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Welcome back. Sometimes, when creating an inherited class,

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you want to create a method that calls the super classes method,

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but just does a little bit extra.

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Now let me explain an example.

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So, let's suppose that I have this pet class and inside of the pet class,

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I have a method, feed.

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Now suppose that I wanted to create something that inherited from the pet class.

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Let's suppose that I wanted to create a class dog.

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So I'm just going to draw out the beginnings of that class dog,

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and I want it to inherit from pet,

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so I'll say pet in parentheses.

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Now, let's suppose that whenever I fed my dog,

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I wanted it to both called this feed method,

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but then let's suppose that every dog instance also has a little extra message.

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So, let's suppose that a dog says Arf!

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Thanks, whenever pet is called on that dog.

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So, if I did feed itself,

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if I did this and then let's say that I print out Arf!

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Thanks!

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So, if I define my dog class this way,

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whenever I create a new instance of dog,

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and then call feed on that dog,

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then all that happens is Arf!

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Thanks gets printed out.

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Because when I define this feed method,

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I am overwriting the feed method in the parent class.

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So, this would never get called,

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at least not by default,

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but there is a way to call it.

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So, the way they call the feed method for

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the pet class in addition to

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printing out this little extra bit of code that I wanted to run.

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The way that I would do that is,

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I would say pet, capital P, pet.

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So, the name of the class,

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and then I would say.feed,

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and I would call this, on self.

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Now, notice that this is calling feed on the pet class.

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Normally, when we call feed,

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we create a new instance,

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so I might say D1 equals new dog,

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and normally I would say d1.feed,

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then I would pass in no arguments here because D1 is an instance.

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Here instead, I'm saying the name of the class directly,

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so I'm saying pet.feed,

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and since I haven't specified which pet instance I'm calling feed on,

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then I need to pass in that particular instance as the first argument to pet dog.

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If I tried to say something like self.feed,

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here then self.feed is just going refer to the dogs' feed method.

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So, again, the way that I'll create a feed method on a subclass,

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that has everything that the superclass feed method has,

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and more I'll cloud call

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the superclass feed method by saying the name of the superclass in this case pet,

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and then I'll say whatever method I want to call,

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and then I'll pass in that instance directly.

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So, in code this is what that might look like.

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So, here I have a dog class,

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and dog class inherits from pet,

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but it has its own feed method.

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Inside of that feed method,

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I first call pet dog feed,

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with whatever instance this feed method is being called on,

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and then I print out our thanks after the pet.

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dog feed method has been called.

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Now this also works for constructors.

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So, let's suppose that I have another subclass of pet called, bird.

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Let's suppose that the bird class has it's own constructor,

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but I want to call the pet classes constructor as well.

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Well, the way that I would do that,

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is I would say pet._init,

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and I would call it with whatever arguments I want to call.

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So, in this case, I'll pass in self,

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and then I'll pass in the name.

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What this is going to do is it's going to call the pet classes constructor,

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on this new instance bird.

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So, it's going to set name,

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hunger, boredom, and sounds.

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So, let's do some multiple choice questions.

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Okay. So, this question asks,

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what would print one print out b1.sounds is run?

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So, B1, here is an instance of bird,

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and if I'm calling b1.sounds,

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then you'll see that's instance variable,

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that gets set right here.

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So, here we're setting self.sound equals to a copy of self.sounds.

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By default every pet has sounds list of [inaudible].

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You'll see that the bird class has a class variable sounds set to chirp.

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So, when I call pet._init,

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when it said self.sounds equal the self.sounds is copy self.sounds,

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what that's going to do because B1 is a bird,

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is it's going to create a copy of this list of sounds,

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and so b1.sounds ends up being chirp.

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So again, the reason that it's creating a copy of this list of sounds,

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is because we have the superclass pet,

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then we have a subclass bird.

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So, every bird is a pet,

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and then we have an instance of bird,

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and that instance is called B1.

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Bird has its own value per sounds,

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and pet has its own different value for sounds.

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Now when we say b1.sounds,

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or self.sounds in the constructor equal a copy of self.sounds,

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then Python is going to first look for what is the value of self.sounds.

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So, let's go back to the constructor.

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So when said self.sounds,

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here again self is B1.

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So, we're setting b1.sounds equal to b1.sounds,

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and we're slicing it so that we create a copy of b1.sounds.

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When Python is searching for b1.sounds,

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here it first searches inside of the instance.

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This instance does it have sounds instance variable yet,

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and then it searches inside of that instances class,

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and so we find chirp,

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or a list that only contains chirp as the list of sounds.

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So, here we end up creating a copy of that.

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So, I mentioned all of that,

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just to say that the answer to this,

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is C. Let's do another question.

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This question asks, for the dog class defined in the earlier window,

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what would happen when d1.feed is run,

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if the pet.feed self line was deleted?

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So, in other words, if we deleted,

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or commented out this line,

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then what would happen when we called D1,

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which is a new instance of dog,

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what would happen, if we call d1.feed?

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So, if this pet.feet(self) line was deleted,

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then the only thing in the dog feed method that actually runs,

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is this print statement prints Arf!

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Thanks! So, remember that the pet feed method is never called by default,

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because this method overrides that method entirely.

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So, the only thing that would happen is print Arf!

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Thanks! would run.

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So, that means that the answer here,

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is C. That's all for now, until next time