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Hey, welcome back.

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In this video, you're going to learn what inheritance is and how to use it in your programs.

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So previously in the previous video, we added this credit card feature.

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So now we check if the credit card is validated and then we let the user book the hotel.

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Now, sometimes, though, some credit cards have another security layer, so it's not enough to enter

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the credit card number and the expiration date, the holder name and the CVC.

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But you also need to provide some sort of passwords and sometimes even a username and password through

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an additional web page.

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However, our program doesn't support those credit cards yet.

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So what should we do then?

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Should we modify the class credit card?

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So maybe add some more methods here or modify by the valid methods?

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Well, the answer is no, because when you want to add some new features like we're doing now, we're

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adding this new secure credit card feature.

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When you want to add more features, you don't want to modify existing classes.

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So this is one of the principles of o.P.

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What you want to do instead is add more classes.

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Of course, it's fine to add methods to a class when you are coding that class, so it was fine to add

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a defined pay method in the previous video.

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If we wanted to do this in a more real life situation.

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But once you have completed the class, once you have completed the feature and then you want to add

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more features, then it's good to create new classes.

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That's what we're going to do now.

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Class secure credit card, right?

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Now, the thing is that this secure credit cards would be checked for that username and passwords,

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which for them where we have created this CSV file, you can find it attached in the lecture resources.

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So we have the cards dot CSV file, which we covered in the previous video, and then we have this additional

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CSV data.

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So in here we have like the card numbers one, two, three, four.

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Actually it's this number.

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So that's a number of the cards and that is the password of that court, basically.

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So the user will have to enter the password of their card in the program and then we'll check against

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these data.

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So to check against those data means we need a method that checks the password, but we also need to

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validate the expiration holder and CVC values as well.

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So what we're talking about here is basically having the same class credit card, but with some extended

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capabilities.

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This extended capability is the capability to check if the password is correct.

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So in that case, we don't want to write that class again like in it and then have define, validate

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and then have another define authenticate.

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Maybe we don't want to rewrite these methods.

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What we do instead is we inherit that credit card class from this class.

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To inherit a class.

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When you define the class in the first line, you use some parentheses, and then there you enter the

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credit card class, the name of the class you want to inherit.

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And to now this secure credit card class is known as the child, and this is known as a parent class.

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So the parent and the child.

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So the child is inheriting all these methods from the parents, but the child will be more advanced

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than the parent because the child will have this authenticate method.

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In addition, you'll see exactly what I mean when I say the child has inherited these methods.

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So first let me implement the authenticate method.

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So this again would need access to that card, that security, the CSV file.

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So for that we say to F here on top cards, security is equal to pandas that read c v cards underscore

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security point C is V and let's load the types as str.

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And then we go down here in authenticate and we want to get the passwords from the DF cards security

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variable, which is a data frame that lock.

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So we get DFF number which should be equal to self.

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That number.

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I'll explain you what self that number is doing here.

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So sell that number and then we extract the password.

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Right.

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Password is this field.

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So we have a number column and the password column as well.

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Two columns.

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So that is the number.

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Column.

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That is the password column.

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So we are saying oh give me the password value where the value of this number column is equal to that.

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Now what is that?

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Well, that is this here.

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So basically, as I told you, this child class inherits everything from this class.

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And that is the first example of inheriting.

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And you'll see that this class also inherits the methods.

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So not only the instance variables such as this self, that number, but also the methods.

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And let's add squeeze here.

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To get the actual value for the passwords and not just the series.

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Then we say if password is equal to given passwords, which is going to be a parameter of the authenticate

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method given password.

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Return.

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True.

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Sorry, this has to be an equal comparison.

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Operator.

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Else return false.

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Although you can omit this and it's it will still work.

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So then we can go down here and instead of using the credit card class we use, we use the C, Q credit

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card class.

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Right?

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So now credit card, this variable holds an instance of secure credit cards.

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And this secure credit card takes the number as an argument, although we don't have an init methods

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here, which gets a number as an argument.

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We do have an init method in the parent class.

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And since this secure credit card class inherits everything from the credit card class.

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That means.

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It's also inherits that in its methods.

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So we can provide all the parameters that the init method takes in the credit card class.

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Right.

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And then the validate method also can be applied to this secure credit card instance.

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Right.

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Which is represented by this variable.

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So the validate method is also inherited.

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Therefore we can pass the methods and all its arguments.

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So everything else remains the same.

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And of course, we also need to add the authenticate method call here.

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So first we validate the codes and then we add another conditional layer if credit codes that authenticate.

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And then we end we enter the passwords.

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Actually, it's given passwords.

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That's the name of the parameter.

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So that is equal to the password, which you can get it from the user using like a variable here with

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an input function.

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Or if you want to keep it simple, you can just provide the passwords which is here my pass.

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So write my pass here as a string.

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And then if that turns true, which is the case with the password is equal to the given password, then

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we want to book the hotel, ask the user for their name, create a reservation ticket instance, print

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out the reservation ticket

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and perhaps add a new LS statement here ls print.

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Credit card authentication failed.

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So that else statement is part of this.

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If conditional block and this one here belongs to that.

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So be careful with the indentation level.

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Right.

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And that is paired with that.

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So we can run the program now run main.

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Perhaps an error.

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Yep.

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Directory cards security not found.

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So I had.

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My typo here.

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Card security does CSC.

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It's not cards, it's card security.

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So let's run again.

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And to the idea of the hotel.

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Now all the hotels have no availability.

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So we're going to get hotel is not free.

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So let's change one of these to yes save the hotels that see the file run again.

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188.

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Another error.

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Let's see what this is telling us.

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So look at the line, which is pointing to your file, not to the other files in our other modules.

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So that's our main that's p file.

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I click there, it takes me here and this is complaining that number was not found in the data frame.

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So a key error is part of PANDAS data frames, errors.

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So we take a look at the court security seal we file.

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Now here I can see that the column number is actually called number.

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So that should be some other issue here.

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What I can see is that I didn't use the FS card security here in here, but I use DF.

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So DF should be replaced DF card security.

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Yeah.

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Run again.

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188 This hotel.

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Press enter into your name, audit.

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And yeah, we've got.

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The reservation ticket if this given passwords here.

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Was not my past, but something else like my past one.

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And we run against the code.

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So let's change this back to Yes, run main 188 and we're going to get credit card authentication failed.

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So the data we have entered in here, which is a password, gave us this problem.

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So that's what we expected to have this error so that it's working fine.

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And yeah, that is about inheritance.

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So inheritance is when you create a new class which inherits from an existing class.

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So the new class will have additional methods.

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So those additional methods are defined in the new class, but you don't need to define the existing

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methods of the parent class.

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You just call them just like we did here validate and you also can use.

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The variables that are passed to.

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The class instance.

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So number was part of the credit cards class, but it is also part of secure credit card.

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And that's about inheritance.

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So thanks a lot for following and I'll see you later.