Magic Methods (Dunder Methods)

1# ============================================================
2# String representation: __str__ and __repr__
3# ============================================================
4class Vector:
5    def __init__(self, x, y):
6        self.x = x
7        self.y = y
8
9    def __repr__(self):
10        """Unambiguous representation (for developers)."""
11        return f"Vector({self.x}, {self.y})"
12
13    def __str__(self):
14        """Readable representation (for users)."""
15        return f"({self.x}, {self.y})"
16
17    def __format__(self, spec):
18        """Custom formatting: f'{v:.2f}'"""
19        if spec:
20            return f"({self.x:{spec}}, {self.y:{spec}})"
21        return str(self)
22
23v = Vector(3.14159, 2.71828)
24print(repr(v))        # Vector(3.14159, 2.71828)
25print(str(v))         # (3.14159, 2.71828)
26print(f"{v:.2f}")     # (3.14, 2.72)
27
28# ============================================================
29# Arithmetic operators
30# ============================================================
31class Vector:
32    def __init__(self, x, y):
33        self.x = x
34        self.y = y
35
36    def __add__(self, other):
37        if isinstance(other, Vector):
38            return Vector(self.x + other.x, self.y + other.y)
39        return NotImplemented
40
41    def __sub__(self, other):
42        if isinstance(other, Vector):
43            return Vector(self.x - other.x, self.y - other.y)
44        return NotImplemented
45
46    def __mul__(self, scalar):
47        if isinstance(scalar, (int, float)):
48            return Vector(self.x * scalar, self.y * scalar)
49        return NotImplemented
50
51    def __rmul__(self, scalar):
52        """Reflected multiply: allows 3 * vector."""
53        return self.__mul__(scalar)
54
55    def __abs__(self):
56        """Magnitude: abs(vector)."""
57        return (self.x ** 2 + self.y ** 2) ** 0.5
58
59    def __neg__(self):
60        return Vector(-self.x, -self.y)
61
62    def __repr__(self):
63        return f"Vector({self.x}, {self.y})"
64
65v1 = Vector(1, 2)
66v2 = Vector(3, 4)
67
68print(v1 + v2)     # Vector(4, 6)
69print(v1 - v2)     # Vector(-2, -2)
70print(v1 * 3)      # Vector(3, 6)
71print(3 * v1)      # Vector(3, 6) — works because of __rmul__
72print(abs(v2))      # 5.0
73print(-v1)          # Vector(-1, -2)
74
75# ============================================================
76# Comparison operators with total_ordering
77# ============================================================
78from functools import total_ordering
79
80@total_ordering
81class Temperature:
82    def __init__(self, celsius):
83        self.celsius = celsius
84
85    def __eq__(self, other):
86        if not isinstance(other, Temperature):
87            return NotImplemented
88        return self.celsius == other.celsius
89
90    def __lt__(self, other):
91        if not isinstance(other, Temperature):
92            return NotImplemented
93        return self.celsius < other.celsius
94
95    def __hash__(self):
96        return hash(self.celsius)
97
98    def __repr__(self):
99        return f"Temperature({self.celsius}°C)"
100
101t1 = Temperature(20)
102t2 = Temperature(30)
103t3 = Temperature(20)
104
105print(t1 < t2)     # True
106print(t1 >= t3)     # True (auto-generated by total_ordering)
107print(t1 == t3)     # True
108print(sorted([t2, t1, Temperature(25)]))  # [20°C, 25°C, 30°C]
109
110# Can use as dict key because __hash__ is defined
111temps = {t1: "comfortable", t2: "hot"}
112
113# ============================================================
114# Container protocol: __len__, __getitem__, __contains__
115# ============================================================
116class Deck:
117    """A deck of playing cards."""
118    suits = ["Hearts", "Diamonds", "Clubs", "Spades"]
119    ranks = ["2", "3", "4", "5", "6", "7", "8", "9", "10",
120             "Jack", "Queen", "King", "Ace"]
121
122    def __init__(self):
123        self.cards = [
124            f"{rank} of {suit}"
125            for suit in self.suits
126            for rank in self.ranks
127        ]
128
129    def __len__(self):
130        return len(self.cards)
131
132    def __getitem__(self, index):
133        return self.cards[index]
134
135    def __contains__(self, card):
136        return card in self.cards
137
138    def __iter__(self):
139        return iter(self.cards)
140
141deck = Deck()
142print(len(deck))                     # 52
143print(deck[0])                       # "2 of Hearts"
144print(deck[-1])                      # "Ace of Spades"
145print(deck[10:13])                   # Slicing works!
146print("Ace of Spades" in deck)       # True
147
148# Iteration works because of __iter__
149for card in deck:
150    if "Ace" in card:
151        print(card)
152
153# ============================================================
154# __call__ — making objects callable
155# ============================================================
156class Validator:
157    def __init__(self, min_val, max_val):
158        self.min_val = min_val
159        self.max_val = max_val
160
161    def __call__(self, value):
162        """Makes the object callable like a function."""
163        if not self.min_val <= value <= self.max_val:
164            raise ValueError(
165                f"{value} not in range [{self.min_val}, {self.max_val}]"
166            )
167        return True
168
169# Use like a function
170validate_age = Validator(0, 150)
171validate_score = Validator(0, 100)
172
173print(validate_age(25))     # True
174print(validate_score(85))   # True
175# validate_age(200)         # ValueError
176
177# Callable check
178print(callable(validate_age))  # True