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Hey, welcome back.

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In this video, you'll learn how to add new classes to existing programs without breaking the program.

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So let's simulate a situation.

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The client, whom we built this app for is complaining that customers are booking a hotel and then they

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are not showing up.

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So the client now is asking that we should add a feature to the program which asks the customers to

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enter their credit card details and pay the booking when they do the booking through the app.

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So as a programmer now we need to extend the program to satisfy that new requirements of the client.

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So we need to add a new feature, a new credit card feature.

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And when your program is in OP, meaning that it's organized in classes, then all you need to do is

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add a new class without breaking the existing classes.

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And that is actually one of the principles of o p.

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You shouldn't change the existing classes, just try to add new ones so we can start the logic by looking

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at the program main loop here.

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So we print out the list of hotels here and then we get the ID of the hotel the user want to book.

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So just to refresh your memory, so the users see this and they enter something like 188 for this row.

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So they want to book that.

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But then let me stop this and then we check if the hotel is available and then we let the user book

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the hotel.

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But now, before we let the user book the hotel, we first want to tell the user, Oh, first let's

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create a credit card instance using a class, which we're going to have to create in here.

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So let's start to create a class class credit card.

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Let's pass for now.

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So in that credit card class, the user will enter some data.

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Or perhaps we need to ask the user first, like, you know, card number.

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We can also user through an input function.

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If this was a web app, then we would show the user some text boxes where they can enter the credit

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card number and the expiration date of the credit card number, the name or the credit card, and also

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the CVC number, which is like a three digit number in Visa and MasterCards.

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So these are the data that the app would need to validate that credit card.

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So we can ask the user for these three like, Oh.

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Enter the credit card number and then another one would be expiration date.

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You know, we could ask again for input, but I would want to.

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Just assume that we got these from the user.

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For example, we got.

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The number se.

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One, two, three, four, five, six.

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Cards have something like 16 digits card numbers.

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So let's assume that the user entered this number.

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So that's ten digits.

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One, two, three, four, five, six.

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That's 16 digits.

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So the expiration date would be 12.

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26 of December 2026.

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That's the year.

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And then the holder, the name of the holder, let's say, John Smith.

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And then finally, the CVC number, which is a three digit number, let's say 1 to 3.

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So let's suppose that we got this from the users using input functions and then those variables we pass

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them in here.

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So we're skipping the input function step just for simplicity.

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So once you have created a credit card instance, then you want to say, if credit card don't validate,

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then you want to book the hotel if this basically is equal to true.

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So that's that would be some a similar method like this.

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So return true if we find that in this card, that CSV file which you can find the touch in the lecture

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resources if we find that that card that the user entered here with this data, if that card is an existing

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records in this database, which is just a CSB file for now, but in real life, this would be some

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remote databases where your program has access to through an API and the program can check if a code

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exists in that database.

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In this example, we want to keep things simple and have those data in a C zip file.

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So if we validate the credit card instance, this credit card instance, then we book the hotel, then

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we ask the user for their name, then we create a reservation ticket, we print out the reservation

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ticket, and we leave this as it is.

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So this else block here is part of this.

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So if the hotel is available, this will happen, right?

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Otherwise, if we are stopped at this point, if the hotel is not available, which is the case when

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this here is.

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No.

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The availability, the available field.

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In that case we print out the hotel is not free.

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If you like here you can add an else statement which belongs to this group.

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If else.

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Right.

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Else.

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Print.

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There was a problem with your payment.

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So if for some reason we couldn't validate the credit card, we showed the use of this message.

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So they know that there's something wrong with their credit card.

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So with that, we can go up to the credit card class here and we start to add methods to this credit

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card class.

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The first method is underscore, underscore, init, underscore, underscore self.

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Now the question is the big question always is what parameters should in it take.

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The way we instantiate its credit cards here suggests that this should take these one, two, three

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four parameters.

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Right so number.

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Expiration.

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Holder and CVC number.

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However, this is debatable because some of these parameters you can also have them in the other method

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validate.

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Right.

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So perhaps you could have only the number there and these three in here.

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And then in here you only provide the number when you create an instance of the credit card and you

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move these three to the validate method because they are now part of the validate method.

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Right.

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Remove this comma as well.

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So the big question is what is better?

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Is it the first version where we had all the parameters in here, or is it this version better?

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Well, to answer this question, you should think of what is the minimum set of parameters you need

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when you instantiate when you create an instance of a credit card.

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So can a credit card be defined using only its number?

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Well, the answer is yes, it can be defined, but it cannot be validated.

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Right?

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Because to validate a card, to make a payment, you need all these data expiration date, the holder

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and the CVC.

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So it does make sense to have only the number here and to have all this other parameters part of the

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validate method.

