PYTHON FUNDAMENTALS | From Basics to Real-World Applications

By Igbojionu Chukwudi for AIGE

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HOW THIS BOOK WORKS

This book is designed as a hybrid learning guide.

Instead of teaching Python by days, it is structured around:

• Real-world problems
• Skills required to solve them
• Practical examples and mini projects

Each chapter builds on the previous one.
You may complete:

• One chapter per day
• Multiple chapters per week
• Or learn entirely at your own pace

All examples, explanations, and code are written for absolute beginners.

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TABLE OF CONTENTS

1. Chapter 1 – Python Foundations
2. Chapter 2 – Working With User Data
3. Chapter 3 – Decision Making With Code
4. Chapter 4 – Automating Repetitive Tasks
5. Chapter 5 – Managing Collections of Data
6. Chapter 6 – Writing Reusable & Clean Code
7. Chapter 7 – Building a Complete Python Application

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CHAPTER 1 — PYTHON FOUNDATIONS

Problem This Solves

How do we communicate clear instructions to a computer?

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What you'll learn in this chapter

By the end of this chapter you should be able to:

• Understand what Python is (and why it's popular)
• Run Python code on your computer
• Use print() to display output
• Write comments
• Create variables (store information)
• Follow basic naming rules

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1) What is Python?

Python is a programming language you use to tell a computer what to do.

Python is used for:

• Websites (Django, Flask)
• Data & AI (Pandas, NumPy, ML)
• Automation (scripts that save time)
• Apps and tools (simple programs)

Python is beginner-friendly because it reads almost like English.

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2) How to Run Python (3 common ways)

Option A: Python Interactive Mode (REPL)

Open your terminal/cmd and type:

python

You’ll see >>> then you can type code directly:

>>> print("Hello")
Hello

To exit:

exit()

Option B: Run a .py file

Create a file like day1.py and run:

python day1.py

Option C: VS Code / Cursor

• Install Python extension
• Create day1.py
• Click Run ▶️

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3) Your First Python Program (print)

What print() does

print() tells Python: show this on the screen.

print("Hello, world!")

Code explanation

• print is a function
• () means “call/execute the function”
• "Hello, world!" is a string (text)
• Strings usually sit inside quotes " " or ' '

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4) Comments (Notes Python will ignore)

Comments help you remember what your code does.

Single-line comment

# This is a comment
print("This line will run")

Why comments matter

If you come back after 1 week, comments help you understand your own code.

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5) Variables (Storing information)

A variable is like a container that stores a value.

Example:

name = "Jane"
age = 17

Explanation

• name is the variable name
• = means “store this value inside the variable”
• "Jane" is stored in name
• 17 is stored in age

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6) Common Data Types

String (text)

business_name = "Snap Pro"

Integer (whole number)

items_in_stock = 25

Float (number with decimal)

price = 2500.50

Boolean (True/False)

is_open = True

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7) Printing Variables (Combining text + variables)

Method 1: Comma separation (easy)

name = "Jane"
print("Hello", name)

Explanation: Python automatically adds a space between items.

Method 2: f-strings (modern and clean ✅)

name = "Jane"
age = 17
print(f"My name is {name} and I am {age} years old.")

Explanation of f-string

• The f before the string enables formatting
• {name} inserts the variable value inside the text
• It’s the most common style in modern Python

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8) Variable Naming Rules (Important!)

✅ Valid:

first_name = "Jane"
age2 = 20
total_sales = 5000

❌ Invalid:

2age = 20         # cannot start with number
my-name = "A"     # dash is not allowed
class = "test"    # reserved keyword

Quick rule

• Start with a letter or _
• Use _ for spaces (snake_case)
• Don’t use Python reserved words (like class, def, if)

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9) Real-Life Task: Personal Profile Script ✅

Task

Write a script that prints your:

• Full name
• Business name
• Location
• Skill
• A short message

Full Code (Day1 Project)

# Day 1 Project: Personal Profile Script

full_name = "Jane"
business_name = "Snap Pro"
location = "Nigeria"
skill = "Django development"

print("=== MY PROFILE ===")
print("Full name:", full_name)
print("Business name:", business_name)
print("Location:", location)
print("Skill:", skill)

print("\nMessage:")
print(f"Hi, I'm {full_name}. I build digital solutions with {skill}.")

