Your first Python program

Learning outcome

By the end of this lesson, you will write a small Python program that asks for your name, does a calculation, and prints a result — using input, output, variables, and a function.

Before you start

You need Python installed. Open a terminal (Command Prompt on Windows, Terminal on Mac) and type:

python --version

If you see Python 3.x.x, you are ready. If not, download Python from python.org and install it.

Your first line of code

Create a file called hello.py and write:

print("Hello, friend!")

Run it:

python hello.py

You should see Hello, friend! printed. Congratulations — you just wrote and ran a program.

Variables

A variable is a named box that holds a value. Think of it like a labelled jar:

name = "Alex"
age = 25
temperature = 36.5
is_learning = True

Python figures out the type automatically. You do not need to write String or int like in some other languages.

Input and output

Programs are more interesting when they respond to the person using them:

name = input("What is your name? ")
print("Nice to meet you,", name)

input() pauses and waits for the user to type something. print() displays text.

A worked example: temperature converter

Let us write a program that converts Fahrenheit to Celsius:

# Ask for the temperature in Fahrenheit
fahrenheit = input("Enter temperature in Fahrenheit: ")

# Convert to a number and do the math
f = float(fahrenheit)
celsius = (f - 32) * 5 / 9

# Show the result
print(fahrenheit, "F is", round(celsius, 1), "C")

Try it with 98.6 — the answer should be 37.0 C.

Functions

A function is a reusable chunk of code. You give it a name and some inputs, and it returns a result:

def greet(name):
    return "Hello, " + name + "!"

message = greet("Sam")
print(message)

The def keyword defines a function. return sends a value back. Everything indented under def belongs to the function.

Common beginner mistakes

• Forgetting the colon after def and if lines. Python uses : to know a block is coming.
• Mixing tabs and spaces. Python relies on indentation to know where blocks start and end. Use 4 spaces consistently.
• Forgetting to convert input. input() always returns a string. If you need a number, you must convert it with int() or float().

Practice task

Write a program that:

1. Asks for your name
2. Asks for your favourite number
3. Prints "Hello, [name]! Your number doubled is [number * 2]."

Recap

• print() displays output. input() reads keyboard input.
• Variables store values. Python infers the type.
• Functions group reusable code with def.
• Convert input strings to numbers with int() or float() before math.

Next step

Now that you can write a working program, the next lesson teaches you how to make your code readable, testable, and safe from common errors.