Python basics - Day 15

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Day 15 – Functions (def, return, parameters)

Project: Build a “Simple Calculator” using user-defined functions

01. Learning Goal

By the end of this lesson, you will be able to:

Define and call your own functions using def
Pass values into functions using parameters
Return results using return
Understand default parameters and multiple return values

02. Problem Scenario

You often repeat the same code for arithmetic operations.

Your goal is to modularize your code by creating reusable functions to handle these tasks.

03. Step 1 – What is a Function?

A function is a reusable block of code that performs a specific task.

You must define it first, then call it.

def say_hello():
    print("Hello!")

say_hello()   # Function call

04. Step 2 – Using Parameters

Functions can receive data through parameters.

def greet(name):
    print(f"Nice to meet you, {name}!")

greet("Sabin")   # Nice to meet you, Sabin!

05. Step 3 – Returning Values

Functions can return results using return.

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

result = add(3, 5)
print(result)   # 8

06. Step 4 – Returning Multiple Values

Python functions can return more than one value at once (as a tuple).

def calc(a, b):
    return a + b, a - b, a * b

s, d, m = calc(10, 5)
print(s, d, m)   # 15 5 50

07. Step 5 – Default Parameters

A parameter can have a default value, used when no argument is provided.

def greet(name="Guest"):
    print(f"Hello, {name}")

greet()          # Hello, Guest
greet("Sabin")   # Hello, Sabin

08. Step 6 – Functions with Loops

You can combine loops with functions for repetitive calculations.

def square(n):
    return n * n

for i in range(1, 6):
    print(square(i))

09. Step 7 – Practice Examples

Example 1: Basic Calculator Functions

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

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

print("Add:", add(10, 5))
print("Subtract:", sub(10, 5))

Example 2: String Function

def shout(text):
    return text.upper() + "!!!"

print(shout("python"))   # PYTHON!!!

10. Step 8 – Mini Project: Simple Calculator

Build a mini calculator that performs multiple operations using functions.

def add(a, b): return a + b
def sub(a, b): return a - b
def mul(a, b): return a * b
def div(a, b): return a / b if b != 0 else "Error: Division by zero"

a = float(input("Enter first number: "))
b = float(input("Enter second number: "))

print("Add:", add(a, b))
print("Subtract:", sub(a, b))
print("Multiply:", mul(a, b))
print("Divide:", div(a, b))