Magic Methods

In This Chapter

• What magic methods are and why Python uses them
• The difference between __repr__ and __str__
• How methods like __len__, __eq__, and __iter__ change object behavior
• Why magic methods let custom classes participate in Python protocols
• Common interview mistakes around dunder methods

What Are Magic Methods?

Magic methods, also called dunder methods , are methods with names like __init__, __repr__, or __len__.

Python calls them automatically in specific situations:

class Box:
    def __init__(self, items):
        self.items = items

    def __len__(self):
        return len(self.items)

box = Box([1, 2, 3])
len(box)   # 3

You do not call box.__len__() directly in normal code. Python calls it when len(box) runs.

__repr__ vs __str__

These are two of the most commonly asked interview methods:

class User:
    def __init__(self, name: str):
        self.name = name

    def __repr__(self) -> str:
        return f"User(name={self.name!r})"

    def __str__(self) -> str:
        return self.name

• __repr__ is for developers
• __str__ is for human-friendly display

If __str__ is missing, Python falls back to __repr__.

Comparison Methods

Methods like __eq__ and __lt__ define how objects compare:

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

    def __eq__(self, other: object) -> bool:
        if not isinstance(other, Point):
            return NotImplemented
        return self.x == other.x and self.y == other.y

Returning NotImplemented for unsupported types is the right protocol behavior.

Container and Iteration Methods

Python also uses magic methods for container behavior:

• __len__ for len(obj)
• __contains__ for x in obj
• __iter__ for for x in obj
• __getitem__ for obj[key]

That is why custom classes can feel like built-in types when designed well.

Protocol Thinking

The most useful interview idea is this: magic methods let your class participate in Python's protocols.

If your object implements __iter__, it behaves like an iterable. If it implements __enter__ and __exit__, it behaves like a context manager.

This is more important than memorizing a long list of dunder names.

Key Questions

Q: What are magic methods in Python?

They are special methods with double underscores that Python calls automatically to support language features like printing, comparison, iteration, and arithmetic.

Q: What is the difference between __repr__ and __str__?

__repr__ is for developers and should ideally be unambiguous. __str__ is for human-friendly display.

Q: Why does len(obj) work on custom classes?

Because Python checks whether the class defines __len__ and, if so, calls it.

Q: Why should __eq__ sometimes return NotImplemented?

Because that tells Python the comparison is unsupported for that type pair, which is better than making incorrect assumptions.

Q: What is the bigger idea behind magic methods?

They let your class participate in Python protocols, so custom objects can behave like built-in objects when appropriate.