Code walkthrough

• The first 4 lines create variables to store info
• print("=== MY PROFILE ===") prints a title
• \n means new line (it drops to the next line)
• The last line uses an f-string to combine text + variables nicely

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10) Practice Exercises

1. Create variables for:

• Your favorite food
• Your favorite app
• Your dream country
  1. Print them in a neat format.
  2. Change your variables and run again.

Example:

food = "Jollof rice"
app = "Instagram"
country = "Canada"

print(f"My favorite food is {food}.")
print(f"My favorite app is {app}.")
print(f"I would love to visit {country}.")

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Mini Challenge (Optional ⭐)

Make your script look nicer:

• Add separators like ----------
• Add emojis inside print (Python supports it)

print("🔥 Welcome to my profile 🔥")
print("--------------------------")

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CHAPTER 2 — WORKING WITH USER DATA

Problem This Solves

How do programs collect information from users and work with it?

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What you'll learn in this chapter

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

• Understand user input
• Accept data from users
• Convert input to numbers
• Perform calculations
• Build simple real-life interactive programs

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1️⃣ Understanding input()

What is input()?

input() allows your program to receive information from the user while it’s running.

Example:

name = input("Enter your name: ")
print("Hello", name)

Explanation

• Python pauses and waits for the user
• Whatever the user types becomes a string
• The value is stored in name

⚠️ Important rule:

input() ALWAYS returns a string

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2️⃣ Strings vs Numbers (Very Important)

Example

age = input("Enter your age: ")
print(age)

Even if the user types 25, Python sees it as:

"25"  ← string, not number

Why this matters

You cannot do math with strings.

❌ This will cause an error:

age = input("Enter age: ")
next_year = age + 1

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3️⃣ Type Conversion (Casting)

Convert string to number

age = int(input("Enter age: "))
print(age + 1)

Common conversions

Function	Converts to
int()	Whole numbers
float()	Decimal numbers
str()	Text

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4️⃣ Practical Example: Simple Calculator

num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))

total = num1 + num2
print("Total:", total)

Code explanation

• User enters two numbers
• int() converts them
• Python adds them
• Result is printed

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5️⃣ Basic Math Operators

Operator	Meaning
+	Addition
-	Subtraction
*	Multiplication
/	Division
//	Whole division
%	Remainder
**	Power

Example

price = 2500
qty = 3

print(price * qty)  # 7500

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6️⃣ Real-Life Example: Product Total Price

product = input("Product name: ")
price = float(input("Price per item: "))
quantity = int(input("Quantity: "))

total = price * quantity

print(f"\nProduct: {product}")
print(f"Total cost: ₦{total}")

Explanation

• float() allows decimal prices
• int() ensures quantity is a whole number
• f-string formats output nicely

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7️⃣ Mini Project: POS (Point of Sale) Calculator ✅

Task

Build a small POS system that:

• Accepts product name
• Accepts price
• Accepts quantity
• Calculates total amount
• Displays receipt-like output

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Full Code (Day 2 Project)

print("=== SIMPLE POS SYSTEM ===")

product_name = input("Enter product name: ")
price = float(input("Enter product price: "))
quantity = int(input("Enter quantity: "))

total = price * quantity

print("\n--- RECEIPT ---")
print(f"Product: {product_name}")
print(f"Price: ₦{price}")
print(f"Quantity: {quantity}")
print(f"Total Amount: ₦{total}")

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8️⃣ Code Walkthrough (Line by Line)

print("=== SIMPLE POS SYSTEM ===")

Prints a title.

product_name = input("Enter product name: ")

Receives product name as text.

price = float(input("Enter product price: "))

Converts user input to decimal number.

quantity = int(input("Enter quantity: "))

Converts user input to whole number.

total = price * quantity

Calculates total cost.

print(f"Total Amount: ₦{total}")

Displays final result using f-string.

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9️⃣ Common Beginner Mistakes ⚠️

❌ Forgetting conversion:

price = input("Price: ")
print(price * 2)   # wrong

✅ Correct:

price = float(input("Price: "))
print(price * 2)

---

🔁 Practice Exercises (Do All)

1️⃣ Ask user for:

• Name
• Year of birth

2️⃣ Calculate:

• Current age

year = int(input("Enter year of birth: "))
age = 2026 - year
print(f"You are {age} years old")

3️⃣ Modify POS to include:

• Customer name
• Discount (optional)

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⭐ Mini Challenge

Add a 10% discount automatically if total is above ₦20,000.

if total > 20000:
    discount = total * 0.10
    total -= discount

(Don’t worry if if looks new — you’ll master it on Day 3.)

