pitfalls.md

Chapter 16 — Pitfalls: Operator Overloading

---

Pitfall 1: Raising NotImplementedError in an Operator

Because they sound similar, developers frequently confuse the Exception with the Singleton.

class Vector:
    def __add__(self, other):
        if not isinstance(other, Vector):
            # BUG: This crashes the program!
            raise NotImplementedError 
        return Vector(self.x + other.x)

v = Vector()
# This crashes instantly with an Exception trace.
result = v + (10, 20)

Why it fails: By raising an exception, you halt the execution engine. Python is supposed to catch NotImplemented (the singleton) and try tuple.__radd__(v). By raising the Error, you rob Python of its fallback mechanism. Fix: return NotImplemented

---

Pitfall 2: Mutating self inside __add__

Infix operators (+, -, *) are universally expected to be immutable operations that produce a brand new value.

class BankAccount:
    def __init__(self, bal):
        self.bal = bal
        
    def __add__(self, other):
        # BUG: __add__ should NEVER mutate self!
        self.bal += other.bal
        return self

a = BankAccount(100)
b = BankAccount(50)
c = a + b

print(a.bal) # 150! We accidentally mutated account A just by evaluating a + b!

Fix: __add__ must always return a BankAccount(self.bal + other.bal). Only __iadd__ is allowed to mutate self.

---

Pitfall 3: Forgetting to return self in __iadd__

When you write a += b, Python evaluates a = a.__iadd__(b).

class Cart:
    def __init__(self):
        self.items = []
        
    def __iadd__(self, item):
        self.items.append(item)
        # BUG: Missing `return self`

c = Cart()
c += "Apple"

print(c) # None! 

Why it fails: Because __iadd__ didn't return anything, it implicitly returned None. Python then evaluated c = None, completely destroying the Cart object. Fix: Always write return self at the end of augmented assignment methods.