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Welcome back, everyone, to part four of our discussion on object oriented programming, or we're talking

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about inheritance.

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Often Jenga will have classes with many useful methods and attributes already defined for us.

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This is kind of the whole point of using Django is that they've already done the hard work of defining

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many different Python files, modules, packages and classes within them.

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What we would like to do in order to create our own custom sites is create our own derived variations

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of those classes that Django provides for us through the use of something known as inheritance.

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Inheritance allows us to pass in an existing class into our new custom class, which means we get to

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inherit the use of the methods already defined within the parent class, including the in its method.

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This saves us a lot of time, and it's going to be a very common syntax in Django that we're going to

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discover as we use Django more and more.

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So what we're going to do in this case is explore a couple of ideas around inheritance.

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First, we're going to discover how to create a parent's class.

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This is officially known as a base class.

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Then we'll discover how to inherit from that parent class to create a child class officially known as

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a derived class.

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Then we're going to discuss how to use the inherited methods within the derived class, how to add new

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methods, and then how to overwrite derived methods.

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Let's get started.

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All right.

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Here I am inside our code editor.

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I'm going to create a very simple class, which is called a person class.

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And we're going to say that this person class, when you actually create a new person, the init method

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is going to require a first name and last name to create a new person.

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So we're going to go ahead and say, here it takes itself.

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And then it takes on a first name and it takes in a last name.

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And we're going to sign those provided two attributes by the same name.

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So.

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The first name is equal to the provided first name and then self thought last name is equal to the provided

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last name.

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Then we're going to do is add in a couple of simple methods.

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One method is going to be super simple.

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It's just going to say hello.

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And self.

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And this is just literally going to print out.

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Hello.

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Then I'll have a slightly more complex method here, which is going to be called Report Peasant Self

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here and report.

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It's just going to print out this person and who they are.

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So using an f string literal here, I'm going to say I am.

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And then in curly braces, I can say self that first name.

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SpaceX, another set of curly braces to insert a variable here and then self-taught last name.

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Don't forget to use self here.

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Otherwise, you won't actually be able to connect to the attributes the fine if you just say first name

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and last name down here.

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It would expect you to pass in first name and last name.

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It's a report.

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So this is a pretty simple class again.

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It's just the person who has a first name and last name, and then this person class has two methods

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associated with it.

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The ability to print out hello and the ability to report who they are.

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Notice both of these are simple print function calls.

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Typically, we'd be using something like return.

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But for simplicity here, when I'm running these files, I'll just go ahead and print out the results.

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So let's create an instance of a person I will say X is equal to an instance of person that requires

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a first name and last name.

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So let's have that be John Smith.

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And then what I'm going to do is say X, that's.

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Hello.

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Open close parentheses.

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And then I'm going to run this code.

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Oops!

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And I get to see the output here is hello.

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And let's try a report to make sure that's also working.

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Open close parentheses, because it's a method.

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Run that.

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Now I see.

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I am John Smith.

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So clearly, I'm able to, as we've discussed, create an instance of the person class, the find them

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of a first name and last name and then use methods either just hello or methods that actually use the

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attributes similar to the way we calculated the area and perimeter of a circle.

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Now let's imagine I'm going to now create a new class, which is kind of similar to a person, but I

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also wanted to have other functionalities through different method calls.

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What I can do is I can inherit some of the methods from the person class or really all of them.

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So what I'm going to do here is I'm actually going to use Visual Studio Code Editor to collapse this

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person class.

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Just keep in mind, you can see I've collapsed based off the line numbers here on the left just to save

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space on the screen.

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Later on, I to show you how to import from other Python files so you don't have to have some huge Python

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file.

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Now I'm going to create a new class and this is going to be an agent class.

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Agents are going to be really similar to a person class.

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However, we want them to actually have this method where if I give them the correct passcode, they'll

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report back, Hey, I'm a secret agent.

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So we're to say class agent and we are going to inherit a person class.

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And not how we do that, I simply pass in person.

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Also, keep in mind, I'm not passing in person, open close parentheses, just saying person, not

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like this.

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OK, so we pass in person, and the act of passing in an instance of the person class now means I will

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have access to the methods of the person class within agent, which is super useful because now I just

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need to worry about the methods that I care about that are specific to agent.

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So let's go ahead and create a method here called reveal and reveal takes and self, but also takes

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in a passcode.

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And we'll say if the passcode is equal to one, two three.

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I will go ahead and print.

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I am a secret agent.

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Else, what can we do here?

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I can go ahead and print.

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I am.

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And then say, self-taught first name and self-taught last name, however, remember I already had a

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method really similar to that called report.

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So because I inherited person, I can actually now use report anywhere I want.

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I could say self-taught report to save myself some time.

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So I'm certain instead of this, I'm going to say else self-taught.

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Report.

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Open close parentheses notice report is not defined anywhere within agents.

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What it is, it's it's using the inherited person class to understand that self.

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That report means call person that report, which I have up here.