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✅ Chapter 2 Outcome

✔ You can collect real data
✔ You understand data types
✔ You can perform calculations
✔ You've built an interactive program

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CHAPTER 3 — DECISION MAKING WITH CODE

Problem This Solves

How does software decide approval, rejection, or status?

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🎯 What you'll learn in this chapter

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

• Make your program think and decide
• Use if, elif, and else
• Compare values correctly
• Build logic for real-life situations (approval, rejection, status checks)

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1️⃣ What Are Conditions?

Conditions allow your program to make decisions based on data.

Real-life examples:

• If balance is enough → allow payment
• If age ≥ 18 → allow registration
• If stock is 0 → show “Out of stock”

In Python, we do this using:

if
elif
else

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2️⃣ The if Statement (Basic Decision)

Example

age = 20

if age >= 18:
    print("You are allowed")

Explanation

• Python checks the condition: age >= 18
• If it is True, the code inside runs
• If it is False, Python skips it

⚠️ Indentation matters

• Everything inside if must be indented (usually 4 spaces)

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3️⃣ if and else (Two Possible Outcomes)

Example

balance = 3000

if balance >= 5000:
    print("Transaction approved")
else:
    print("Insufficient funds")

Explanation

• Python checks the condition
• If True → runs if block
• If False → runs else block

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4️⃣ Comparison Operators (Very Important)

Operator	Meaning
==	Equal to
!=	Not equal
>	Greater than
<	Less than
>=	Greater or equal
<=	Less or equal

Example

score = 75

if score >= 50:
    print("Passed")
else:
    print("Failed")

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5️⃣ Using elif (Multiple Conditions)

Example

score = 85

if score >= 80:
    print("Grade A")
elif score >= 60:
    print("Grade B")
elif score >= 50:
    print("Grade C")
else:
    print("Failed")

Explanation

• Python checks from top to bottom
• The first true condition runs
• Others are ignored

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6️⃣ Conditions with User Input

Example

age = int(input("Enter your age: "))

if age >= 18:
    print("You are eligible")
else:
    print("You are not eligible")

Explanation

• User input is converted to int
• Python compares the value
• Decision is made automatically

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7️⃣ Real-Life Example: Business Discount Logic

amount = float(input("Enter total amount: "))

if amount >= 20000:
    discount = amount * 0.10
    print(f"Discount applied: ₦{discount}")
else:
    print("No discount applied")

Explanation

• If customer spends enough → reward them
• Else → normal pricing

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8️⃣ Mini Project: Loan Eligibility Checker ✅

Task

Build a program that:

• Asks for monthly income
• Approves loan if income ≥ ₦100,000
• Rejects otherwise
• Shows clear message

---

Full Code (Day 3 Project)

print("=== LOAN ELIGIBILITY CHECKER ===")

income = float(input("Enter your monthly income: ₦"))

if income >= 100000:
    print("✅ Loan Approved")
    print("You qualify for the loan.")
else:
    print("❌ Loan Rejected")
    print("Income below required threshold.")

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9️⃣ Code Walkthrough (Line by Line)

print("=== LOAN ELIGIBILITY CHECKER ===")

Displays program title.

income = float(input("Enter your monthly income: ₦"))

• Receives income
• Converts to number

if income >= 100000:

• Condition check

print("✅ Loan Approved")

• Runs only if condition is True

else:

• Runs when condition is False

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🔥 Common Beginner Errors & Fixes

❌ Using = instead of ==

if income = 100000:   # wrong

✅ Correct

if income == 100000:

---

❌ Forgetting indentation

if income > 50000:
print("Approved")  # error

✅ Correct

if income > 50000:
    print("Approved")

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🔁 Practice Exercises (Do All)

Exercise 1: Voting Eligibility

age = int(input("Enter your age: "))

if age >= 18:
    print("You can vote")
else:
    print("You cannot vote")

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Exercise 2: Stock Checker

stock = int(input("Enter available stock: "))

if stock > 0:
    print("Item available")
else:
    print("Out of stock")

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Exercise 3: Grading System

score = int(input("Enter your score: "))

if score >= 70:
    print("Excellent")
elif score >= 50:
    print("Good")
else:
    print("Needs improvement")

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⭐ Mini Challenge (Optional)

Build a Payment Status Checker

• Ask for balance
• Ask for price
• If balance ≥ price → Payment successful
• Else → Insufficient balance

balance = float(input("Balance: "))
price = float(input("Price: "))

if balance >= price:
    print("Payment successful")
else:
    print("Insufficient balance")

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✅ Chapter 3 Outcome

✔ You can make decisions in code
✔ You understand conditions & comparisons
✔ You can build approval/rejection logic
✔ You're ready for loops

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CHAPTER 4 — AUTOMATING REPETITIVE TASKS

Problem This Solves

How do we avoid doing the same work repeatedly?

