Hey, everybody. How's [it] going in this series of videos? We'll be learning how to create and use classes within python and how object-oriented concepts are applied within the language now There's a lot to cover when working with classes, so I'm going to break these up into several different videos We'll cover the basics of creating and instantiating classes will learn about inheritance class and instance variables static methods and class methods and Several other topics So breaking these up in several videos will allow us to focus on specific topics in each video so in this video We'll be learning the basics of creating and instantiating simple classes, but first Why should we even use classes now this isn't just specific to Python you can see classes being used throughout most modern programming languages, and there's a good reason for that they allow us to Logically group our data and functions in a way that's easy to [reuse] and also easy to build upon if need be Now just a quick side note when I say data and functions that are associated with a specific class We call those attributes and methods, and you'll hear me use those terms a lot throughout these videos, so when I say methods I mean a function that is associated with a class [so] let's go ahead and get started so say we had an application for our company And we wanted to [represent] our employees and our python code now This would be a great use case for a class because each individual employee is going to have specific Attributes and methods so for example each employee is going to have a name and email address [a] pay and also Actions that they can perform so it'd be nice if we had a class That we could use as a blueprint to create each employee so that we didn't have to do this manually each time from scratch So let's go ahead and create a simple employee class and see what that would look like so to create a class It's just as easy as saying class employee now I'm going to leave this empty for now, and if we just left it like this and we get an error so if you ever have a class or a function [that] you want to leave empty for the time being then you can simply put In a past statement and python will know that you just want to skip that for now So now we have a simple employee class with no attributes or methods And I wanted to stop here because I wanted to explain the difference between a class and an instance of a class So our class is basically a blueprint for creating Instances and each unique employee that we create using our employee class will be an instance of that class so for example if I said employee 1 equals employee and EmPLoyee 2 Equals employee then each of these are [going] to be their own unique instances of the employee class so for example if I go ahead and print both of these out and Copy and paste that There and you can see that both of these are employee objects, and they're both unique They both have different locations here in memory now This is an important distinction because you'll hear me talk a lot about instance variables and class variables and it's important to know the difference between those and I'll go more in depth into class variables in the next video but for this video We're going to be looking at instance variables so instance variables contain Data that is unique to each instance Now we could manually create instance variables for each employee by doing something like this So let's say we wanted employee 1 to have a first name and a last name so I could just do employee 1 not first Equals Corey and then I could do employee last is equal to Schaefer and I could also give it an email address, so I'll do employee 1 dot email equals and I'll just do the first name with the last name at Company comm and lastly let's go ahead and add a pay [onto] there too, so I'll do employee 1 not pay and we'll just do 50,000 there now let's give employee to Some of these same attributes so for this one.
I'll just do test User and then I'll do test User and I will make that 60,000 ok so now each of these instances have attributes that are unique to them so if I print it out, let's say I could print out the employee [1] dot mail and Also print out the employee to email On it looks like whenever I gave these the instance variables I forgot to make this employee to here so now let's run that so now you can see that that email Was created for each employee? [let's] say that we wanted to set all of this information for each employee When they're created rather than doing all of this manually like we did here So we wouldn't want to have to Manually set these variables every time you can see it's a lot of code and it's also prone to mistakes Just like I did whenever I forgot to change the [to] employee, [too] So we don't get much benefit of using classes if we did it this way, so to make these Set up automatically when we create the employee we're going to use a special init method So now inside of our employee class I'm going to create this special init method Now you can think of this method as initialize and if you're coming from another language Then you can think of this as the constructor now when we create methods within a class they [receive] the instance as the first argument Automatically and by convention. We should call the instance self now.
You can call it Whatever you want But it's a good idea to stick to convention here and just use self So after self we can specify what other arguments that we want to Accept so let's go ahead and accept the first name the [last] name and the pay and I know that we had email, too But we can create the email using the first name and the last name so now within our and Knit method We're going to set all of these instance variables So let's do self dot first equals first and now I'm just going to do this for the rest of them as well So [I'm] going to do self dot pay or self dot last and self dot pay and for the email I can do self dot email equals first plus and then we'll put a dot between those and then last and then we'll add on to the end at company comm Okay, so whenever I say that self is the instance what I mean by [that] is that when we self dot first? equals [two] first here It's going to be the same thing as us saying down here that employee one dot first equals [Korey] except now instead of doing this manually It'll be done automatically when we create our employee objects Now these don't need to be the same as our arguments so for example. I could make this Self dot f name equals first, but I usually like to keep these similar if possible So I'm going to go ahead and set that back [to] self dot first equals first, okay?
