Pure Functions

Definition

1. No Side Effects

# Pure
def add(x, y):
    return x + y

# Impure
def add_and_print(x, y):
    print(f"Adding {x} and {y}")  # Side effect
    return x + y

2. Same Input → Same Output

# Pure
def square(x):
    return x ** 2

# Impure
import random
def random_square(x):
    return x ** 2 + random.randint(0, 10)

Benefits

1. Testable

# Easy to test
def multiply(x, y):
    return x * y

assert multiply(3, 4) == 12

2. Cacheable

@lru_cache
def fibonacci(n):
    if n < 2:
        return n
    return fibonacci(n-1) + fibonacci(n-2)

Summary

• No side effects
• Deterministic
• Easy to test
• Cacheable