Python Day #19: Python Dunder Methods Explained Simply

Learn Python dunder methods like __init__, __str__, and __len__. Master object-oriented programming with clear examples and practical use cases.

You’ve been following along my Python Roadmap to Success, recently we’ve just covered the four pillars of object-oriented programming.

These four pillars are in order for how you should learn them. These are: Encapsulation, Inheritance, Polymorphism, and Abstraction. These help us write cleaner, smarter programs in Python.

Today, I want to shift gears just a little and talk about something I did not introduce in these four pillars, that was intentional as I want us focusing on the core concepts at hand. But you’ve made it this far, so it’s time!

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In this lesson, I am going to cover Dunder methods. These are basically “magical functions” in a sense for working with classes. The term dunder comes from the meaning: double underscore methods.

These are the special methods in Python that start and end with two underscores. Which at this point you already have been using one for OOP as we’ve built out classes over the last four lessons, you’ve used __init__ countless times!

At first glance, they can look intimidating, especially if you have not been using them much. But once you understand them, they’re not only approachable, they’re downright powerful.

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What Are Dunder Methods?

Think of dunder methods as Python’s secret way of letting you customize how your objects behave. They’re hooks into the language itself. Ususally without them we can’t work with objects directly like we could with other variables in Python.

For example if we tried to just print off an object, we get a termianl output that looks something along the lines of: <__main__.Class object at 0x7f3a1c2b7d30>

Pretty much, that is useful to use in no way whatsoever. So we need a way to interact with objects on a basic level with Python just like we can with everything else in our scripts.

For instance if we want our class to work with len() we can use __len__. Or maybe we want to print something meaningful when we run print(my_obj). So instead of getting a. meaningless line back we can use __str__ to do this for us.

Python is already full of these behaviors for its built-in types (strings, lists, dictionaries). Dunder methods let us bring that same useful functionality into our own classes.

There are countless dunder methods from: __add__, __sub__, __enter__, __exit__, __lt__, __gt__, __delete__. The list goes on and on, but in this lesson I want to cover the most commonly used ones.

Once you grasp how these play into our code, picking up new ones will feel like a breeze.

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The Top 5 Most Commonly Used

There are a lot of dunder methods in Python, but you’ll find yourself coming back to the same handful over and over again. Let me break down the five most commonly used ones.

One thing to point out here, is these are useful as they all “call themselves”. You never have to call them like you would a normal class method linked to an object, no. As soon as you create an object, these will call themselves as you need them.

You just need to use your object like you would any other variable in your code.

__init__: The Constructor

This is the one you’ve already seen the most. It runs every time you create a new object, and it sets everything up. This is hands down our most common one we see and it’s needed most of the time we are setting up a class.

What’s the appeal? Well this is automatically called when we make an object to initialize (set up) our class and is often the home to all of our class properties.

We can think of creating an object like moving into a new house, __init__ is the process of putting furniture inside so the house is livable. We are passing in outside information from the object, into the class through __init__ to get everything ready to “live in.”

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__str__: Human-Readable String

We can add in the ability to actually print off an object. Becasue when we do this we want something readable to us and natively we can not just print an object off.

If we print off an object we would normally get something like: <__main__.Class object at 0x7f3a1c2b7d30>.

But when we introduce the __str__ method then the whole game changes.

Without __str__, printing an object just gives you a cryptic memory address. With it, you control what gets displayed. So when I try to print off my passenger object above this time it will actually return to me the string I want with the output in this case of:

Departing from Saigon with Air France!

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__repr__: Developer-Friendly String

While __str__ is for humans, __repr__ is for developers. It’s what shows up when you type an object into the shell or use the Python repr() function.

We use this more commonly in development so we don’t have to go around using print all the time, spamming up our codebase. We use this just through the terminal or using the repr() function to tell apart from print().

So while __str__ is like a casual nickname you’d use in conversation, __repr__ is more like an official ID card. It’s precise, unambiguous, and mainly there to help you (or another developer) quickly understand what’s going on under the hood.

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__len__: Length of an Object

You already know and use Python’s built-in len() function all the time. You call len() on a list and it tells you how many items are inside. You call it on a string and it tells you how many characters are in it.

But what if you make your own class and you want it to work the same way? That’s exactly what the __len__ method is for.

Think of __len__ as your way of teaching Python how to measure the size of your object. Without it, Python has no idea what len(my_object) should mean.

Now the Airplane class is holding onto a list of passengers. By adding __len__, we’ve told Python: “If someone asks how long this airplane is, just tell them how many people are inside.”

So when we run len(airplane), it doesn’t blow up with an error—it works exactly like you’d expect, giving us back 4.

__getitem__: Indexing an Object

Normally, when you think of lists in Python, you just grab something with square brackets. This is what we call indexing. It looks like this:

That works because lists already know how to handle those brackets. But if you create your own class, it doesn’t magically know how to do that. Python is dumb… It’s up to us to tell it how to act.

That’s where __getitem__ comes in. It basically teaches your object how to respond when someone writes thing[0] or thing["key"].

Now let’s load this plane up with some very “special” passengers:

See what happened? Without __getitem__, trying airplane[0] would have crashed, pun intended. But because we defined it, Python knows, “Oh, this object is pretending to be like a list, so I’ll hand it over to __getitem__.”

This trick is useful whenever you’re building your own custom containers, maybe a deck of cards, a playlist, or even your own kind of database. You get to decide how the square brackets should work.

Wrapping Up Lesson Nineteen

Dunder methods might look a little strange when you first see them, but they’re really just built-in shortcuts that let your classes act like real Python objects.

We walked through how __init__ sets things up when you make an object, how __str__ and __repr__ make it easier to actually see what’s inside, and how __len__ and __getitem__ let your custom classes work with things like len() and square brackets.

The main takeaway is that dunder methods aren’t anything extra or complicated. They’re simply the way you tell Python, “Hey, treat my object the same way you treat strings, lists, or dictionaries.” Instead of writing everything from scratch, you can tap into what Python already knows how to do.

As your projects get bigger, don’t shy away from these. They’ll save you time, make your code cleaner, and help it feel more natural to work with.

So the next time you write a class, ask yourself: “What should happen if someone prints it, measures it, or tries to index into it?” More often than not, there’s already a dunder method that handles exactly that.

Let’s keep moving forward with Day #20 next week.

Hope you all have an amazing week nerds ~ Josh (Chief Nerd Officer 🤓)

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