Please Master This MAGIC Python Feature... 🪄

Key Takeaways

today I'm going to show you one of the coolest features that python has and I guarantee that after you watch this video you're going to look at python code completely differently so let's get into it and talk about Dunder or magic methods now Dunder stands for double underscore and they look something like this double underscore a knit and then double underscore now you've probably seen this method before if you've done object-oriented programming in Python but there are a ton of other ones and these are reserved or special methods in Python that map to some kind of behavior so in the case of a nit you probably know what this does it's used to construct or create a new object so when we Define this inp method we're defining what we want to happen when a new instance of an object is created here look at this quick example we have a rectangle we Define some a nit and this now means that when I Define something like rect and I pass values 2 and three we're going to implicitly call this a knit function or this a knit method and we're going to create or initialize the object okay so this double underscore method maps to the behavior when we create or construct initialize whatever you want to call a brand new object but the thing is we have many other double underscore methods and I'm going to explain to you how they work okay so let's have a look at some basic types in Python let's have something like string one equal to hello and something like string 2 and let's fix our quotation marks here equal to world now in order for you to really understand how these magic methods work work what you need to know is that everything you create in Python is really an object so something like this string is an object something like an integer five this is an object of type int something like a function yes even a function this is an object as well of type function now if you don't believe me you can simply print out the type of anything that you want so we can even print out the type of something like a function so let's define one Define Funk let's just make a very simple one here and you'll notice that if I print the type of funk and I run my code here that it says class function this tells us that this is an object of instance of the class function okay it's just important to understand this because this is how python deals with behavior and actually performing different operations okay so now that we understand that everything in Python really is an instance of a class or an object what we want to look at is how we perform different operations with these types so why is it in Python that I'm able to add string one and string two well we just take for granted the fact that we can perform this operation but the reason why we can do this is because these objects are of the same type and Python's actually implemented this behavior in something known as a double underscore method so I can do something like you know newcore string is equal to string 1 plus string 2 we know this is going to give us hello world but did you know that when I use this plus operator what this is actually doing is mapping to a double underscore method something that's actually defined and written on the string class remember that everything we have in Python is an object that means there's some kind of class behind it defining all of its behavior and well on the string class which these are both instances of there's actually a special method called add and this add method defines what happens when you try to add two strings together now in this case what it does is concatenates them and let me show you how it works so I'm going to say print newcore string okay we're going to run this and we get hello world but there's another way that we can execute this rather than using this plus operator we can actually call the double uncore method we can call the do add okay let's go here and watch and notice I get the exact same thing now let's take it a step further of course we know that we can get something like the length of a string so I can look at the length of string one right and if we do that we see that we get five but did you know that this lend function actually maps to a double _ method as well the reason why we can use this Len function is because on the string class again kind of the blueprint for this object there is aore uncore Lenore uncore method let me show you what I mean I can actually go string onecore Lenore I can explicitly call this method and when I run this we get five now I know this isn't 100% clear yet but what I'm trying to illustrate to you is that every single operation all of the behavior that exists in Python exists because all of the things in Python are objects those objects have some class behind them defining their behavior and on that class we have these special magic methods like Len add a knit Dell I'm going to show you a bunch of them in this video and that maps to The Operators or the behavior that we use in our python code so even adding two numbers together 1 + 12 the reason why this works is because there's a special ad method that exists on the int class that defines what we should do when we add these two types together so now what I want to do is I want to show you how we write our own custom double underscore methods so you can really understand how this works and how we can use it to make some really cool functionality now before we go any further let's talk about boot dodev today's sponsor and an online platform that's redefining how you learn backend development what makes this platform special is that it's designed to keep you engaged boot dodev uses techniques from Modern game design to make learning backend skills not just effective but genuinely fun and it's also free to view all of the content now here's how it works you progress through Hands-On lessons that guide you step by step building real world projects along the way there's no endless video watching here boot dodev is all about Interactive Learning and coding directly in your browser plus it's focused entirely on backend development so you'll Master skills like databases apis and serers side programming in depth specifically in languages like python and go one thing that I love is that it's structured like a game you'll level up along the way unlocking new content to stay motivated and hit your goals you'll do tons of exercises and projects and that's so important because you can't learn how to code without writing a ton of code and working on real world projects if you're serious about becoming a software engineer or leveling up your backend skills then go to boot dodev and use my code Tech with Tim for 25% off your first year if you