dunder（マジック）メソッドとは何ですか？

class Vector:
    def __init__(self, x, y):       # constructor — Vector(1, 2)
        self.x, self.y = x, y

    def __repr__(self):             # repr(v) and the REPL — for developers
        return f"Vector({self.x}, {self.y})"

    def __str__(self):              # str(v) / print(v) — for users
        return f"({self.x}, {self.y})"

    def __add__(self, other):       # enables the + operator: v1 + v2
        return Vector(self.x + other.x, self.y + other.y)

    def __eq__(self, other):        # enables ==
        return self.x == other.x and self.y == other.y

v1, v2 = Vector(1, 2), Vector(3, 4)
v1 + v2          # Vector(4, 6) — calls __add__
print(v1)        # (1, 2) — calls __str__
v1 == Vector(1, 2)  # True — calls __eq__

__init__   → constructor (object creation)
__repr__   → unambiguous developer representation (debugging, REPL)
__str__    → readable user-facing string (print, str())
__len__    → len(obj)
__getitem__→ obj[key]  (indexing/subscripting)
__iter__ / __next__ → make an object iterable (for loops)
__eq__, __lt__, __gt__ → comparison operators (==, <, >)
__add__, __sub__, __mul__ → arithmetic operators
__call__   → make an instance callable like a function: obj()
__contains__ → the `in` operator
__enter__ / __exit__ → context managers (with statement)

class Deck:
    def __init__(self): self.cards = [...]
    def __len__(self): return len(self.cards)        # len(deck)
    def __getitem__(self, i): return self.cards[i]    # deck[0], and iteration/slicing!

deck = Deck()
len(deck)           # works
for card in deck:   # __getitem__ makes it iterable too
    ...