Introduction to Python

1. Getting Started (your first program)

Python is a programming language that reads almost like English, which makes it one of the easiest languages to learn and one of the most widely used.

A program is a list of instructions that the computer follows one line at a time, from top to bottom. Think of a recipe: each step is simple, and the order matters. Python's job is to read your recipe and carry it out.

Below is a tiny program saved in a file called hello.py (.py is the ending used for Python files). Press Step to run it one line at a time; the box under the picture explains each line.

What Python is and where people use it

Python is a general-purpose language: the same language is used for very different jobs. It's free, it runs on Windows, macOS and Linux, and it comes with a huge collection of ready-made tools.

Installing Python and choosing an editor

1. Download Python from python.org and run the installer. On Windows, tick "Add Python to PATH", so you can start Python by typing python in a terminal.
2. Open a terminal: a window where you type commands instead of clicking (on Windows it's called Command Prompt or PowerShell; on macOS it's Terminal). Check Python is installed: python --version (on macOS or Linux it may be python3 --version).
3. Pick a place to write code:

Running a script vs. the interactive shell (REPL)

There are two ways to run Python. A script is a file such as hello.py that you run all at once with python hello.py. The interactive shell, or REPL (Read, Evaluate, Print, Loop), starts when you type python on its own: you type one line at a time after the >>> prompt and see the answer immediately.

Where it shows up

• Every Python program you'll ever write is run the same way: line by line, top to bottom.
• print() is the first tool for checking what your program is doing (you'll use it again for debugging in lesson 9).
• The REPL is a handy calculator and the quickest way to test an idea.

2. Variables and Data Types (names for values)

A variable is a name that refers to a value. Every value has a type that decides what you can do with it.

Think of sticky labels on boxes. age = 21 puts the number 21 in a box and sticks the label age on it. Later, writing age means "the value in that box". Give the label to a new value and it simply moves.

The program below asks for your name and age, stores each answer under a name, and makes a few more variables of different types. Step through it now; the sections after it explain each new word.

Variables and naming rules

• Names use letters, digits and _, and can't start with a digit: score2 is fine, 2score is not.
• No spaces or dashes: use user_name, not user name or user-name.
• Names are case-sensitive: age and Age are two different names.
• You can't use Python's own reserved words, called keywords, such as if, for or class.
• Choose names that say what the value means: price beats p.

Core types

The type names are short: int = integer (a whole number), float = floating-point number (a number with a decimal point), str = string (a string of characters, in other words text), bool = Boolean (True or False, named after the mathematician George Boole).

Ask Python for the type of any value with type(): type(3.14) gives <class 'float'>.

Type conversion and user input

input() pauses the program, waits for the user to type something and press Enter, and always gives back a str, even if they typed digits. To do maths with it, convert it first with int() or float(). Use str() to turn a number into text.

Where it shows up

• Every program stores data in variables: a score, a user's name, a price, a setting.
• Data from users, files and websites usually arrives as text and must be converted.
• None marks "not set yet", for example a value you haven't looked up.

3. Operators and Expressions (working values out)

An expression is anything Python can work out to a single value, like 2 + 3 * 4. Operators are the symbols that do the work.

It's the maths you know from school, with the same rule: multiplication happens before addition unless brackets say otherwise. Python also has operators that compare values and combine yes/no answers. Working out an expression's value is called evaluating it.

Pick an expression below and press Evaluate to watch Python work it out one operator at a time. The tables after the demo list every operator and the order they go in.

Arithmetic, comparison and logical operators

Operator precedence

When an expression has several operators, Python does them in this order, from first to last. Operators on the same row go left to right. When in doubt, add brackets: they make your intent clear to readers too.

Where it shows up

• Totals, averages and prices: total = price * quantity.
• Even or odd checks: n % 2 == 0 is True for even numbers.
• Decisions (lesson 5): if age >= 18 and has_ticket:.

4. Strings (working with text)

A string (str) is a sequence of characters (single letters, digits, spaces or symbols). Each character has a numbered position, so you can pick out one character or a slice (several characters in a row).

Think of a string as a row of letter tiles with numbers underneath. The number under a tile is that character's index (its position number). Counting starts at 0 on the left. Python also numbers from the right: -1 is the last tile, -2 the one before it.

To pick a position, write the index in square brackets: s[0] is the character at index 0. Type a word below, choose the numbers, and press a button.