choose an annual plan I tried it myself and it's genuinely a fantastic way to learn how to code and I can't recommend it enough okay so welcome back and we're going to look at this simple example now notice that I have a class counter on the class I have an a knit double underscore method we already know how this one works and then we have a count up in a countdown method now this is fine this allows us to count up and countdown and keep track of some kind of value but what happens if I want to do something like add two counters together or view the value of a counter maybe as a string or maybe I want to subtract counters there's all kinds of operations I might want to perform on this type and right now I'm not able to do that so let me show you what I mean we have two counters we count both of them up so they're both going to be at two because we start the value at one we can then print them out and we can try to add them together now watch what happens when I run this you can see that we get this main counter object and then we get another main counter object that's kind of useless doesn't really tell us anything and then when I try to add these two counters together well of course we get an error it says we don't have have a supported oper end for counter and counter so let's see how we can fix this by using double underscore methods so first things first when I print out my counts I don't want to see just this counter object I want to see what the count actually is so to do that we can Implement a special method called uncore string now this string method is automatically going to be called whenever we try to print out some type of object and this is kind of a human readable version of our object and what we can do is return some string that represents this object or that kind of shows us some information that's human readable right that would make sense in the terminal or the console so what I'm going to do here is I'm just going to put an F string and I'm going to say return and I'll just say count is equal to and then self. value okay so now let's go back here let's run and notice that we get count equals 2 and count equals 2 Okay so this string method is called whenever the string function is used on some kind of type so in this case string count one will map to this string double uncore method but what the print function does is it will call this string function on anything it's trying to print to First convert it to a string so it's going to use this double underscore method okay so that's great but we're still going to get that issue when we try to add the two types together so what we're going to do now to implement that behavior is we're going to define the ad method now what I can do in the ad method is I can take self and I can take what the right side operand is pretty much what that means is the thing that comes after the plus sign so in this case we have count and then we do plus whenever we see a plus coming after some type of object it's going to go and look at this class so the class of whatever this object on the left is and look for this add double uncore method okay then the parameter that we put here which is typically denoted as other is going to be whatever is on the right side of the plus okay so we always look to the left side operon to find this add method and then we have other which is passed as a parameter and that's the right side operand okay hopefully that makes sense so here if I want to add two counts together I have some different options and I can implement this behavior however I want but I can return and then I can say self. Value Plus other. value now this assumes that the other type is going to be a count object or a counter object we can look at that in 1 second but now Watch What Happens let's bring up our terminal and run our code and you see that we get four okay so now we don't get an issue because we implemented this add method which now means this plus sign or the addition operator is valid to use on our counter type now of course if we try to do something like count 1 + 2 we're going to get an issue here because it says attribute error in object has no attribute value because int gets passed in as other and then we try to access value in that that doesn't exist so typically what you do if you're implementing these types of methods where you're dealing with multiple objects together is you check the type before you perform this operation so you can say something like if and then this is is instance and you can say other and then this is type counter then it's fine to do this otherwise you can raise some kind of exception so you can say raise exception invalid type okay so now if we run this you'll see that we get the exception invalid type but if we changed it back to be the counter object then this would be fine because this is an instance of counter okay so I just want to show you a simple example here of how you write your own double underscore methods and how they can be useful because here you see we just added this plus functionality now I'm going to go through a bunch more examples that show some more complex situations so stay tuned so here's a quick example that explains the differences between the string and the wrapper double uncore methods now you may have seen these before but these are very popular and almost anytime you write a python class you'll want to be writing these methods on them now the string method we already looked at this is a userfriendly output something that will maybe print to the terminal and by default when you call the print function on some type of object it's going to look for this string double uncore method so in this case we have some car and we just print out the year the make and the model kind of in a human readable format right something that makes sense however we have another method that's pretty important as well and this is the repper or the representation method now what this is used for is debugging okay so this is something you would see while you're debugging the code this is meant to be for kind of like a developer output and something that defines what the type is and maybe all of the different fields or values okay so if you use repper you can actually trigger this double underscore method by calling the repper function it's going to give you a developer friendly output at least that's the way you should implement it so we would get car make equals Toyota model equals Corolla and year equals 2021 this is kind of the standard format you're going to write whatever the name of the class is and then you're going to write all of the different properties and their values okay I just wanted to quickly show this to you I'm not going to run the code the point is you