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Well, in the previous lectures, I mentioned that I'll be creating a program, an application where

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we'll be handling sort of bank account so that we will have a balance and then we will be applying

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some operations, such as withdraw money from this bank account object and deposit money as

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well.

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I'll be storing the value, so the balance in a simple text file.

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So that's not one of the 10 programs we're building in the course.

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This is just to get you familiar with classes and to explain them a little bit more so.

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I'll do some housekeeping here.

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So let me create a folder here and put all these existing files.

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In this folder, so we have the previous files in here and I'll create a new folder now.

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Let's call this account and in this folder, I'll have my Python script.

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So let's call it like this.

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So under account, we have this Python file and we also need this text file.

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We will be saving the current balance of our bank account.

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So this will be a trivial application because in real life, you don't want to save the bank account information,

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the balance in a text file, in real life you'd want to save that in a real database

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such as PostgreSQL for instance.

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But for the sake of practicing object oriented programming, we will just say the balance value in this

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file.

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So this is our storage and also for the sake of simplicity, we will be working only on one single account.

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So our program will be able to process one account object because if you have multiple accounts, then

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the solution could be different from this.

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So you'd probably have to think about having a table and maybe a database would be necessary here.

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So let's think about our personal accounts.

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You could use this program for your personal notes, so you may want to keep track of your balance and

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you can use this Python program. Now the object

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here is the bank account, and now about this object,

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we should start thinking about what attributes this bank account object should have as an initial state.

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So we talked about this minimal object and a bank account would have, of course, an initial balance.

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So let's start to create this account object and call it account.

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You can call it whatever you want, but I'll call it account.

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So in it, here's the first function we want to define and then we pass self in there.

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And let's keep it like that for a moment.

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And we said our bank account object will have a balance attribute.

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And in our case, we have this balance.txt. Let's create an initial value there.

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And now, to create an object instance, we need to grab this value and pass it to our init function

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so that we construct this minimal object.

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And because we have a file, what we need to do is we need to read this number from this file.

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And really what you want to pass

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here is some parameter like filepath.

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So this is how I'm going to call that and now if you remember in the Bookstore Application, in the

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backend script here, we also pass a parameter to the Init function and then we created a connection

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object out of this database file.

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In this particular case, what we want to create is an integer.

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So a number out of this file, out of this path.

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So you could do something like with open.

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So we're doing file handling with the with method, with open filepath.

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And this is a text file.

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So we need to go with 'r' mode, which means read.

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So we're opening the file path,

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this file in read mode as a file.

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So this is just a temporary variable that exists in this with statement.

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And then we need to read from this file.

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So something like file.read.

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And yeah.

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That's it.

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This will read the file, but you're not storing that anywhere.

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So what you want to do is say: self.balance=file.read().

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So what that will do, it will save the value inside the text file.

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In this instance, variable.

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So this is an instance variable, that is how it is called.

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Actually this is the instance variable and this is the object.

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So the account object, good.

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This is almost good.

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And we also need to add the integer function there because this will be read as a string by default.

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So we need

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to convert that value to an integer and yeah, that should work and that should construct the object

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for us, the account object with a balance attached to it.

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And before going and creating the withdrawal method here and the deposit method,

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I'd prefer to call an instance of this class first. So I would do that here.

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And let's say saw that in the account variable.

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So this will be the object, the object that will be created out of these blueprints, the account blueprints.

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And there you need to pass this argument, the argument for this parameter, which is balance.txt.

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Great and for self python will automatically pass the account object instance.

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Good.

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Now, this will create an object instance.

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And let me print out that object here so we see some output in action python and that's inside the account

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folder.

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And acc.py. We got an error: No such file or directory for balance.txt.

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And the reason for that is that we need to pass the account directory there as well.

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So.

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And yeah, this time it's working, so we had to pass the directory there because we are in this folder, in

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the demo folder, and you need to pass the account folder first and then point 

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to balance.txt.

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And this is the output.

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So it says __main__.Account object.

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So the account object of the main module and what main is, is when you execute a script like we did

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here.

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So you point to the script with Python.

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Python will assign this name to the module.

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So this module will get this name and the script, actually.

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But it's it's treated as a module.

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But if you import that module.

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So if I run, I open an interactive shell now.

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So from the account folder import acc.

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What happens is that you get the print function executed as well, but this time when you import

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a file.

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So when you don't executed like we did previously with Python account.acc.py, when you import the

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module, the module gets the name of the script that it is written to.

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So in this case, this acc and without the extension.
Here, you also get the name of the package of

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the module.

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So packages are made of modules and a package is like a folder.

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There you have several module modules or one module and like we have in this case and our package here

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is account.

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Then it has this module and then this module has this class, the account class.

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So that was something I wanted to show you.

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Packages, modules and classes.

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Great.

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Now I'll exit this Python Shell and play around with this script a little bit.

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And what we did here, we printed all the object namespace and we didn't get the actual balance

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as an output because we are not returning the balance here.

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But if you want to get the balance, you'd want to point to the balance.

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And that uses the dot notation to point to the object that holds the balance attribute.

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So let's see what we get this time. Yeah 1000.

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So that's the current balance.

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As you can see, you are able to access attributes of your object instance using the dot notation.

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Good.

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Now, let's add these other methods there.

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And what can we do to a bank account?

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Well, we can withdraw money and you always need to pass self in all of your methods.

