Module 6: First-Class Functions

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Learning outcomes

• treat functions like ordinary values;
• write a function that returns another function;
• explain what a closure is;
• use function values in simple examples.

Prerequisites

Read Module 5 first. You should be comfortable with function syntax and lambda.

The idea in one sentence

First-class functions are functions that Python treats like ordinary values.

Everyday analogy

Think of a recipe card. You can store it in a drawer, hand it to someone, or reuse it later. A function in Python can be handled the same way.

Syntax

def make_adder(n):
    def add(x):
        return x + n
    return add

Worked example

add_five = make_adder(5)
result = add_five(10)

Step by step:

1. make_adder(5) creates a new function.
2. That function remembers n = 5.
3. add_five(10) uses the remembered value.
4. The result is 15.

Why this matters in practice

• In college notes, it explains why functions can be passed around like data.
• In real code, it supports callbacks, decorators, pipelines, and factory functions.
• It makes programs more reusable and easier to organize.

Important concepts

First-class function

A function is first-class if you can:

• store it in a variable
• pass it as an argument
• return it from another function

Closure

A closure is a function that remembers values from the environment where it was created.

def outer(n):
    def inner(x):
        return x + n
    return inner

Function factory

A function factory is a function that creates and returns another function.

def power(exp):
    return lambda x: x ** exp

Technical meaning

Python functions are first-class objects. That means they have identity, can be assigned to variables, and can carry along remembered state through closures. This supports higher-order programming patterns.

Memory rule

• store a function in a variable
• pass a function as an argument
• return a function from another function

Quick check

• Why is make_adder useful?
• What value does add_five remember?
• What is a closure?
• How is a function factory different from an ordinary function?

Short exam answer

First-class functions are functions that Python can store, pass, and return like any other value. A closure is a function that remembers values from the surrounding scope. This makes function-based design more flexible and reusable.

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