if I go ahead and run this code once more,

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and we go to the URL that is our server,

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so this we know is the home route or the homepage,

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but if we go to /bye

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then we go to the bye route and this method gets rendered.

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Now in order to make sure that you get a chance writing decorator functions

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yourself, if you head over to the next lesson,

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I've got a coding exercise for you to challenge you

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to create your own decorator functions.

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And it's going to be a really useful decorator function because it's going to

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measure the amount of time that it takes a particular function to run.

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So for all of that and more, I'll see you on the next lesson.