dunder_methods

Dunder Methods (Magic Methods)

Double-underscore methods define how objects interact with Python operators, built-ins, and syntax.

Files

File	Description
example.py	Vector, Stack, Sentence with operator overloading

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Descriptive Example

Scenario

Build a 2D vector class that supports +, ==, indexing, and readable printing.

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

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

    def __getitem__(self, index):
        return (self.x, self.y)[index]

v1 = Vector(1, 2)
v2 = Vector(3, 4)
print(v1 + v2)      # Vector(4, 6)
print(v1 == v2)     # False
print(v1[0])        # 1

repr vs str

print(repr(v1))   # Vector(1, 2) — unambiguous, for developers
print(str(v1))    # falls back to __repr__ if __str__ not defined

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Interview Q&A

Q1: What is the difference between __repr__ and __str__?
A: __repr__ is unambiguous, developer-facing (goal: valid Python to recreate object). __str__ is human-readable, used by print(). __str__ falls back to __repr__ if not defined.

Q2: If you define __eq__, should you define __hash__?
A: If objects are equal they must hash equal. Mutable objects with custom __eq__ should set __hash__ = None (unhashable). Immutable objects should define both.

Q3: How do you make an object callable?
A: Define __call__. Then obj() invokes it. Used for callable classes, decorators, and function-like objects.

Q4: What triggers __getitem__?
A: obj[key] syntax. Also used for slicing if __getitem__ handles slice objects.

Q5: What is operator overloading?
A: Defining dunder methods like __add__, __lt__, __len__ so built-in operators work with custom classes.

Q6: What makes an object a context manager? Iterator?
A: Context manager: __enter__ + __exit__. Iterator: __iter__ + __next__. These are protocol-based dunder patterns.

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Run

python3 example.py