Introduction to Python

Python is a general-purpose programming language known for code that reads almost like plain English. It's used for web back ends, data analysis, machine learning, automation scripts, scientific research and teaching — and it's one of the most popular languages in the world.

A first look

Here's a complete Python program. Press Run to execute it right here in your browser:

The first run downloads a Python interpreter into your browser (a few seconds); after that, examples run instantly. You can edit the code and run it again as often as you like.

What makes Python different

• Readable syntax. No semicolons or curly braces. Indentation shows the structure of the code, so every Python program looks tidy.
• Batteries included. The standard library covers files, dates, JSON, maths, regular expressions, networking, testing and much more, with nothing extra to install.
• A huge ecosystem. Over half a million third-party packages are available — pandas, NumPy, Django, Flask, FastAPI, PyTorch and more.
• Interpreted and dynamic. You run code directly, without a separate compile step, and variables don't need declared types.

The same task in two languages

Compare reading the lines of a file and printing the long ones. In Java:

import java.nio.file.*;
import java.util.List;

public class LongLines {
    public static void main(String[] args) throws Exception {
        List<String> lines = Files.readAllLines(Path.of("notes.txt"));
        for (String line : lines) {
            if (line.length() > 40) {
                System.out.println(line);
            }
        }
    }
}

In Python:

with open("notes.txt") as file:
    for line in file:
        if len(line) > 40:
            print(line, end="")

Less ceremony means you spend more time on the problem and less on the language.

Where Python is used

CodeAtlas has follow-on courses for several of these — NumPy, pandas, Django and Data Science.

Python versions

This course uses modern Python 3 (3.12 and later), which is what you should install. Python 2 reached end of life in 2020; you may still see its print "hello" syntax (without parentheses) in very old tutorials, but it won't run in Python 3. New Python versions are released every October — lesson 55 summarises recent additions.

How this course works

• Examples show their real output underneath. Every example has been run to check that output is exactly right.
• Try it yourself editors run Python in your browser — press Run or Ctrl+Enter.
• When you finish, practise with the exercises and the quiz.