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🎯 What you'll learn in this chapter

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

• Understand what loops are
• Use for loops and while loops
• Repeat tasks automatically
• Build real-life programs that run multiple times
• Control when a loop stops

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1️⃣ What Is a Loop?

A loop allows Python to repeat an action multiple times.

Real-life examples

• Recording daily sales for 7 days
• Printing a receipt for 10 customers
• Counting stock items
• Repeating until user says “stop”

Instead of writing code again and again, we use loops.

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2️⃣ The for Loop (When You Know How Many Times)

Basic Example

for i in range(5):
    print("Hello")

Explanation

• range(5) means 0 to 4 (5 times)
• i is a counter (0, 1, 2, 3, 4)
• Code inside runs 5 times

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3️⃣ Understanding range()

Code	Meaning
range(5)	0 → 4
range(1, 6)	1 → 5
range(1, 10, 2)	1, 3, 5, 7, 9

Example

for day in range(1, 6):
    print("Day", day)

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4️⃣ Looping Through Data (Real-Life Friendly)

products = ["Rice", "Beans", "Oil"]

for item in products:
    print(item)

Explanation

• Python takes one item at a time from the list
• No need to count manually

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5️⃣ Real-Life Example: Daily Sales Tracker

for day in range(1, 6):
    sales = float(input(f"Enter sales for Day {day}: ₦"))
    print(f"Sales recorded: ₦{sales}")

What’s happening?

• Loop runs 5 times
• Each time, user enters sales
• Python labels each day automatically

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6️⃣ The while Loop (Repeat Until Condition Is False)

Basic Example

count = 1

while count <= 5:
    print("Count:", count)
    count += 1

Explanation

• Loop runs as long as condition is True
• count += 1 prevents infinite loop

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7️⃣ Infinite Loop (⚠️ Be Careful)

while True:
    print("This will run forever")

To stop it manually:

• Press Ctrl + C

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8️⃣ Using break (Exit a Loop)

while True:
    name = input("Enter name (or 'exit'): ")

    if name == "exit":
        break

    print("Hello", name)

Explanation

• Loop keeps running
• Stops only when user types exit

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9️⃣ Using continue (Skip One Round)

for num in range(1, 6):
    if num == 3:
        continue
    print(num)

Output

1
2
4
5

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🔥 Mini Project: Daily Sales Recorder ✅

Task

Build a program that:

• Records sales for 7 days
• Shows total sales at the end

---

Full Code (Day 4 Project)

print("=== DAILY SALES RECORDER ===")

total_sales = 0

for day in range(1, 8):
    sales = float(input(f"Enter sales for Day {day}: ₦"))
    total_sales += sales

print("\nWeekly sales summary")
print(f"Total sales for the week: ₦{total_sales}")

---

🔍 Code Walkthrough (Line by Line)

total_sales = 0

Starts total at zero.

for day in range(1, 8):

Loop runs 7 times.

total_sales += sales

Adds each day’s sales to total.

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⚠️ Common Beginner Mistakes

❌ Forgetting to update counter in while

count = 1
while count <= 5:
    print(count)

✅ Fix

count += 1

---

❌ Wrong indentation

for i in range(3):
print(i)

✅ Fix

for i in range(3):
    print(i)

---

🔁 Practice Exercises (Must Do)

Exercise 1: Print Numbers 1–10

for i in range(1, 11):
    print(i)

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Exercise 2: Repeat Until User Stops

while True:
    msg = input("Type something (or 'stop'): ")

    if msg == "stop":
        break

    print(msg)

---

Exercise 3: Sum of Numbers

total = 0

for i in range(5):
    num = int(input("Enter number: "))
    total += num

print("Total:", total)

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⭐ Mini Challenge (Optional)

Modify the sales recorder:

• Stop early if user types 0
• Count how many days were entered

(Hint: use break and a counter)

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✅ Chapter 4 Outcome

✔ You can automate repetitive tasks
✔ You understand for and while loops
✔ You can control program flow
✔ You're ready for Lists & Data Collections

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CHAPTER 5 — MANAGING COLLECTIONS OF DATA

Problem This Solves

How do applications store and manage multiple records?

