Python: What are Python magic methods (dunder methods)?

Magic methods (dunder — double underscore) are special methods that start and end with __.

Common magic methods:

Object lifecycle:

• __init__(self, ...) — constructor.
• __repr__(self) — representation for developers.
• __str__(self) — representation for users.
• __del__(self) — destructor.

Comparison:

• __eq__(self, other) — ==.
• __lt__(self, other) — <.
• __le__(self, other) — <=.
• __hash__(self) — for use in dict/set.

Arithmetic:

• __add__(self, other) — +.
• __mul__(self, other) — *.
• __len__(self) — len().

Example:

1class Vector:
2    def __init__(self, x, y):
3        self.x = x
4        self.y = y
5
6    def __repr__(self):
7        return f"Vector({self.x}, {self.y})"
8
9    def __add__(self, other):
10        return Vector(self.x + other.x, self.y + other.y)
11
12    def __len__(self):
13        return (self.x**2 + self.y**2) ** 0.5
14
15v1 = Vector(1, 2)
16v2 = Vector(3, 4)
17print(v1)           # Vector(1, 2)
18print(v1 + v2)      # Vector(4, 6)
19print(len(v1))      # 2.23