First-Class Functions in Python

In Python, functions are first-class citizens, meaning functions can be treated like any other object (int, string, list, etc.).

This means you can: - Assign functions to variables - Pass functions as arguments - Return functions from other functions - Store functions inside data structures

---

1. Assigning Functions to Variables

A function can be assigned to a variable and used through that variable.

Example:

def greet():
    return "Hello"

say_hello = greet

print(say_hello())

Output:

Hello

Real-world example: A payment system where different payment methods are assigned dynamically.

def pay_by_card():
    return "Paid using Card"

def pay_by_upi():
    return "Paid using UPI"

payment = pay_by_upi
print(payment())

Output:

Paid using UPI

---

Common Mistake

def greet():
    return "Hello"

g = greet()
print(g())

Error: - Because greet() is executed immediately instead of assigning function

Correct:

g = greet

---

Homework 1

Create a function add() and assign it to a variable operation. Call it using the variable.

Solution:

def add():
    return 5 + 3

operation = add
print(operation())

---

2. Passing Functions as Arguments

Functions can be passed into other functions.

Example:

def add(a, b):
    return a + b

def calculate(func, a, b):
    return func(a, b)

print(calculate(add, 10, 5))

Output:

15

Real-world example: Different discount strategies in e-commerce.

def discount_10(price):
    return price * 0.9

def discount_20(price):
    return price * 0.8

def apply_discount(func, price):
    return func(price)

print(apply_discount(discount_10, 1000))

Output:

900.0

---

Common Mistake

def add(a, b):
    return a + b

print(calculate(add(), 10, 5))

Problem: - add() is called instead of passing function reference

Correct:

calculate(add, 10, 5)

---

Homework 2

Create: - multiply function - pass it into a calculator function

Solution:

def multiply(a, b):
    return a * b

def calculator(func, a, b):
    return func(a, b)

print(calculator(multiply, 4, 5))

---

3. Returning Functions

A function can return another function.

Example:

def outer():
    def inner():
        return "Inner function called"
    return inner

f = outer()
print(f())

Output:

Inner function called

Real-world example: Login system based on user roles.

def role_checker(role):
    def admin():
        return "Admin Access"

    def user():
        return "User Access"

    if role == "admin":
        return admin
    else:
        return user

access = role_checker("admin")
print(access())

Output:

Admin Access

---

Common Mistake

def outer():
    def inner():
        return "Hello"
    return inner()

f = outer
print(f())

Issues: - Function not called correctly

Correct:

f = outer()

---

Homework 3

Create a function that returns a function which multiplies by 2.

Solution:

def outer():
    def inner(x):
        return x * 2
    return inner

double = outer()
print(double(10))

---

4. Storing Functions in Data Structures

Functions can be stored in lists, dictionaries, etc.

Example (List):

def add(a, b):
    return a + b

def sub(a, b):
    return a - b

operations = [add, sub]

print(operations[0](10, 5))
print(operations[1](10, 5))

Output:

15
5

---

Example (Dictionary - Real World Use Case):

def deposit():
    return "Money Deposited"

def withdraw():
    return "Money Withdrawn"

bank_ops = {
    "d": deposit,
    "w": withdraw
}

print(bank_ops["d"]())

Output:

Money Deposited

---

Real World Analogy

Concept	Real World Example
Function variable	Remote control button mapped to TV action
Passing function	Choosing payment method
Returning function	Role-based access system
Function storage	Menu system in ATM

---

Graph (Conceptual Flow)

Function Object
      |
      +--> Assigned to variable
      +--> Passed as argument
      +--> Returned from function
      +--> Stored in list/dict

---

Common Mistakes Summary

Mistake	Problem
add() instead of add	function executed instead of passed
returning func() instead of func	returns result instead of function
calling function too early	breaks higher-order logic

---

Homework 4 (Mixed Challenge)

Create a calculator system: - add, subtract, multiply functions - store them in dictionary - take input key and execute function

Solution:

def add(a, b):
    return a + b

def sub(a, b):
    return a - b

def mul(a, b):
    return a * b

ops = {
    "+": add,
    "-": sub,
    "*": mul
}

print(ops["+"](10, 5))

---

Next Advanced Topics to Learn

• Closures in Python
• Decorators
• Lambda Functions
• Higher Order Functions in depth
• Functional Programming in Python
• functools module (map, filter, reduce) ```