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OK, so I'm going to save this.

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And let's create first an instance of person just to show you that they're not really being mixed and

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matched upwards, they're only going through the inherited class.

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So if you create a person, you can say X that report run this python example that pie and forgot to

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define last name and first name, let's say again, John Smith here.

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Save that.

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OK, now let's go ahead and run this, OK, so I see I am John Smith.

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Now let's try to call reveal we should expect a problem because I'm trying to call reveal on an instance

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of person.

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However, reveal is only defined for agent, which does inherits from person.

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But this really only goes one way.

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This relationship is just agent can now use the stuff with in person, but person has no real association

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with agent.

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So now if I were to save this, in fact, Visual Studio code tries to warn you by the fact it's not

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actually coloring this method.

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We should be able to see an error here.

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And it says Attribute error person object has no attribute called reveal, essentially warning you,

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hey, there's no method or attribute called reveal.

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Now let's try this by creating an agent so instead of person here.

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I'm going to call agents and an agent has to be constructed the same way a person does, which is to

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say it inherits its in its method to construct an agent with a first name and the last name.

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So I'm going to create an instance of agents called John Smith.

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And let's actually call hello on this agent, know how this actually should work, because the coloring

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is different here.

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And the most important thing to note here is that hello is not directly defined with an agent.

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That's a method agent is inheriting from person.

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So if I run this code, I see I get back.

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Hello.

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I am also able to run report, which is another method the fine of then person.

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And I get back.

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I am John Smith.

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Now what I'm going to do is use the method that is only available for agents, which is reveal, remember,

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reveal those require a passcode.

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If I just run it like this, I would expect an error saying, Hey, passcode not provided.

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So it says Hey, required positional argument passcode.

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Let's actually provide the correct passcode, which is reveal passing in one two three there.

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I run this ad out, says, Hey, I am a secret agent.

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OK, so just to be clear of what's going on here, I have the methods available inside my person class

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by passing in person class into agents.

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I have no access to all these methods and I can add on additional methods that I only want to be specifically

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associated with an instance of this child class, otherwise known as a derived class.

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Now, let's imagine I want to actually change what report does within agents.

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How do I do that?

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It's actually easy.

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You can simply overwrite the inherited methods by saying the F and in the name of the method you want

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to overwrite, you essentially redefine it like report.

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Let's say a secret agent should probably not just report back their name John Smith.

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Instead, we're going to say report for an agent is simply saying, I am here.

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Instead of saying I am John Smith.

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So now when we say reveal and we were passing the wrong passcode, we would expect report to simply

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say, I am here for the case of an agent.

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So if I run this, I won't get anything different.

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He'll still say I'm a secret agent, but let's say the code is now, let's say one two three four.

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We i that in wrong.

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I run this and it simply says, Now I am here.

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It no longer says I am John Smith.

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And in fact, if we say x dot and simply call report now, it just says I am here, and that's a way

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you can overwrite old methods in your child or derived class.

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You just call that method and then overwrite it so you have the ability to use all these methods.

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And then if you want it to, you can add new methods and overwrite the existing methods.

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The last thing I want to point out here is there may be some instances where you actually want to add

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in more attributes beyond first name and last name.

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We ourselves won't really see this of jingo, but I do want you to be aware of the syntax for this.

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If in fact you do need to add in more attributes than what is provided for us with the parent class.

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You would essentially start overwriting the in its method itself, you would say underscore, underscore

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in its underscore, underscore and self and then say first name, last name.

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And let's imagine for some reason, an agent also needs a code name.

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Now I still am inheriting person, so if I want to create an instance a person and then just add on

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these attributes, the syntax for this is to say person dot underscore underscore in its underscore,

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underscore and then pass in self.

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First name, last name, and then we tack on the other attributes that we added here, like code name,

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I would then say self code name is equal to code name.

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I should point out and be clear here, this isn't really something we do that often within Django,

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but it is possible.

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And the reason you're doing this is if you have more attributes that you yourself need to add beyond

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what's provided for you by the base class.

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So then when you define an agent, it would also ask you for a code name like Mr. X or something.

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And we can see what report is that actually shouldn't change anything.

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So we see still here I am John Smith.

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I think we said here report Oh, we actually didn't overwrite anymore.

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I deleted that.

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Let's actually just ask for its code name.

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So I'm going to print out the code name Prince X thought, let's say first name.

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And now let's also print out X thought.

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Codename.

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Save those changes, and now we see, OK, first name.

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Is this give ourselves a little more space so we can see the whole thing?

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First name is John and then codename was Mr. X.

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All right.

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So again, as a quick overview, you create the original class, have the init method.

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Any other methods you want that can use the functionality of the attributes.

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And once you inherit that class again, typically we're not going to be doing something like this,

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overwriting it.

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So instead, typically what you're going to see is maybe we add some more methods to it and we have

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access to those methods within the child class.

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All right.

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That's it for this lecture.

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I'll see you at the next one.