Lesson 3: Your First Python Program

The Tradition of "Hello, World!"

In the programming world, writing a "Hello, World!" program is a tradition that dates back to the 1970s. It's the first program almost every programmer writes in a new language. It may seem simple, but it proves that your environment is set up correctly and that you can write, save, and run code.

Here's your first Python program:

print("Hello, World!")

That's it—one line of code! Let's break it down:

• print — tells Python you want to display something on screen
• ( and ) — parentheses wrap what you want to print
• "Hello, World!" — the text you want to display, enclosed in quotes

Creating and Running a Python File

There are two ways to run Python code: in a file (a "script") or interactively (which we'll cover in Lesson 4). Let's learn the file-based approach first.

Step 1: Create a Python File

Open your text editor (IDLE, VS Code, or any editor) and create a new file. Type the following:

# My first Python program
print("Hello, World!")

Step 2: Save the File

Save the file as hello.py. The .py extension tells your computer this is a Python file.

Step 3: Run the File

Using IDLE: With the file open in IDLE, press F5 or go to Run > Run Module.

Using the command line: Open your terminal, navigate to the folder where you saved the file, and type:

python hello.py

Exploring the print() Function

The print() function is more versatile than just printing one message. Let's explore what it can do.

Printing Multiple Lines

Each print() statement creates a new line of output:

print("Hello!")
print("My name is Python.")
print("Nice to meet you!")

Printing Numbers

Numbers don't need quotes:

print(42)
print(3.14)
print(2 + 3)

Printing Multiple Items

Separate items with commas—Python adds spaces automatically:

print("The answer is", 42)
print("Python", "is", "fun!")

Empty print()

A print() with nothing inside creates a blank line:

print("Line one")
print()
print("Line three (with a blank line above)")

Single vs. Double Quotes

Python accepts both single quotes (') and double quotes (") for text. They work the same way:

print("Hello with double quotes")
print('Hello with single quotes')

print("It's a beautiful day!")   # Works
print('It\'s a beautiful day!')  # Also works (with escape)

print('She said "hello" to me.')

Comments: Notes to Yourself

Single-Line Comments

Start a comment with the # symbol. Everything after # on that line is ignored:

# This is a comment - Python ignores this line
print("This runs")  # This comment is after code

# print("This won't run because it's commented out")

Why Write Comments?

• Explain your thinking: Why did you write the code this way?
• Help future you: You might forget what your code does weeks later
• Help others: If someone else reads your code, comments guide them
• Debugging: Temporarily "turn off" code by commenting it out

# Bad comment - just restates what the code does
print("Hello")  # prints hello

# Good comment - explains WHY
# Greet the user when the program starts
print("Hello! Welcome to the Grade Calculator.")

Common Beginner Errors (and How to Fix Them)

Making errors is completely normal! Here are the most common mistakes beginners make and how to fix them.

Error 1: Missing Quotes

print(Hello, World!)

Fix: Text must be wrapped in quotes: print("Hello, World!")

Error 2: Mismatched Quotes

print("Hello, World!')

Fix: Use the same type of quote to open and close: print("Hello, World!")

Error 3: Missing Parentheses

print "Hello, World!"

Fix: In Python 3, print requires parentheses: print("Hello, World!")

Error 4: Wrong Capitalization

Print("Hello, World!")

Fix: Python is case-sensitive. Use lowercase print, not Print or PRINT.

Putting It All Together

Let's create a more interesting program that uses everything we've learned:

# My About Me program
# This program displays information about me

print("================================")
print("       About Me Program")
print("================================")
print()
print("Name: Alex")
print("Age:", 20)
print("Major: Computer Science")
print("Favorite number:", 7)
print()
print("Fun fact: I can count to", 10 * 10, "in one second!")
print("================================")

Ready for More?