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🎯 What you'll learn in this chapter

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

• Understand what lists are
• Store multiple values in one variable
• Add, remove, and access items
• Loop through lists
• Build simple real-life data management programs

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1️⃣ What Is a List?

A list is a collection of items stored in one place.

Real-life examples

• List of products in a shop
• List of patients in a clinic
• List of tasks for a day
• List of customer names

Instead of creating many variables:

p1 = "Rice"
p2 = "Beans"
p3 = "Oil"

We use a list:

products = ["Rice", "Beans", "Oil"]

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2️⃣ Creating a List

items = ["Bread", "Milk", "Eggs"]
print(items)

Explanation

• Square brackets [] define a list
• Items are separated by commas
• Lists can contain text, numbers, or both

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3️⃣ Accessing List Items (Indexing)

products = ["Rice", "Beans", "Oil"]

print(products[0])  # Rice
print(products[1])  # Beans
print(products[2])  # Oil

Important Rule ⚠️

Python starts counting from 0, not 1.

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4️⃣ Changing List Items

products = ["Rice", "Beans", "Oil"]
products[1] = "Garri"

print(products)

Explanation

• Index 1 (Beans) is replaced with Garri

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5️⃣ Adding Items to a List

Using append() (most common)

products = ["Rice", "Beans"]
products.append("Oil")

print(products)

Explanation

• Adds item to the end of the list

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6️⃣ Removing Items from a List

Remove by name

products.remove("Beans")

Remove last item

products.pop()

---

7️⃣ List Length (len())

products = ["Rice", "Beans", "Oil"]
print(len(products))  # 3

Real-life use

• Count number of items in stock
• Count number of customers
• Count number of days recorded

---

8️⃣ Looping Through a List (Very Important)

products = ["Rice", "Beans", "Oil"]

for item in products:
    print(item)

Explanation

• Python picks each item one by one
• Cleaner than using indexes manually

---

9️⃣ Lists with User Input (Dynamic Data)

items = []

for i in range(3):
    product = input("Enter product name: ")
    items.append(product)

print(items)

Explanation

• Starts with an empty list
• User adds items dynamically

---

🔥 Mini Project: Inventory Manager ✅

Task

Build a program that:

• Allows user to add products
• Stores them in a list
• Displays all products
• Shows total number of products

---

Full Code (Day 5 Project)

print("=== INVENTORY MANAGER ===")

inventory = []

while True:
    product = input("Enter product name (or 'exit'): ")

    if product == "exit":
        break

    inventory.append(product)
    print("Product added.")

print("\nInventory List:")
for item in inventory:
    print("-", item)

print(f"\nTotal products: {len(inventory)}")

---

🔍 Code Walkthrough

inventory = []

Creates an empty list.

while True:

Keeps program running.

inventory.append(product)

Adds each product.

len(inventory)

Counts total items.

---

⚠️ Common Beginner Mistakes

❌ Accessing invalid index

print(products[5])  # error if list has only 3 items

✅ Fix

if len(products) > 5:
    print(products[5])

---

❌ Forgetting quotes around strings

products.append(Rice)  # wrong

✅ Fix

products.append("Rice")

---

🔁 Practice Exercises (Must Do)

Exercise 1: Favorite Foods

foods = []

for i in range(3):
    food = input("Enter a food: ")
    foods.append(food)

for f in foods:
    print(f)

---

Exercise 2: Remove an Item

items = ["Pen", "Book", "Bag"]
items.remove("Book")
print(items)

---

Exercise 3: Count Items

names = ["Jane", "John", "Mary"]
print("Total names:", len(names))

---

⭐ Mini Challenge (Optional)

Modify Inventory Manager:

• Prevent duplicate products
• Show message if item already exists

if product in inventory:
    print("Item already exists")
else:
    inventory.append(product)

---

✅ Chapter 5 Outcome

✔ You can store multiple data values
✔ You can manage dynamic data
✔ You can loop through lists
✔ You're ready for Functions

---

CHAPTER 6 — WRITING REUSABLE & CLEAN CODE

Problem This Solves

How do professionals organize code for reuse?

