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Hi guys, and welcome back!

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In this video, we're going to start talking

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about class inheritance.

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Inheritance allows one class to take some methods

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and properties from another class.

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Let's have a look.

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Imagine you've got a class called Device,

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such as a USB device or something like that.

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And you want it to represent just anything

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you might plug into your computer.

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So I'm going to define my init method,

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and then we're going to be able to give these devices names

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and how they are connected, such as by a USB.

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So I'll say, connected by.

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Then we'll say self.name=name,

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self.connected by=connected by,

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and we're also going to assume

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that when you create a device object,

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that is going to represent a connected device,

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something that is already connected to your computer.

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So we'll say self.connecte=true.

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We are going to assume that it's connected

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and we don't have to pass in a parameter for that.

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We can just say true here.

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Then, we will type str and we're gonna say return f

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device self.name, and we're going to put !r

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and we're gonna say self.connected_by.

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The exclamation mark r,

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calls the repr method of self.name,

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so that it shows up as having the quotes already.

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So this is just a little bit nicer

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than putting the quotes in yourself.

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Then we're also going to add a disconnect method,

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which just says self.connected=false

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and the prints out disconnected.

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So you have this class device here.

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This device doesn't do much at the moment,

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but you could do something like printer=device

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and call it Printer and say it's connected via USB,

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and then you can print it out.

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Finally, you can also disconnect it.

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All right, let's press play.

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And you can see that you get Device, Printer, USB,

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and then disconnected, so very good.

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Now, let's say you want to add functionality to this class

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that allows it to print things out.

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For example, it can print a certain number of pages,

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or something like that.

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This class here is not a printer, this is a device.

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Could be a printer, but it could also be a webcam,

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or a microphone, it could be anything.

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So we don't want to add

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printer specific functionality to this class.

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You could rename the class and turn it into a Printer class,

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but you can also create another class

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that uses all of these methods and has more,

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and that is where inheritance comes in.

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So we're going to create a printer class now,

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and we're going to do something new,

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which is put a pair of brackets after it,

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and here we're going to put Device.

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And what this tells Python

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is that you're creating a new class called Printer

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and it inherits from Device, which means that it will

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essentially copy and paste all of these methods.

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It won't quite do that,

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but you will be able to use these methods from this class.

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So, let's start by defining an init method.

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Now, this printer object is going to need a name,

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just like a device, e-connected by just like the device,

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and it's also going to have a capacity.

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How many pages left of ink?

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Then, what we're going to do is,

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instead of doing self.name=name,

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self.connect by=connected by, self.connected=true.

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Instead of doing that,

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which as you can see is a copy paste of this method here,

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we can just call the parent class device init method.

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So, what to do in Python 3, is to do super.init.

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So, super and then a pair of brackets,

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and what this does in Python

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is it gets the super class, or the parent class,

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which in this case is Device,

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and it's going to call the init method of that super class..

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Of course, the init-method needs a name and a connected by,

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so we would pass a name and connected by.

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These are two parameters.

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We still have capacity.

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So, we will say self.capacity=capacity

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and self.remaining pages=capacity.

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Why am I doing two properties with the same value?

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The capacity is the maximum amount of pages

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that a printer could potentially print, if it had full ink.

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Whereas the remaining pages is how many are left

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after printing some.

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So, you can think of this as maximum capacity

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and this as current capacity.

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Then, we are going to define an str method,

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and here we're going to return

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the super classes str method.

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And then we're also going to put in self.remaining pages,

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pages remaining, something like that.

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Of course, do remember to put in your quotation marks.

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You will need them and your f-string.

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So now, we're going to print out this entire thing,

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Device printer, USB,

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and then we're gonna say remaining pages 100, or something.

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And finally, we're going to put

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in our printer specific method, which is the print method.

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Here, we're going to be able

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to print a certain number of pages.

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So we will say that if not self.connected,

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then we will print your printer is not connected.

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Otherwise, we're going to say print printing pages that,

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and then self.remaining pages-=pages.

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Of course, we do have to return here,

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so that we exit the function if we're not connected.

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Otherwise, we will print this and then that,

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which is not what we want.

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So, now we have our two classes;

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the printer and the device class.

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You can still use it, Device class on its own if you want.

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It's in no way related to the Printer class,

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so the Device class knows nothing about Printer.

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But the Printer class is related to the Device.

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It knows about it, it is the child of it.

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So, you can now create a printer and say

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Printer, printer, USB, and say 500 pages of capacity.

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And then, you can do printer.print and print 20 pages out,

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and then you can print your printer.

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This is all a bit confusing

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because I've gone for the name printer,

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which may not be the best one, but oh, well.

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So, we forgot an f-string here.

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Make sure to add that in.

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Then, we're gonna run it again.

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So, you get Printing 20 pages,

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and then your Device, printer, USB, 480 pages remaining.

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Here, we've used inheritance to build out our methods,

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like our init methods and our str method

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by using the parent-class-methods, such as here and here.

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In addition, this printer

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can still use the disconnect method

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that was defined in the Device class.

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And what happens when you do printer.disconnect

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is that Python will try to look for the disconnect method

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inside the Printer class.

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But upon not finding it,

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it will try to look in the next parent class,

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which is Device, and it will find it there.

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The Device class actually inherits from another class,

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as well, which is the Default inheritance,

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and that is Python's object class.

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So, if you call a method that is not in the Printer

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or the Device class, Python will then go to the Object class

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and see if it's there.

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And if it's not there, then you'll get an error.

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So those are the three levels of calling

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that you will see when you use the Printer class.

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First, the printer will be checked.

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Then, the Device will be checked.

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Finally, the object will be checked,

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and if none of them has the method

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or the property you're trying to access,

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then you'll get an error.

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Let me just run that,

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and you can see that you get disconnected there.

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Of course, if you do printer.print, and you try to print

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30 after you've disconnected,

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then you will see your printer is not connected.

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Hopefully, all of this makes sense.

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Inheritance in Python is much simpler

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than in other programming languages.

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All that happens is there is a hierarchy for calling things.

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When you try to call a method,

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or access a property of a class,

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that just goes up the hierarchy, and that's it.

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You can always use the Super class within a class.

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So, you can access that class' methods,

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such as what we're doing here and here.

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Thanks for joining me in this video.

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I hope you've enjoyed it,

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and I'll see you in the next one.