Indexing and slicing

• s[0] is the first character and s[-1] is the last.
• s[start:stop] takes characters from start up to but not including stop: "Python"[0:2] is 'Py'.
• Leave a number out to mean "from the start" or "to the end": s[:3], s[2:].
• A third number is the step: s[::2] takes every second character, s[::-1] reverses the string.
• Strings can't be changed in place: s[0] = "J" is an error. Make a new string instead.

Common methods and f-strings

A method is a command that belongs to a value and works on it. You use it (programmers say you call it) by writing a dot after the value: text.upper(). A function is a named command such as print() or len(); lesson 7 covers them. String methods never change the original; they give back a new string.

Some methods work with a list: an ordered row of values in square brackets, like ['a', 'b'] (lesson 6 covers lists). Run the program below with your own text. Its last line uses an f-string, explained just below the table.

Where it shows up

• Cleaning what users type: answer.strip().lower() (you'll see this in the final project).
• Reading data: line.split(",") splits a line of a CSV file into fields.
• Building messages and reports with f-strings.

5. Control Flow (decisions and loops)

Control flow decides which lines run and how many times. if makes decisions; for and while repeat work.

Think of a road with junctions and roundabouts. At a junction (if) you take one road depending on a condition. On a roundabout (a loop) you go around again and again until it's time to exit (break), and sometimes you skip ahead to the next lap (continue).

The program below counts from 1 upward. For each number it decides what to print (if / elif / else). Two settings let you skip one number (continue) or stop early (break). The sections after the demo explain each keyword.

Decisions and loops at a glance

A condition is an expression that is True or False, such as age >= 18 (lesson 3). An if checks one condition; elif and else add more choices underneath it. The lines under each one form its block, and they are indented (4 spaces): indentation is how Python knows what's inside.

Python checks the conditions from the top. It runs the block under the first condition that is True, then skips the rest. else runs only when none of them were true.

A loop repeats its block. Each pass through a loop is called a round (or iteration), and the indented lines that repeat are the loop's body. range(5) gives the numbers 0, 1, 2, 3, 4; range(1, 5) gives 1, 2, 3, 4.

Loops: for, while and range()

Use for when you know what to loop over: a range() of numbers, the items of a list (lesson 6), or the characters of a string. Use while when you repeat until something happens.

Here is a while loop that counts down from a number you choose and stops at zero.

Where it shows up

• if: checking a password, a score, or whether a file exists.
• for: processing every item in a list, every line in a file, every row in a spreadsheet.
• while: game loops, menus and asking again until the user types a valid answer (see the final project).

6. Data Structures (list, tuple, dict, set)

Python has four built-in (ready to use, nothing to install) ways to keep many values together, called collections. Each one is good at a different job.

A list is like a shopping list: ordered, and you can add or cross off items. A tuple (a maths word for a fixed group of values) is like a date of birth: fixed once written. A dict (short for dictionary) is like a phone book: you look up a value (the phone number) by its key (the person's name). A set is like a bag of unique stickers: no duplicates allowed.

Below, four collections are already in memory. Each button runs one line of code on them, and the box under the picture explains what that line did.

list, tuple, dict and set

Ordered means the items stay in the order you put them in (not that they get sorted).

When to use each one

Pick the collection you'd reach for in each situation, then check the answer.

List comprehensions (a light intro)

A list comprehension builds a new list in one line: [n * n for n in range(1, 6)] reads as "n times n, for each n in range(1, 6)". It does exactly what the three-line loop does. Step through both below.

Where it shows up

• list: rows of data, to-do items, scores, anything in an order you can change.
• tuple: fixed pairs such as coordinates (x, y) or RGB colours.
• dict: settings, word counts, a user's profile ({"name": ..., "email": ...}), and JSON data from the web.
• set: removing duplicates and fast "have I seen this?" checks.

7. Functions (reusable blocks of code)

A function is a named block of code that does one job. You define it once with def and call it as many times as you like.

Think of a coffee machine. You press a button and give it inputs (the cup size). It does its work inside, then hands you a result (the coffee). You don't need to know what happens inside to use it, and you can use it again and again.

The program below defines two functions, greet and area, and then uses (calls) them. Step through it: a purple box of the function's own names appears for each call. The sections after the demo explain what you see.