should always Implement a repper and a string method when you're defining your own custom classes so that you can debug this easily and you can also have some kind of useful output if you print this to something like the console all right so now we're going to look at an example that uses all of the different arithmetic operators that you can Implement as double underscore methods now before we just looked at the ad method but of course we can Implement double underscore methods for pretty much every single operator so of course we have a nit here we have repper and what we're doing is just defining a simple inventory item okay now what we've done is we've added add sub mole true div equals LT GT I'm going to explain what all of these are on this inventory item to allow us to use the standard operators in Python with this type so the ad method is pretty straightforward this tells us what happens if we use the plus op operator so notice that what we've done here is we've just made sure that it's safe to perform an operation before we actually return a new inventory item so we've said if is instance of other an inventory item and self. name is equal to other dotes if these objects have the same name indicating that they're the same inventory item then what we can do is return a new inventory item with the same name and the quantity plus one so if we have an inventory item that has seven quantity and another inventory item with the same name that has five quantity we would return a new inventory item with the quantity 12 okay otherwise we have a value error and we say it cannot add items of different types great next thing we have sub now this is subtraction right this is the minus operator so same thing here we do our checks we make sure that the quantity is greater than the other quantity and then we return a new inventory item with the subtracted quantities we then have mole this stands for multiplication or the asterisk operator right same thing you can see that now we mult multiply by some kind of factor and this Factor can be an INT or a float so we don't need to be doing operations with the same types but we should Define what types are valid to perform these operations on then we have true division now true division is just the one slash you also have the ability to do floor division which is two slashes but for now we just did one okay so this is just normal Division and then we move into the comparison operators so just like we can perform addition and subtraction multiplication we can also compare objects using the compar operators now the equal here is simply the double equal sign the less than is the less than operator the greater than is the greater than operator and you also have the ability to add other ones we can say Define underscore GTE this is greater than or equal to okay we can double underscore here and that's perfectly fine same thing with less than or equal to and then that would give us the less than or equal to operator okay I don't want to go through all of them because I think it's fairly straightforward but if we go here you can see some example Usage Now of this inventory item because of the fact that we've added these double underscore methods we have three items notice that two of them have the same name this means that we can actually add these together and now we will get a new inventory item that has name apple and quantity 880 because we added these two values together same thing for subtraction you could see that if we subtracted then we would get a quantity of 20 in a new inventory item we can multiply them you'd see we would get apple with 100 and then we can do our different comparisons so we can compare items and see which one has a higher quantity okay now if we try to do something like add item one and item three which don't have the same names you see that we would get a value error we could catch that and then handle it same thing if we do a subtraction here and then we have a negative quantity just a quick example here showing you that there's a bunch of useful methods for the arithmetic operators as well as for the comparison operators there's a few other ones I didn't show you here like for example we could do the not equal to so rather than underscore EQ this is going to be NE okay that stands for not equal to and then you could check the comparison here okay so there you go let's move on to another example now if you thought what I just showed you was cool wait until you have a look at an implementation of a linked list that actually uses the index operator now you've probably seen before that if you define something like a list in Python so like LST is equal to 1 2 3 then you can perform operations like a slice or you can grab maybe the negative 1 index you can grab the zero withth index and you can use that on this type what if I told you even this operation is actually mapped back to double uncore method that you can Implement yourself okay so let's have a look at this we've just made a linked list so I have a node I have a linked list class I Define my initialization and then notice I Define my lend method now this allows me to get the size of the linked list so we just return self. size so if we were to call now the Len function that would map back to this Lenore method but now we've added what's known as the index syntax or kind of the index um double underscore methods so we have a get item set item and delete item this allows us to do something like object and then a set of square brackets and whatever we put inside of the square brackets will be passed as our index so here if we want to index our link list that's what we're trying to do we can check if the index is valid if it is we can then Loop through the linked list and get that element now same thing with setting an item we can do object at index is equal to some kind of value and the syntax looks like this not going to run through all of this this but you get the point next we have delete item so if I want to Dell something using the Dell keyword same thing we can grab the index we can make sure it's valid and we can delete that from the linked list and then we have even contains now contains is what's going to be mapped if you use the in operator so if I say something like you know for in and then some linked list assuming LL is an object of type linked list this will allow me to check if four is in the linked list okay and it will return true or false and go into this Dunder method now we added a normal method notice it's highlighted in green not blue this is not a d method this is just an a pen method so we can add something to the linked list and then we've added our string method so that when we print this out we can see what the linked list looks