Python introduction: what it is, why it's so popular, who uses it and your first programs

Python is one of the most popular programming languages in the world and often the first language taught in schools, universities and bootcamps. It reads almost like English, so you can focus on solving problems instead of fighting symbols. Python powers data analysis, artificial intelligence, automation, web servers, scientific research, and countless scripts that save people hours of work. This unit starts from absolute zero.

What is Python?

Compare the same task in two languages:

// Java
public class Hello {
  public static void main(String[] args) {
    System.out.println("Habari!");
  }
}

# Python
print("Habari!")

A short history

• 1989–1991: Guido van Rossum started Python as a hobby project in the Netherlands; version 0.9 was released in 1991. The name comes from the British comedy group Monty Python, not the snake.
• 2000: Python 2.0.
• 2008: Python 3 (not backwards compatible, but cleaner). Python 2 reached end of life in 2020; always learn Python 3.
• Today: Python consistently ranks at or near the top of programming language popularity rankings (such as the TIOBE index and GitHub's reports), driven by data science and AI.

Why is Python so popular?

1. Easy to read and write: clean syntax, few symbols, indentation instead of braces.
2. Huge standard library ("batteries included"): dates, maths, files, CSV, JSON, email, web requests and more, built in.
3. Massive ecosystem: over half a million packages on PyPI (the Python Package Index) for almost anything: data (pandas), AI (PyTorch, TensorFlow, scikit-learn), web (Django, Flask, FastAPI), automation (Selenium, requests), charts (matplotlib).
4. Versatile: scripts, websites, data analysis, AI, games, hardware (Raspberry Pi), desktop apps.
5. Free and open source, running on Windows, Mac, Linux and even phones.
6. Big, friendly community: tutorials, forums and answers everywhere.

What is Python used for?

Who uses Python?

Ways to run Python

1. This editor: examples run in your browser. (The first run downloads Python into the page, so it may take a few seconds.)
2. Your computer: install Python 3 from python.org (on Windows, tick "Add Python to PATH"). Write code in VS Code with the Python extension, save as hello.py, run with python hello.py (or python3).
3. Google Colab (colab.research.google.com): free Python notebooks in the browser, great for data and AI, works on modest laptops.
4. Phone: apps like Pydroid 3 (Android) let you practise on the go.

The Python shell

Typing python in a terminal opens the interactive shell (>>>), where each line runs immediately: perfect for quick experiments.

Your first program

print("Habari! Welcome to Python.")
print("I am learning at Marzley Tech")
print(2 + 3)
print("Total:", 180 * 3)

print() shows output. Text goes in quotes (single '...' or double "..."); numbers don't. print can take several values separated by commas; it puts spaces between them.

Comments

# This is a comment: Python ignores it
print("Comments explain code to humans")   # a comment at the end of a line

# Use comments to explain WHY, not obvious WHAT:
vat_rate = 0.16   # Kenya's standard VAT rate

Variables

A variable is a name that refers to a value.

customer = "Wanjiku"
price_per_kg = 180
kilos = 3

total = price_per_kg * kilos
print(customer, "pays KSh", total)

kilos = 5              # variables can be changed
total = price_per_kg * kilos
print("New total: KSh", total)

Rules and conventions:

• Names use letters, digits and underscores; they can't start with a digit; no spaces.
• Case-sensitive: total and Total are different.
• Python style (PEP 8) uses snake_case: price_per_kg, first_name.
• Choose meaningful names: total_price, not tp or x.
• No let/const needed: just assign with =.

Basic maths

print(10 + 5)    # 15 addition
print(10 - 5)    # 5 subtraction
print(10 * 5)    # 50 multiplication
print(10 / 4)    # 2.5 division (always gives a decimal)
print(10 // 4)   # 2 whole-number (floor) division
print(10 % 3)    # 1 remainder
print(2 ** 10)   # 1024 power
print((2 + 3) * 4)   # 20 brackets first

// and % are very useful: 100 days = 100 // 7 weeks and 100 % 7 days.

Input and f-strings

input() asks the user for text. It always returns a string, so convert numbers with int() or float().

name = input("What is your name? ")
town = input("Which town are you from? ")
print(f"Karibu {name} from {town}!")

age_text = "19"            # imagine this came from input("How old are you? ")
age = int(age_text)        # convert text to a whole number before doing maths
print(f"Next year you will be {age + 1}.")

(In this editor, input is simulated with sample text, so try it on your computer for real interaction. On your computer, age = int(input("How old are you? ")) works the same way; if someone types something that isn't a number, int() raises an error, which you'll learn to handle later.)

f-strings

An f-string (formatted string) starts with f and puts values inside {}:

item = "Unga 2kg"
price = 180
qty = 3
print(f"{qty} x {item} = KSh {price * qty}")
print(f"Total with VAT: KSh {price * qty * 1.16:,.2f}")   # :,.2f = commas + 2 decimals

Indentation: Python's most important rule

Python uses indentation (spaces at the start of lines) to group code, where other languages use { }. The standard is 4 spaces.

marks = 72
if marks >= 50:
    print("Pass")          # indented: belongs to the if
    print("Well done!")
print("Results printed")   # not indented: always runs

Wrong indentation causes an IndentationError or changes what your program does. VS Code inserts 4 spaces when you press Tab in Python files.

Reading errors

Errors (called exceptions or tracebacks) tell you what went wrong and where:

# This example fails on purpose so you can read the error
price = 180
print("Total:", price * qty)    # qty was never defined

The last line of the traceback, NameError: name 'qty' is not defined, is the key: what type of error and why. Common ones:

Good habits from day one

• Run code after every small change.
• Read the last line of errors first.
• Use clear snake_case names and 4-space indentation.
• Type code yourself; experiment by changing examples.
• Use AI assistants to explain errors, but make sure you understand the fix.

Practice tasks

1. Print your name, school or workplace and town on three lines.
2. Store the price of bread (65) and quantity (4) in variables and print the total with an f-string.
3. Convert 100 days into weeks and remaining days using // and %.
4. Ask for two numbers with input(), convert them, and print their sum (on your computer).
5. Make three deliberate errors (missing quote, wrong indentation, undefined name) and read each message.

Summary

• Python is a readable, general-purpose language created by Guido van Rossum (1991); always use Python 3.
• It's popular for data, AI, automation, web back ends, science and education, with a huge library ecosystem.
• Run it here, on your computer (python.org + VS Code), or in Google Colab.
• print() shows output; # starts comments; variables use snake_case; / gives decimals, // whole numbers, % remainders.
• input() returns strings; convert with int()/float(); f-strings format output.
• Indentation (4 spaces) groups code; read the last line of tracebacks to understand errors.