Dunder (Magic) Methods

Introduction

Dunder methods (__xxx__) hook into Python's protocols: __len__ for len(), __iter__ for loops, __eq__ for ==, etc.

Understanding the topic

Core concepts to understand:

• __init__, __repr__, __str__.
• __eq__, __hash__ — equality.
• __len__, __getitem__, __iter__.
• __enter__/__exit__ — context managers.

Syntax reference

Visual flow / code:

class Bag:
    def __init__(self, items):
        self.items = list(items)
    def __len__(self):     return len(self.items)
    def __iter__(self):    return iter(self.items)
    def __getitem__(self, i): return self.items[i]
    def __repr__(self):    return f"Bag({self.items!r})"

b = Bag([1, 2, 3])
print(len(b))      # 3
for x in b: print(x)
print(b[0])
print(b)           # Bag([1, 2, 3])

Execution workflow

Real-world use

Implementing the right dunders makes your class behave like a built-in — usable in for, len, in, JSON pretty-print, etc.

Best practices

• Implement __repr__ for debuggability.
• __eq__ + __hash__ together.
• Use dataclasses to auto-generate them.

Hands-on exercise

Interview preparation — practice these questions:

• Difference between __str__ and __repr__?
• When add __hash__?
• What are protocols in Python?

Summary

In summary: Dunders = duck-typed protocols. dataclasses auto-generate many.