---

🎯 What you'll learn in this chapter

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

• Understand what functions are
• Write your own functions
• Pass data into functions
• Get results back using return
• Organize code like a professional developer

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1️⃣ What Is a Function?

A function is a block of code that does a specific job and can be reused.

Real-life example

Instead of calculating profit again and again:

• You create one function
• Call it whenever you need it

Think of a function like:

“A machine that takes input → does work → gives output”

---

2️⃣ Why Functions Are Important

Without functions:

• Code becomes long
• You repeat yourself
• Bugs are harder to fix

With functions:

• Code is cleaner
• Easier to read
• Easier to reuse
• Easier to maintain

---

3️⃣ Creating a Function (def)

Basic Function Example

def greet():
    print("Hello, welcome!")

Explanation

• def means define function
• greet is the function name
• () means no input yet
• Indentation defines the function body

---

4️⃣ Calling a Function

greet()
greet()

Output

Hello, welcome!
Hello, welcome!

Explanation

• Function runs only when called
• You can call it many times

---

5️⃣ Function with Parameters (Input)

Example

def greet_user(name):
    print(f"Hello {name}, welcome!")

Calling it

greet_user("Jane")
greet_user("Chuks")

Explanation

• name is a parameter
• Value passed is called an argument

---

6️⃣ Function with Multiple Parameters

def add_numbers(a, b):
    print(a + b)

add_numbers(5, 3)

Explanation

• a and b receive values
• Function performs calculation

---

7️⃣ Using return (Very Important)

Example

def calculate_profit(cost, selling_price):
    profit = selling_price - cost
    return profit

Calling it

result = calculate_profit(500, 800)
print("Profit:", result)

Explanation

• return sends value back
• Code after return does NOT run
• Returned value can be stored in a variable

---

8️⃣ Difference Between print() and return

print()	return
Displays value	Sends value back
Cannot be reused	Can be reused
For output only	For logic

❌ Bad practice

def total(a, b):
    print(a + b)

✅ Better practice

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

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9️⃣ Function with User Input

def calculate_total():
    price = float(input("Price: "))
    qty = int(input("Quantity: "))
    return price * qty

total = calculate_total()
print("Total:", total)

Explanation

• Function handles logic
• Main program handles output

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🔥 Mini Project: Profit Calculator ✅

Task

Build a program that:

• Uses a function to calculate profit
• Accepts cost price and selling price
• Displays profit or loss

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Full Code (Day 6 Project)

print("=== PROFIT CALCULATOR ===")

def calculate_profit(cost_price, selling_price):
    return selling_price - cost_price

cost = float(input("Enter cost price: ₦"))
selling = float(input("Enter selling price: ₦"))

profit = calculate_profit(cost, selling)

if profit > 0:
    print(f"Profit made: ₦{profit}")
elif profit < 0:
    print(f"Loss incurred: ₦{abs(profit)}")
else:
    print("No profit, no loss")

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🔍 Code Walkthrough

def calculate_profit(cost_price, selling_price):

Defines function with two inputs.

return selling_price - cost_price

Calculates and returns result.

profit = calculate_profit(cost, selling)

Stores returned value.

if profit > 0:

Decision based on function result.

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⚠️ Common Beginner Mistakes

❌ Forgetting to call function

calculate_profit

✅ Correct

calculate_profit(500, 700)

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❌ Forgetting return

def calc(a, b):
    a + b

✅ Correct

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

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🔁 Practice Exercises (Must Do)

Exercise 1: Greeting Function

def welcome(name):
    print(f"Welcome {name}")

welcome("Jane")

---

Exercise 2: Area Calculator

def area(length, width):
    return length * width

print(area(5, 4))

---

Exercise 3: Reuse Function

def square(num):
    return num * num

print(square(2))
print(square(5))

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⭐ Mini Challenge (Optional)

Create a Simple POS Function

• Function to calculate total
• Function to apply discount
• Combine both

def apply_discount(amount):
    if amount > 20000:
        return amount * 0.9
    return amount

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✅ Chapter 6 Outcome

✔ You can write clean reusable code
✔ You understand functions & return values
✔ You can separate logic from output
✔ You're ready for Building a Complete Python Application

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CHAPTER 7 — BUILDING A COMPLETE PYTHON APPLICATION

Problem This Solves

How do we combine all Python fundamentals into a real system?