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So think of this like, Oh, this is like the user name of this object and these are like the passwords.

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So I think it does make sense to have it like this, although some programmers would think otherwise.

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So there's no concrete way a definite technique to do this.

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So let's keep it like this and I'll do self number is equal to number.

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I did this because we're going to need the number and this method, right?

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So we need to access that number.

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And the way to access it is by assigning number as an attribute of the instance.

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So now we can access that as self, that number, right.

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So how do we validate the credit card?

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Well, first of all, you need to load the data frame out of this CSB file.

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So let's do that.

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We can do that outside here.

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So you can say DF for four cards is equal to pandas.

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Dot three C is V cards dot C as V?

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That's the name of that file.

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I'll do D type is equal to SDR.

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So I want to load all the fields as strings because none of them has any mathematical meaning.

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Right.

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So this is a number, but it just an idea.

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This is also just an expiration date which can be treated as a string.

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The CVC field doesn't also have any mathematical meaning.

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We're not doing mathematical operations, so it's fine to load all these as strings.

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Now, I would like to load this dataframe actually, as a dictionary.

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Why?

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Well, I'll explain you why.

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So let me import pandas in a python console to show you how this dictionary will look like.

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Import pandas DF cards.

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DF cards looks like this.

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So a dictionary?

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Sorry.

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A list of dictionaries and a dictionary represents one row of the cards data.

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So.

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One, two, three, four.

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That role that I selected here reflects that dictionary here.

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I did convert the dataframe into a dictionary because it's easier to check if all these.

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Data are part of that list of dictionaries.

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Right.

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You'll see how it works.

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Exactly.

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So the idea is that now I can I can make a dictionary with.

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The data which should look like this, let's say.

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We have the number first rate as a string number.

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We just sell that number and then we have expiration.

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So that's the key.

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The value would be expiration.

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The variable, this parameter which comes directly in the validate method.

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Holder.

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Holder.

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CVC.

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CVC let me split this because it's too long, just for readability and following the PEP eight style

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guide.

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So once I have these data as dictionary, so I get the data from the user, right?

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I get the number in here, I get the expiration holder and CVC in here and then these are transported

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to here.

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And then we create a dictionary and then we say if cart data in DF cards.

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So that data frame actually it's not a data frame, as you see, it's a list of dictionaries.

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So we're checking if that dictionary is one of those dictionaries in the DF cards list.

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Pretend to.

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Right.

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And you can say ls return false, although it will work even if you don't do that because this will

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return none by default.

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Same goes with this available method of the hotel class.

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But if you want to be explicit and make the code more readable, you can do the return false.

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So some people prefer this method of doing things more explicit.

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So that means that this validate methods in here will return.

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True.

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And that means this will be evaluated to true.

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And that means we book the hotel and so on.

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So, yeah, that's it.

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Let's run the program.

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So run it.

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Now, all these hotels are currently not available, so if we do a 188, we're going to get hotel is

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not free as expected.

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So this was executed, this line.

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Now let's go to hotels as we change the second one snow palace in New Delhi to yes, they do have some

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availability currently because it's not snowing in Delhi.

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So they don't have lots of customers.

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Usually it snows there and anyway, it's run and enter the idea.

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So we said Snow palace, which is.

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Yes.

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Now 188 press enter.

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There was a problem with your payment.

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So something is not going as smoothly.

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That's because the number here doesn't reflect the number there.

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So let me just add it.

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The number of the cards here plays that there are.

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Try again.

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So Snow Palace is still available.

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So 188 enter.

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Oh, enter your name.

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Now we're being asked for our name ordered.

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And thank you for your reservation.

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Here are your booking data or this hotel name and so on.

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So now our card was validated and we successfully booked the hotel.

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Of course, you can have this database more complex, so you could add like a balance here and then

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have.

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How much money each of these cards have.

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Right.

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And then you could.

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Extend the class credit card by adding another methods here like define pay.

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And then this pay method would modify this data frame, right, and subtract the payment the price of

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the hotel.

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If the hotel had a price, which also is not the case because our hotel's database was also simple,

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but you get the idea so you can add more methods to a class when you want to extend the class.

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So it's better to add the methods here than to modify this validate methods.

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So each method should be responsible for one and one thing only.

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So this method validates the card and that's its duty.

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The other method would pay, which means subtracts something from that hotel.

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Right.

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And so on.

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Anyway, I don't want to go into the rabbit hole with so many details.

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I want to keep the focus on P.

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So that's the cards.

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And yeah, that's how you can extend an existing program.

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So you add a new class.

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And then you modify the main loop.

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Where do you create instances of the classes and where the logic of how the program will run is?

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So I hope this is helpful for you to understand classes even better.

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And yeah, we have some more videos today, so I'll see you in the next one.