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When it comes to using classes in Python, and in most programming

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languages, actually, we can inherit other classes, which

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means we can use one class as a template for another class

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and automatically inherit the goodies.

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So we can actually really increase our reusability of any given class.

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Now, a good example of this would be a class for 'cats' and

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'dogs'. Now, cats and dogs are very different animals, but

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they also have a lot of similarities.

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For example, they both know how to speak.

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They say something.

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One says "Meow", one says, "Woof".

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They have fur, most of them have fur.

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The fur usually has some sort of color.

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There are different sub species of cats and different variations of dogs.

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And these are data points that sort of come together to create

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a base class.

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So what we're going to do, is create a class called 'Animal',

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and in this class we need to instantiate this class with

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specific attributes.

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So let's give an animal a 'name'.

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Let's say it always needs to have a name, just so, you know,

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we can call it something.

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And let's just assume we're using pets, not wild animals

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here. And every animal needs a 'weight', because if it is a

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thing, it probably has some weight.

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So 'self.name =', whatever the 'name' is going to be, and

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'self.weight =', whatever the 'weight' is going to be.

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Now, every animal says something,

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so let's give this a 'speak' method.

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And this is just going to say, "What does the animal say?"

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Because, I mean, it's a generic animal.

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We don't know which animal we're talking about here.

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We don't even know what the species is yet.

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So we just have to implement something very generic.

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So we have this generic 'Animal' class, and it comes with two

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attributes, a 'name', and a 'weight', and a 'speak' method.

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Now let's say we wanted to have a class for 'Cats'.

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Well, we could copy and paste this whole thing.

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Copy and paste, and the cat says, "Meow", and we could do that.

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There's nothing really technically wrong with that,

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but there is a nicer way. Because we're already using the

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entire '__init__' method,

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it's literally the exact same.

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What we can do is let's go ahead and get rid of this,

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and let's just inherit from 'Animal'.

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This 'Animal' matches the name right there.

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[no audio]

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And so we use 'class', our new class name, an opening parentheses,

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the class that we want to extend,

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and if we had other classes, we could add other classes as well. We don't,

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so we only have the one to inherit from, the closing parentheses

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and then a ':', colon.

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Now, honestly, at this point in time, we could just say 'pass',

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and it's still going to ask us for a 'name' and a 'weight',

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and when we do 'speak', it's going to say, "What does the animal say?"

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So as an example, let's go ahead and say, 'henry =',

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actually, I don't want that to be 'Cats', I want it to be 'Cat'.

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'henry = Cat', and it's going to ask for a 'name'.

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So the name is going to be "Henry", and his 'weight' is

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going to be '9 lbs'.

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And we have 'henry'.

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We can actually even see that it is technically a 'Cat' class.

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Now, in our 'Cat' class, we did nothing.

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We just basically said, "Hey, inherit from 'Animal', but just

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call it something different".

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So this is actually a pointless example, which we'll build on in just a moment,

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but this also is a good example in the sense that we have

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all the '__init__' stuff within 'Cat' now.

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And as an example, let's go ahead and get rid of this and just

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try to instantiate the 'Cat' class without 'name', or 'weight',

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and see the error that comes up.

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We get a 'TypeError: missing 2 required

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positional arguments: 'name' and 'weight'.

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So let's go ahead,

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let's undo that, and let's run 'henry.speak()'.

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And 'henry' says, "What does the animal say?"

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As far as I know, my little cat cannot speak human.

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So we need to change that.

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So let's go ahead and get rid of this 'pass', and let's overwrite

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'speak'. Because 'Cat' inherits from 'Animal', it automatically

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got the 'speak' method.

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Now, that 'speak' method is not quite correct.

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It's cool that we inherited it, but we needed to actually change it.

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So we're going to overwrite this.

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And it's going to say, "Meooowwww".

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[no audio]

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And when we rerun each cell, 'henry' now says, "Meooowwww".

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Now to show you the difference here, let's instantiate a

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regular 'Animal' class.

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So the 'animal = Animal' class, and the 'name' is

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"Ani", and the 'weight' is "100 lbs", because whatever animal

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this is, it's big one. 'animal.', 'Tab', 'speak'.

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And it says, "What does the animal say?"

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So this actually has nothing to do with a 'Cat'

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right now. The 'Cat' overwrote 'speak'.

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It inherited from 'Animals',

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so it originally said, "What does the animal say?"

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And then we said, "No. No-No, that's not right.

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Change it. Change it to, "Meooowwww".

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But the 'Animal' class inherits from nothing.

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It is just a base class,

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and so by default, it has nothing to do with 'Cat'. Does not

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really know that it exists.

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And by default, it's 'speak' method is going to say, "What does the animal say?"

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So now we're actually taking this, and applying this part

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inside of it, and overwriting the 'speak' method.

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Now, why is this useful?

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Well, because we can do the same thing with a 'Dog' class.

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'Dog' is going to inherit from an 'Animal', and we can do 'def

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speak(): print("Woof woof")'.

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[no audio]

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And now we can say there is a dog, his name is 'veto'. Dog

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his 'name' is "Veto", and his 'weight' is about "120 lbs".

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Sure, we'll use an integer there.

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'veto.', hit 'Tab', 'speak',

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"Woof woof".

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And so now what we've done, is we've really reused some of

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our code here. We said, 'Animal' is just a base template.

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We don't actually need to use it.

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But because a 'Cat' and a 'Dog' do have similar things, such as

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a 'name' and a 'weight', we don't have to write this inside

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of the 'Cat' class.

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We don't have to write it in there.

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We also don't have to write inside of the 'Dog' class either.

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And all we have to do is overwrite the things that we want to change.

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It's a lot like cloning, science fiction cloning

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anyways, where you have the person that you want to clone,

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and then you're going to clone that person, and then

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after that person is cloned, they're going to have different life

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experiences. And because they have different life experiences,

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they are going to eventually become their own

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people. They're going to be a little bit different, but they will

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still have certain things that are fundamentally the same.

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And so this is basically how we clone, in a sense, classes in Python.

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Now, what I would like you to do, is create a base class, and

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then inherit that class, and instantiate the inherited class

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with something that you've overwritten.

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Much like what I've done in this video.