So now when we create our instances of our employee class right here now we can pass in the values that we specified in our anit method now our net method takes the instance which we call itself and the first name last name and Pay as arguments, but when we create our employee down here the instance is passed automatically So we can leave off self we only need to provide the names and the pay so we could create these by Passing in first and we have to do this in order So I will pass in all of the same information that we did manually down there and for the second one I'll do [test] and User and I think I had that at [sixty] [thousand] okay so what would happen on this line when we create this employee the anit method will [be] run automatically, so Employee one will be passed in as self and then it will set all [of] these attributes So it'll set employee one dot first is equal to first which we passed in as quarry Employee one that last equals what we passed in is last and so on so now that we have that an it method in place We can go [ahead] and delete these manual assignments that we made down here And you can see by deleting that that we got rid of a lot of code So I'm going to go ahead and comment out those lines [we're] printing [the] employees, and I'm just going [to] go ahead and print out the email So if I run that then you can see that that still works, okay? So everything that we have so far [like] the names and email and the pay are all attributes of our class now Let's say that we wanted the ability to perform some kind of action not to do that We can add some methods to our class So let's say [that] I wanted the ability to display the full name of an employee now This is an action that you'd likely need to do a lot with a class like this So we can do this manually outside the class if I was to come down here and do Print and I could get the full name just by putting in two placeholders there and doing a format and saying employee one dot first and Employee one dot last and if I go ahead and print this out And you can see that we got the full name there But that's a lot to type in each time that you want to display the employees full name, [so] instead Let's create a method within our class that allows us to put this functionality in one place, so within our class here I'm going to create a method called full name and we can do that with just doing a death of Full name now [like] I said before each method within a class Automatically takes the instance as the first argument and We're going to always call [that] self and the instance is the only argument that we'll need in order to get the full name So within this method here I'm just going [to] take the same logic that we had down here and cut that out and I'm just going to go ahead and return that but we have to be careful here because now instead of printing Employee [1] [first-name] and [lastname], [I'm] going to use self so that it will work [with] all instances So I'm going to do self dot first and self dot last So now that we created that method instead of printing like we did before now I can just come down [here] and do employee 1 dot full name and Print that out And if I run that then you can see that we got the same [thing] and notice that we need the parentheses here Because this is a method instead of an attribute So if I left the parentheses off and printed this then you can see that it prints the method instead [of] the return value of the method so we're going to have to put those parentheses on in order to run that properly so now we have full advantage of Code reviews here, so instead of typing this out for each full name that I want to print now I can just use that method so now if I wanted to print employee two's full name It's just as easy as replacing employee one with employee two and running that and we get the correct answer Okay, and one more quick thing that I wanted [to] point out here Now one common mistake that I see when creating methods is forgetting the self argument for the instance So let's take a quick look at what that would look like if we left that off so now before I run this if I just comment it out this printing of full name down here and Ran this then you can see I'm actually going to remove these print statements here as well Now [you] can see that this runs without any errors But if I was to try to run this method that we accidentally left self off of then Run this and you can see that we get an error and the error that we got was a type error full-name takes zero positional arguments But one was given now this can be confusing because it doesn't look like we're passing any arguments here into full name But the instance which in this case is employee two is getting passed automatically So we have to expect that instance argument in our method, and that's why we added self so I'm going to come back up [here] to full name and Put self back in and now running this you can see that [it] runs Correctly now we can also run these methods using the class name Itself which makes it a bit more obvious of what's going on in the background because whenever we do that, so I'll do employee Dot full name now [when] we run it from the class we have to manually pass in the instance as an argument So in this case I'll pass in employee one so you can see how these are similar, but not exactly the same So I'm going to put these side-by-side just so that we can compare them here So these two lines here do the exact [same] thing, but here when I do employee one Which is a instance and I call the method. I don't need to pass in self It does it automatically and when we call the method on the class And it doesn't know what?
instance that we want to run that method with so we do have to pass in the instance and that gets passed in as self and if I go [ahead] and Print this out and run it then you can see that it works Just like if we were to print out the employee one dot full name And I wanted to show you that because that's actually what's going on in the background when we run employee one dot full name it gets transformed into this here employee dot full name and Passes in employee one as self and that's why we have self for these methods, so I hope that makes sense to you That's not extremely important to know when we're just getting started with working with classes But we need to understand that [in] later videos once we start inheriting from other classes and things like that So I figured it would be a good thing to go ahead, and show you that now okay?
