Magic / Dunder Methods

Python's protocol-based design: every operator and built-in maps to a "dunder" (double-underscore) method.

The ones you'll use most

Syntax	Method
len(x)	__len__
x[i]	__getitem__
for x in y	__iter__ (or __getitem__ as fallback)
x + y	__add__
x == y	__eq__
str(x) / print	__str__ then __repr__
x()	__call__
with x:	__enter__, __exit__

The principle

Implement the dunder, get the syntax for free. Your class plugs straight into all of Python.

Try it

1. Implement __eq__ on Vec so Vec(1,2) == Vec(1,2).
2. Add __mul__ to scale a vector by a number.