Dunder Methods & the Python Data Model

18 min 45 XP

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Lesson 4 of 6 · View course roadmap

Step 1

Learn the idea

Why does len("abc") work? Because str implements __len__. Python's operators and built-ins are a thin layer over dunder (double-underscore) methods — implement them and your own classes plug straight into the language:

__repr__ — how the object prints (aim for something a developer could paste back into code)
__add__ — powers a + b
__eq__ — powers == (by default Python compares identity, not value!)
__len__, __getitem__, __contains__ — make objects sliceable, iterable and in-testable

This is called the data model, and it's the most Pythonic idea in the language: instead of inventing .equals() or .plus() methods, you teach your objects to speak Python's native vocabulary.

Where you'll use this

pathlib overloads / to join paths, NumPy overloads every operator for arrays, Django models compare by value — Python's most loved libraries feel native precisely because they implement the data model instead of inventing method names.

Common mistakes

Defining __eq__ without thinking about __hash__ — Python sets __hash__ to None, and your objects can no longer live in sets or dict keys. Implement both or use @dataclass(frozen=True).
Returning a plain tuple from __add__ instead of a new instance of your class — addition should stay closed over the type.
Writing __repr__ that hides information: aim for eval-able output like Vector(4, 6), not '<Vector object>'.

Pro tip

Implement __repr__ on every class you write, first thing. Debugging a list of '' is misery; a list of Order(id=7, total=99.5) reads itself.

Step 2

Try it yourself

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The lesson example is loaded and ready — press Run, then change something and run it again. Breaking it is part of learning. Want a clean slate? Tap “New blank”.

PYexample.py

⌘ + Enter to run

Output appears here…

Step 3

Pass the challenge +45 XP

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Build a Vector class with x and y. Implement __repr__ returning Vector(x, y), __add__ for coordinate-wise addition, and __eq__ for value equality. Print Vector(1, 2) + Vector(3, 4), then print Vector(1, 2) == Vector(1, 2).

Target output

Vector(4, 6)
True

PYchallenge.py

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):
        return f"Vector({self.x}, {self.y})"

    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)

    def __eq__(self, other):
        return self.x == other.x and self.y == other.y

print(Vector(1, 2) + Vector(3, 4))
print(Vector(1, 2) == Vector(1, 2))

Run your code to check it…

Step 4

Check your understanding

1. Without __eq__, what does obj1 == obj2 compare? All attributes Their identity (are they the same object?) Their string forms It raises TypeError

2. Which dunder does a + b call? a.__plus__(b) a.__add__(b) a.__sum__(b) add(a, b)

Last step

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