Your First Python Program

#What Is Python and Why Should You Care?

Python is a programming language — a way to write instructions that a computer can understand and execute. It was created in 1991 by Guido van Rossum (he named it after Monty Python, the comedy group, not the snake).

Here is why so many people choose Python:

1. It reads like English. Compare these two lines that do the same thing:
   • Java: System.out.println("Hello, world!");
   • Python: print("Hello, world!")

   Python wins. Every time.

2. Somebody has usually done the hard part already. Want to read a spreadsheet, send an email, make a chart, or download a web page? A free, ready-made tool exists for each, and using one is a single line. Thousands of people have built these and given them away.
3. You are never stuck for long. Because so many people use Python, almost every question you will hit has already been asked and answered by someone else.

#Run Your First Python in 30 Seconds

print("Hello, world!")

You just ran your first Python program. Output appears under the code. No install, no terminal, no setup.

• print(...) is a built-in function that displays whatever you give it.
• "Hello, world!" is a string — text wrapped in quotes.
• The result shows up below the block (your "stdout").

Try changing the text inside the quotes — click in the code, edit, and Run again. Make it say your name. This is your laboratory for the next 53 lessons.

#The Python REPL: Your Interactive Playground

You're already using one. Every Run button in this course IS a REPL — paste any Python expression, hit Run, see the result. Try a few:

print("Hello, world!")
print(2 + 2)
print(100 * 365)

The REPL is for experimentation. Calculator math, string fiddling, quick sanity-checks. By lesson 53 you'll use it constantly — to test a snippet before pasting it into a real file.

#A Multi-Line Python Program

A real program has multiple lines. Python runs them top-to-bottom, one at a time. Try this:

# My first Python program
print("Hello, world!")
print("I am learning Python.")
print("This is going to be awesome.")

You should see three lines of output. Let's break it down:

• # starts a comment. Python ignores everything after # on that line. Comments are notes for humans
• print() is a function that displays text on the screen. Whatever you put inside the parentheses gets printed
• "Hello, world!" is a string — a piece of text wrapped in quotes. You can use single quotes 'like this' or double quotes "like this" — Python treats them the same

#Python as a Calculator

Python is incredible at math. Here are the basic operators:

Let us try a calculation with some big numbers:

# Underscores make long numbers readable -- Python ignores them
people_on_earth = 8_100_000_000
print("People on Earth:", people_on_earth)

# If everyone drank 2 litres of water a day, how many litres per day?
litres_per_day = people_on_earth * 2
print("Litres per day:", litres_per_day)

# That is a big number. How many Olympic pools (2,500,000 litres each)?
pools = litres_per_day / 2_500_000
print("Olympic pools per day:", round(pools))

# Python follows order of operations (PEMDAS)
result = 2 + 3 * 4
print("2 + 3 * 4 =", result)  # 14, not 20

# Use parentheses to change the order
result2 = (2 + 3) * 4
print("(2 + 3) * 4 =", result2)  # 20

Notice a few things:

• You can use underscores in numbers to make them readable: 1_700_000_000_000 is the same as 1700000000000
• Python follows standard math order of operations (multiplication before addition)
• You can print multiple things by separating them with commas

#Strings — Working with Text

Most programs handle text somewhere — names, messages, addresses, whole documents. Here is how Python handles text:

# Strings are text wrapped in quotes
greeting = "Hello"
name = "Aditi"

# Concatenation -- joining strings together
message = greeting + ", " + name + "!"
print(message)  # Hello, Aditi!

# String multiplication -- yes, this works!
print("Ha" * 3)  # HaHaHa
print("-" * 40)  # prints a line of 40 dashes

# The len() function tells you how many characters a string has
prompt = "Explain quantum physics like I am five"
print("Prompt length:", len(prompt))  # 38

# Strings are case-sensitive
print("python" == "Python")  # False
print("python" == "python")  # True

#Code Playground

Time to experiment. Run this code, then try the challenges in the comments:

#Common Beginner Mistakes

Everyone makes these. Here is how to fix them:

# Wrong -- Python thinks Hello is a variable name
print(Hello)  # NameError: name 'Hello' is not defined

# Right -- wrap text in quotes
print("Hello")  # Hello

# Wrong -- started with double, ended with single
print("Hello')  # SyntaxError: mismatched quotes

# Right -- match your quotes
print("Hello")  # or print('Hello')

# Wrong -- Print with capital P does not exist
Print("Hello")  # NameError: name 'Print' is not defined

# Right -- lowercase print
print("Hello")

# Wrong in Python 3 (this is Python 2 syntax)
print "Hello"  # SyntaxError: Missing parentheses

# Right -- print is a function, needs parentheses
print("Hello")

These errors are not failures — they are how you learn. Every programmer in the world has made every one of these mistakes. The key is reading the error message. Python tells you what went wrong and even points to the exact line.

#Key Takeaways

• Python is a good first language — it reads like English, ready-made tools exist for most jobs, and help is easy to find
• The REPL lets you experiment instantly — type python3 in your terminal and start coding. No setup, no files needed
• print() displays output. It is the most basic and most useful function. You will use it in every program you write
• Python is a powerful calculator — it handles +, -, *, /, // (floor division), % (remainder), and ** (power)
• Strings are text in quotes — you can join them with +, repeat them with *, and measure them with len()

Hello, world!
I am learning Python 3.

#Appendix: Installing Python Locally (Optional)

#On Mac

python3 --version

#On Windows

1. Go to python.org/downloads and click Download Python 3.x
2. Run the installer. Check the box "Add python.exe to PATH" at the bottom — this is the #1 mistake beginners make on Windows
3. Open Command Prompt or PowerShell and run python --version

#On Linux

Most distros ship with Python 3. Open a terminal:

python3 --version

#Verify your install

python3 -c "print('Hello from local Python!')"

#Once you have Python locally: virtual environments

python3 -m venv .venv
source .venv/bin/activate     # macOS / Linux
.venv\Scripts\activate        # Windows
pip install requests          # installs into THIS project only