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And then the parameter that applies to that particular method in this case, when you withdraw some

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money, well, you have an amount in mind that you want to withdraw.

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So you pass amount there as a parameter.

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And just to be clear that you understand what I'm doing, what I'm doing here is I have an initial balance

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that is being read from this file.

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So for this text file and then I want to allow the user to apply operations or withdraw money from this

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balance.

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And once the user executes the withdrawal method, I want to update the balance number inside the object

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instance.

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And I don't want yet to write that balance to two to my balance file.

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So I just want to calculate something here like self.balance=self.balance.

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Well, this is a withdrawal, so minus the amount that I want to withdraw.

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That means this will do

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self.balance equals to 1000 minus 100 let's say, and I'll end up with an updated balance of

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900

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in this case.

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And that if I want, I can go ahead and open that file again here and then write this updated balance

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in that file, so that would be one solution.

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But a more constructive solution would probably be to have specific methods that commits the changes

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to the file.

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So you may want to withdraw some money and then you deposit some money in the same session and then

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you apply the committee method to save the changes.

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So this is the solution

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I want to go for. We are done with the withdrawal method.

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Now deposit(self, amount) again, so this is a local variable.

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This is a local variable.

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They happen to have the same name, but they don't have to do much with each other.

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And self.balance equals to self.balance plus the amount in this case.

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So let's see what we have this far.

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And I'll keep this instance object here that is being generated.

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So we are instantiating an object instance, in other words, and print out the current balance and

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then I want to withdraw some money from my account object.

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That means I apply, withdraw to the account object and withdraw takes two arguments.

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So the object instance which is passed automatically and then the amounts which could be one hundred.

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And once I do that, I want to print out the updated balance.

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So let's see what it's going to do.

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Yeah.

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1000.

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So an object is created and you get 1000 printed out because here you have 1000 and then you take 100

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out of that,

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and so you end up with 900, great.

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So if you run that again, you will notice that a new object instance is being created again.

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So you don't have 900 as the current balance anymore, you have 1000.

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So that means you need to write the changes to the file

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if you wanted to save that balance, and we can do that using a method and let's call it commit.

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And pass self there and I don't think of any parameters that I need to pass through this commit method

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because all we will work with are the existing variables there.

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So this would be something like: with open.

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Well, we need to open the file path, but we have a small problem in here because the file path

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Variable now is just a local variable in this init method.

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So if we pass it here, this function, this method, we will not recognize it.

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So this is a bit different from the other the backend script that we wrote, because here we have this

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parameter like we do in here.

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But in this case, this parameter, it was only used inside the init method, so we didn't use

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it anywhere else.

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So there is a solution to that.

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And one solution would be to go ahead and pass another.

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Let's call this path here.

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So another parameter to the committee methods.

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Then when we call the committee method like we were doing with withdrawal, we'd have to pass the balance.txt.

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Actually this path to the commit method while that would work, it's still not the best practice

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of object oriented paradigm.

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So we need to seek for another solution.

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And that would be to make this local variable as an instance variable.

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And to do that you just do something like self.filepath=filepath.

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And what this does is... So file path, don't confuse this with this, this happened to be the same, but

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you can also write another thing here, like anything.

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So this is the parameter of the init function.

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So the balance.txt will go to this parameter.

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So the path of this file will go to this parameter and then you create this instance variable and that

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instance variable will be equal to this path.

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But you can use this thing here, you can use it wherever you like inside your class.

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And in our case, we would like to use it in here.

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So we open themselves up to the fall path in the write method this time because at this time we need

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to write or more accurately to overwrite the current balance, which should be this one.

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For example, we want to overwrite that to this file.

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So as file, this is just a temporary variable. File and file.write.

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And self.balance.

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Good.

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Let's try this out.

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And we didn't get any of the in the file here because I forgot to actually call the commit method and so once

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we withdraw money, let's bring that out in the console.

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And then account.commit without passing any arguments explicitly, because implicitly we are passing

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the object instance. Good. We got an error there invalid literal for integer with base 10.

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So this would be the integer in line six here and so on.

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The value that is being read by this method is not converting to an integer.

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And the reason is that this value has disappeared from balance.txt, and this is reading

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an empty string.

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So this is an empty string that is being read from this file.

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And then you try to convert that and it doesn't work.

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Now, the reason that we got this empty string and empty file here is when we got the previous error,

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so here a right argument must be string, not integer.

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So what Python did was it opened the file with the method, but then it wasn't able to actually write

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a value inside that file.

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So the file ended up empty.

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Hmm.

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So let me write the initial value there and then now we should be able to run that again.

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And yeah, now we have the updated value.

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So, again, what we did is we created the object instance and then we put it in the current balance,

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which was read by from this fall, which was one thousandth of the beginning, and then we withdrew

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100.

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So the balance was updated and then we committed those changes to this file.

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So we get 900 there now.

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Same thing if you want to add, let's say 200 and deposit.

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Let's see if this will work.

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Yeah, this was printed out.

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Let's check the file.

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Yeah, so this is our new balance and I hope this clarifies a few concepts about object oriented programming.

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If you are creative, you can go ahead and expand the script so you can add more methods there and maybe

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make this more of a real life application.

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So great.

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And this is about this lecture.

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And in the next lecture, I'll talk about inheritance and we will be expanding this script by applying

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inheritance in there.

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So talk to you in the next lecture.