Classes - CS50P Shorts

Key Takeaways

[Music] well hello one and all and Welcome to our short on classes we'll see here why we should create classes and the syntax we can use to do so now I have here a program called packages. py whose goal is to really track packages we're sending between different users that we have I have here a list called packages and notice how it has two packages represented as strings inside of it I one package one Alice to Bob 10 kg and another one package two Bob to Charlie 5 kog so two different packages here inside of my list now there are a few different reasons you probably don't want to be using strings to represent these packages here now one of them is strings are flexible but arguably too flexible I could very easily change the ordering that I have for each of these pieces of information about a package here I did follow this same pattern of maybe package number colon uh sender and then recipient comma weight but I could very easily stray from that as I add more and more packages I might want something that's a little more rigid can store information in a better way so thankfully I do have at my disposal this thing called a class now a class operates a bit like a template that I can use to create various objects in my my code I could think of each of these packages as being some object in fact I could look in the real world and see a package as some object and I could think of my class as being a template to later create some object that looks a lot like a package but a particular package so here I have some pieces of information about this package I probably want to encapsulate to put inside my template uh one might be the ID of the package is it 1 2 3 some unique idea to refer to it later on I might also want to store the sender of the package as well as the recipient alongside of the weight as well so classes these templates allow me to encapsulate this information in a single place and not worry about all this different formatting that I have to maintain consistently in my code if I were to use something like a string for instance so to create a class toate this template for a package I could literally type class just like this followed by the name for my class in this case package and by uh by uh convention these class names tend to begin with uppercase so here we have capital P package and I could later on fill in this package but here I'm getting started on creating this template for some package that I can then use later on to create particular packages so let's think about what a template for a package might ideally store well to do so I need to First create this particular method one called Dunder in it which allows me to use my template my class here to later on instantiate or create some particular object from this template from this class so here I'll Define in this case Dunder in it Dunder stands for these double underscores here and it turns out that this by convention must take at least one argument one called self when I create later on a new object this self refers to the instance that new object I'm creating when I were for inst to call something like package we'll see that later on the other thing this really wants to probably take as input here is all of the pieces of information I need to create some new package what we said down below we want to encapsulate or really combine a few different pieces of information one is the package ID d one is the uh sender and one is the recipient and one is the weight of the package here so I could go ahead and say as input uh to this particular template that I have here creating some new object later on I'll go ahead and say each package has a number or some ID it also has a sender a recipient and in this case a weight and this is my Dunder init method allows me in this case to specify what information I'm encapsulating as part of this template in this case all packages have some number some sender some recipient and some weight well now we might see where self comes in within this Dunder init method I could actually go ahead and try to assign to this new instance of my object everything I've passed in down below I could say self. number equals number self. sender equals sender self. recipient equals recipient and self. we equals weight now this might be a little bit confusing at first especially seeing self and especially having made up some of these actual pieces of information down below but let's see what it could do for us and maybe walk through what's happening each step along the way so I ideally I want to rewrite my code online 10 here this is not what I should be doing to represent packages but I do now have this template that I'm able to use to instantiate to create various objects in this case individual packages well down below here let's try doing just that I'll remove these strings and I'll then go ahead and as input to my list well I'll create some new package and I can do so in this case by typing my class name package followed by parentheses and now here is where I can actually use those arguments I gave to Dunder in in it when I use package parentheses I'm actually at least on one step of the way calling Dunder in it to create for myself some new instance of this template we said here that this should take Beyond self uh number sender recipient and weight as arguments so I can do this I could say my first package's number is one and it's the sender the sender is Alice uh the recipient in this case is Bob and and the weight the weight in this case will be 10 and so this is the way we've now defined some new instance of this particular class a given package that has a number one the sender Alice the recipient Bob and the weight 10 for 10 kg let's go ahead and add in another package another instance of this class I could type package again and I'll say number equals 2 sender equals Bob uh maybe recipient here was Charlie as before and the weight the weight of this package was five and so this is this is here our way of representing that second package in our list and now we have a much more methodical way of storing or encapsulating this information thanks to the class we've created above but now let's see what this was doing here we said self. number self. sender self. recipient self. weight we later on we'll actually see if we can access these things called instance variables but we'll see that in another short this then was our short on classes how to create them and the syntax to do so we'll see you next time