Defining and calling functions

• def name(parameters): starts a definition. The indented lines below are the function's body.
• Defining a function does not run it. The body runs only when you call it: greet("Ada").
• Calling pauses the caller, jumps into the function, and comes back when the function finishes.

Parameters, default values and return values

Scope: local vs. global

Names created inside a function are local: they exist only while that call runs, in its own box of names (shown in purple above), and disappear when it returns. Names created at the top level of the file are global. A function can read a global name (like greeting), but the code outside can't see the function's local names.

Where it shows up

• Everywhere: print(), len() and input() are functions someone else wrote.
• Splitting a program into small named steps, as the final project does with ask(), play() and main().
• Avoiding copy-and-paste: fix a bug once inside the function and every call gets the fix.

8. Modules (the standard library)

A module is a file of ready-made Python code. import loads it so you can use its functions instead of writing them yourself.

Think of a toolbox. Python comes with a big one, the standard library, with a drawer for each kind of job: math for calculations, random for chance, datetime for dates. import opens a drawer; it's free and already installed.

The program below imports modules in three different ways (the Import styles table after it explains each one). Each button runs one line.

What a module is and why we use import

• A module is just a .py file. Every file you write is a module too, so you can import your own code.
• Modules keep code organised and save you from reinventing things like square roots or calendars.
• Put imports at the top of your file, so readers see what the program depends on.

Import styles

Practical use of math, random and datetime

Where it shows up

• Real programs import a lot: reading files (os, pathlib), the web (urllib), data (json, csv).
• Beyond the standard library, pip install adds packages from the internet, such as requests or pandas. You import them the same way.

9. Errors and Exceptions (when things go wrong)

Errors are normal: every programmer sees them all day. Python tells you what went wrong and where, and try / except lets your program recover instead of crashing. When Python hits a problem while running, it stops and reports an error. Python calls this an exception, and says the line raised it.

Think of a smoke alarm. An exception is the alarm going off: something unexpected happened. A traceback tells you which room it came from, and an except block is your plan for what to do when the alarm rings.

Reading error messages and tracebacks

Click each line of the traceback to see what it means. The program it came from is on the right. It uses two things you haven't met yet: sum(items) adds up all the numbers in a list, and if not items: is True when the list is empty (an empty list counts as False).

try / except / finally

Put code that might fail inside try. If an error happens, Python jumps straight to the matching except block instead of crashing. finally runs at the end either way. Try a good input, then some text, then a zero.

Basic debugging

1. Read the last line of the traceback, then go to the line number it points to.
2. Print what you think you know: add print(x, type(x)) just before the failing line to check names and types.
3. Make it smaller: run the failing piece on its own in the REPL.
4. Use a debugger: in VS Code or PyCharm, click next to a line number to set a breakpoint, then run step by step and watch the variables, just like the Step button in this course.
5. Explain it out loud, line by line ("rubber duck debugging"). You'll often spot the problem as you speak.

Where it shows up

• Checking user input: ask again when someone types letters instead of a number (the final project does this).
• Files that might be missing, and network calls that might fail.
• finally: always closing a file or connection, even after an error.

10. Final Project (a quiz game)

Time to put it all together: a small quiz game that uses variables, strings, loops, a list of dicts (dictionaries), functions, two modules and error handling.

Play it below. The game really runs: type your answers into the console and watch each line of the program light up as it executes. Then read the code and the table underneath to see which lesson every part comes from.

Which lesson each part comes from

New pieces in the game

The game uses a few small things the lessons didn't show:

• score += 1 is short for score = score + 1.
• while True: repeats forever on purpose. The break inside it is the way out: it runs once the player types a valid number.
• 1 <= rounds <= len(QUESTIONS) means rounds is at least 1 and at most 4 (the number of questions).
• random.sample(QUESTIONS, rounds) picks that many different questions in a random order, and round() rounds a number to the nearest whole number.

Make it your own

• Quiz game: add your own questions to QUESTIONS, or a time limit using datetime.
• To-do app in memory: a while True menu with add, list, done and quit, storing tasks in a list of dicts.
• Simple calculator: ask for two numbers and an operator, use if / elif to pick the operation, and catch ValueError and ZeroDivisionError.

Python cheat sheet

Everything from the course on one page. Filter by topic to revise before the final quiz.

Final quiz & certificate

This is the last step of the course: one list of 10 questions covering every lesson. After each answer you'll see the correct option and why it's right. Score 80% or more (8 of 10) to earn a certificate with your name.