Imperative approach

Describe each step and update state along the way.

prices = [10, 20, 30]
discounted = []

for price in prices:
    new_price = price * 0.9
    discounted.append(new_price)

Functional approach

Describe the transformation and produce a new value.

prices = [10, 20, 30]

discounted = [
    price * 0.9
    for price in prices
]

• Depend only on their arguments.
• Return a value instead of modifying external state.
• Easier to test because they are predictable.

# Pure
def add_tax(price, rate):
    return price * (1 + rate)

# Not pure: depends on global state
tax_rate = 0.07

def add_tax_global(price):
    return price * (1 + tax_rate)

Functional code often returns a new value instead of changing an existing one.

• Original data stays available.
• Unexpected side effects are less likely.
• Pipelines become easier to reason about.

def add_item(items, new_item):
    return items + [new_item]

cart = ["book", "pen"]
new_cart = add_item(cart, "notebook")

print(cart)      # ['book', 'pen']
print(new_cart)  # ['book', 'pen', 'notebook']

• Assign a function to a variable.
• Pass a function as an argument.
• Return a function from another function.

def square(x):
    return x * x

def apply(func, values):
    return [func(v) for v in values]

nums = [1, 2, 3, 4]
print(apply(square, nums))
# [1, 4, 9, 16]

A lambda is a small anonymous function for simple expressions.

names = ["Ada", "Grace", "Linus"]

by_length = sorted(
    names,
    key=lambda name: len(name)
)

print(by_length)
# ['Ada', 'Grace', 'Linus']

Functional tools

nums = [1, 2, 3, 4, 5]

squares = list(map(lambda n: n*n, nums))
evens   = list(filter(lambda n: n % 2 == 0, nums))

Pythonic alternative

nums = [1, 2, 3, 4, 5]

squares = [n*n for n in nums]
evens   = [n for n in nums if n % 2 == 0]

A higher-order function works with other functions.

• Takes a function as input.
• Returns a function as output.
• Builds reusable behavior without duplicating control flow.
• make_multiplier returns a closure: multiply remembers factor (see 4.2).

def make_multiplier(factor):
    def multiply(x):
        return x * factor
    return multiply

double = make_multiplier(2)
triple = make_multiplier(3)

print(double(10))  # 20
print(triple(10))  # 30

• A decorator takes a function and returns a wrapped function.
• Use for logging, timing, validation, caching, and access checks.
• @decorator syntax replaces manual wrapping.

def announce(func):
    def wrapper(*args, **kwargs):
        print(f"Calling {func.__name__}")
        return func(*args, **kwargs)
    return wrapper

@announce
def greet(name):
    return f"Hello, {name}"

print(greet("Ada"))

Recap from 4.3 Recursion: a base case stops the calls, and a recursive case calls the function on a smaller problem.

• Fits functional style: repeats work without mutating a loop counter.
• Good fit for self-similar structure such as trees and nested folders.

A context manager handles setup and cleanup around a block of code.

• Open and close files safely.
• Acquire and release resources.
• Keep cleanup reliable even when errors happen.

from contextlib import contextmanager

@contextmanager
def section(name):
    print(f"Start {name}")
    try:
        yield
    finally:
        print(f"End {name}")

with section("demo"):
    print("working")

from functools import reduce, partial, lru_cache

total = reduce(lambda a, b: a + b, [1, 2, 3])

# initializer: needed when items are not the result type
students = [("Alice", 85), ("Bob", 90), ("Charlie", 78)]
grade_sum = reduce(lambda acc, s: acc + s[1], students, 0)   # 253
reduce(lambda acc, x: acc + x, [], 0)                       # 0, no TypeError

def power(base, exp):
    return base ** exp

square = partial(power, exp=2)

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

Use functional style when…

• You are transforming collections.
• A pure helper function makes logic easier to test.
• A comprehension or small pipeline is clearer than a loop.

Prefer another style when…

• The lambda becomes hard to read.
• The code needs several statements or error handling.
• Mutating state is the simplest honest model.