like so you can see here that we can append some values to the linked list we can get the length of it this is because we implemented the Len Dunder method we can grab ll at index one we can do a set here index one is equal to 25 we can do a deletion and we can check if something is in the linked list because we've added all of this functionality so just to prove to you that I'm not crazy let's go python link list and you can see that all of the output is printed here very good let's move on to the next example so now let's go a step further and let's talk about something known as context managers now I have entire videos on my channel breaking this down so I'm not going to get into it too much but I just want to show you that again all of the syntax you see in Python pretty much Maps back to some kind of Dunder method so look you might have seen this before we have some kind of with this is known as a context manager and we use an instance of some kind of class and then we say as some variable in this case as DB now when we do this what we're doing is we're utilizing a context manager and a context manager exists when some class has the following methods uncore uncore enter and underscore uncore exit now by writing these different methods here this allows us to use this with syntax with an object of this class and what it does is it calls this enter function as soon as we enter the width then this exit function is called as soon as the width is exited whether there's an exception or not now you may have seen a context manager before with something like opening a new file but we can write our own context managers to handle resource usage so in this case we're simulating a database connection so if we want to connect to the database okay we're going to go into this enter method we're going to say that yes we've connected we're going to print out some kind of logging message like we connected to the database and then return ourself and when we return ourself that's what this variable will be equal to we also could return an instance of a new class we can return anything that we want okay then we have the exit now the exit is triggered as soon as we exit the context manager and the important thing here is that this will be called whether an exception occurs or not so you can see that we can take the exception type the exception value and also the traceback of the exception and we can either handle that exception or we can raise it if we want so we'll say that we're disconnected now so self. connected is equal to false we'll print out some kind of logging message and then we can handle different exceptions so we can say if there's some kind of exception type that we're going to print an exception occurred and when we return true this means that we're going to suppress the exception so we're not going to crash the program but if we were to return false this would mean that we would raise that exception or kind of pass it up to the context manager so then we would actually have to handle it anyways the point is that this is some more syntax that you can use and some more advanced double uncore methods that map to context managers now continuing we also have an iterator have you ever wondered how certain types in Python are iterable for example you iterate through something like a list or a string and you can write it in a for Loop right you can say four number in and then the list and you can grab all of the values that exist inside of it well the reason why those are iterable is because they have the iter and the next method defined on them now if something defines the method that means it can be used directly inside of something like a for Loop also a list comprehension there's a bunch of other places where the iterator can be used because what happens implicitly when you use a for Loop in Python is you actually call the iter method on whatever you're trying to Loop through that returns some value and then you continually call the next function on that value until stop iteration is raised now I'm going quickly here because I have again entire videos on my channel breaking down how iterators work but the basic idea works like this when you call the for Loop the first thing that happens is we call theore uncore _ uncore Dunder method on whatever it is that we're iterating through so in this case it's countdown with five this then returns something in this case it just Returns the same countdown object but it could return a new object and then what we do is we continually call the next function or the double uncore next uh method on this object okay so countdown of five this then would return five right then we call this again and again and again and we get four three and then eventually what happens is we raise a stop iteration and then the for Loop stops okay I know this might be confusing if you never seen this before but the way the for Loop works is you call this eater Dunder method that returns something in this case it's just returning this countdown object now the thing that's returned needs to have this next method defined on it because then what we do is we continually call next to iterate through that object so a for Loop isn't just magic it's calling this method it returns some value so it returns five we print five right then we call next again it returns four we print four then we call next again returns three we print three and we keep going until eventually we hit this raise stop iteration and as soon as this exception is occurred the for Loop exits that's how iteration Works in Python in this this is why you can actually build your own iterators like we just did here by implementing the iter and the next method now again just to prove to you that I'm not crazy let's run this code and you can see that we get 5 4 3 2 1 and then it stops because we raised stop iteration and that's pretty much it now there are a ton of other double underscore methods that you can use in Python but I wanted to show you some of the more common ones and really instead of getting you to memorize these methods just make sure that you're aware that they do exist and understand how they map back to different Behavior everything in Python is an object those objects have some kind of class that defines their behavior and on the class you have various double underscore methods which allow you to use the different python syntax features things like for Loops context managers addition operators length functions string functions repper functions all of those work because of the underlying double underscore or magic method implementation with that said guys I will wrap it up here if you enjoyed make sure to leave a like subscribe to the channel and I will see you in the next one [Music]