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🎯 Goal of this chapter

By the end of this chapter, you will:

• Combine input, conditions, loops, lists, and functions
• Build a real-life business-style program
• Think like a junior Python developer
• Be confident to move into Django, automation, or AI basics

---

🧠 What You'll Use From Previous Chapters

Chapter	Concept
Chapter 1	Variables, print
Chapter 2	input(), numbers
Chapter 3	if / else
Chapter 4	loops
Chapter 5	lists
Chapter 6	functions

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📦 Mini Project: Simple Business Sales Manager

📝 Project Description

Build a program that:

• Records multiple product sales
• Calculates total price per product
• Stores all products sold
• Calculates total sales
• Allows user to stop when done
• Displays a final summary

This is similar to:

• A shop POS
• A sales tracker
• A basic inventory/sales system

---

🧱 Step 1: Plan the Program (Very Important)

We need:

• A list to store products
• A loop to keep asking for data
• A function to calculate total
• Conditions to stop the program
• A summary at the end

---

🧩 Step 2: Define Helper Function

def calculate_total(price, quantity):
    return price * quantity

Explanation

• Takes price and quantity
• Returns the total amount
• Reusable for every product

---

🔁 Step 3: Main Program Loop

Full Project Code (Day 7 Final)

print("=== SIMPLE SALES MANAGEMENT SYSTEM ===")

sales = []          # list to store sales
grand_total = 0     # total sales amount

def calculate_total(price, quantity):
    return price * quantity

while True:
    product = input("\nEnter product name (or 'exit'): ")

    if product.lower() == "exit":
        break

    price = float(input("Enter price: ₦"))
    quantity = int(input("Enter quantity: "))

    total = calculate_total(price, quantity)
    grand_total += total

    sales.append(product)

    print(f"Total for {product}: ₦{total}")

print("\n=== SALES SUMMARY ===")

for item in sales:
    print("-", item)

print(f"\nNumber of products sold: {len(sales)}")
print(f"Grand Total Sales: ₦{grand_total}")

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🔍 Full Code Walkthrough (Section by Section)

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1️⃣ Program Title

print("=== SIMPLE SALES MANAGEMENT SYSTEM ===")

Displays program heading.

---

2️⃣ Data Storage

sales = []
grand_total = 0

• sales stores product names
• grand_total accumulates total sales

---

3️⃣ Function Definition

def calculate_total(price, quantity):
    return price * quantity

Handles calculation logic cleanly.

---

4️⃣ Infinite Loop

while True:

Keeps program running until user stops it.

---

5️⃣ Exit Condition

if product.lower() == "exit":
    break

• Allows user to exit safely
• .lower() makes input case-insensitive

---

6️⃣ User Input & Calculation

price = float(input("Enter price: ₦"))
quantity = int(input("Enter quantity: "))

Gets numeric input.

total = calculate_total(price, quantity)

Uses function.

---

7️⃣ Data Storage

sales.append(product)
grand_total += total

Stores data and updates total.

---

8️⃣ Final Summary Output

print("\n=== SALES SUMMARY ===")

Prints results neatly.

---

⚠️ Common Errors to Watch Out For

❌ Forgetting to convert input

price = input("Price: ")  # wrong

✅ Correct

price = float(input("Price: "))

---

❌ Not using break

while True:
    print("Running forever")

---

🔁 Practice Improvements (Do These)

1️⃣ Store price + quantity together

sales.append((product, total))

---

2️⃣ Prevent empty product names

if product == "":
    print("Product name cannot be empty")
    continue

---

3️⃣ Add discount logic

if total > 20000:
    total *= 0.9

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⭐ Mini Challenge (Advanced Beginner)

Upgrade the app to:

• Store product, price, quantity, total
• Display full receipt-style output

Example:

Rice - Qty: 2 - ₦5000
Beans - Qty: 1 - ₦3000

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🎉 FINAL OUTCOME

After completing this book, you can:

✔ Write clean Python scripts
✔ Build interactive programs
✔ Use logic, loops, lists, and functions
✔ Create real-life tools for business
✔ Move confidently into Django, automation, or AI

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NEXT STEPS

• Python file handling
• Error handling
• Databases
• Django web development

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🎉 Congratulations

You have completed *Python Fundamentals *.

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