Introduction to Python: What It Is, How to Set It Up, and How to Use It

Before we dive into Python, let's quickly cover two foundational ideas you need to understand. Don't worry · these are very simple.

What is a Program?

A program is a list of instructions written for a computer. Just like a recipe tells a cook what to do step by step, a program tells a computer what to do step by step.

Analogy: Imagine you are directing a robot helper. You cannot just say "make breakfast." You have to say:

1. Open the fridge.
2. Take out two eggs.
3. Turn on the stove.
4. Crack the eggs into the pan.
5. Wait 3 minutes.
6. Turn off the stove.

A computer program works the same way · precise, step-by-step instructions.

What is a Programming Language?

Computers do not understand human language (like English or Yoruba). They only understand numbers (0s and 1s). A programming language is a special language that sits between human language and machine language. You write instructions in a programming language, and a tool called an interpreter or compiler translates those instructions into something the computer can understand.

Python is one of the most popular and beginner-friendly programming languages in the world.

The History and Purpose

Python is a general-purpose programming language created by a Dutch programmer named Guido van Rossum. It was first released to the public in 1991 · over 30 years ago! Since then, it has grown into one of the most widely used languages in the world.

The word "Python" comes not from the snake, but from Guido's love of the British comedy show Monty Python's Flying Circus.

What Can Python Do?

Python is like a Swiss army knife for programmers · it can be used for many different things:

• Web development (server-side): Python can power the behind-the-scenes logic of websites. Big platforms like Instagram and Pinterest were built using Python.
• Software development: You can build desktop applications, games, and tools.
• Mathematics and data analysis: Python is widely used by scientists, engineers, and data analysts to crunch numbers, draw charts, and find patterns in data.
• Automation: Python can automate repetitive tasks, like renaming thousands of files, sending emails, or scraping data from websites.
• Machine learning and artificial intelligence: Python powers most of the AI tools you hear about today · from recommendation systems to image recognition.
• Connecting to databases: Python can read from and write to databases like MySQL and MongoDB.
• Big data: Python handles and processes enormous datasets.
• Rapid prototyping: You can quickly test ideas without writing hundreds of lines of code first.

Why Should You Learn Python?

Here are the key reasons Python is such a great first language:

• Simple syntax: Python's code reads almost like plain English. Compare this to languages like Java or C++ which require much more complex setup just to print "Hello World".
• Fewer lines of code: Python lets you accomplish the same task with far fewer lines than most other languages, saving you time.
• Cross-platform: Python runs on Windows, Mac, Linux, Raspberry Pi, and many other systems without any changes to your code.
• Interpreted language: Python runs code line by line as soon as it is written, so you see results immediately. There is no long "compile" wait like in some other languages.
• Flexible programming styles: Python can be written in a procedural way (step-by-step instructions), an object-oriented way (using classes and objects), or a functional way (using pure functions). You learn more about these styles as you grow.
• Huge community: Millions of programmers use Python. Whatever problem you face, someone has probably solved it and shared the solution online.

Python in the Real World

Thinking Prompt: Which of these use cases interests you most? As you learn Python, keep that application in mind · it will motivate you.

Option A: Try Python Online (No Installation Needed)

If you just want to start experimenting right now without installing anything, you can use an online Python editor. Websites like https://www.w3schools.com/python/trypython.asp let you type Python code in your browser and see the result immediately.

This is a great way to follow along with the examples in this lesson.

Option B: Install Python on Your Computer

To run Python on your own machine, follow these steps.

Step 1: Check if Python is Already Installed

Many computers already have Python installed. Let's check.

On Windows:

1. Press the Windows key, type cmd, and open the Command Prompt.
2. Type the following and press Enter:

python --version

On Mac or Linux:

1. Open the Terminal.
2. Type:

python --version

If you see something like Python 3.12.0, you already have Python! If not, move to Step 2.

Step 2: Download Python

Go to the official Python website: https://www.python.org/

Click Download Python and follow the installer instructions. Make sure you are downloading Python 3 (not Python 2, which is outdated).

Important for Windows users: During installation, tick the box that says "Add Python to PATH". This makes Python accessible from your Command Prompt.

Step 3: Verify the Installation

After installation, open your Command Prompt or Terminal again and type:

python --version

You should now see a version number printed, like:

Python 3.12.0

Congratulations · Python is installed!

Writing and Running Your First Python File

Now let's write our very first Python program.

Step 1: Open any plain-text editor (Notepad on Windows, TextEdit on Mac, or any code editor like VS Code).

Step 2: Type the following:

print("Hello, World!")

Step 3: Save the file as hello.py (the .py extension tells the computer this is a Python file).

Step 4: Open your Command Prompt or Terminal, navigate to the folder where you saved the file, and type:

python hello.py

Expected Output:

Hello, World!

You just ran your first Python program! Let's unpack exactly what happened.

• print is a function · a built-in tool in Python that displays output.
• The parentheses () tell Python "here comes what I want to print."
• "Hello, World!" is the text you want to display. The quotation marks tell Python this is a piece of text (called a string).

Using the Python Command Line (Interactive Mode)

Python also has a special interactive mode where you can type one line of code at a time and see the result immediately. This is perfect for quick experiments.

Open your Command Prompt or Terminal and type:

python

(On some systems, use py instead of python.)

You will see something like:

Python 3.12.0 (main, Oct  3 2023, ...) on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

The >>> symbol is called the prompt. It means Python is ready and waiting for your instruction.

Type this and press Enter:

>>> print("Hello, World!")

Expected Output:

Hello, World!

To exit the Python command line, type:

exit()

Checking Your Python Version From Inside Code

You can also find out which version of Python is running directly from your program:

import sys

print(sys.version)

Expected Output (will vary depending on your version):

3.12.0 (main, Oct  3 2023, 08:56:10) [GCC 11.4.0]

• import sys loads a built-in Python module (a collection of extra tools). sys is the system module.
• sys.version is a piece of information stored in that module · your Python version.

You will learn a lot more about modules later in the course. For now, just know they are extra toolkits you can bring into your program.

Just like English has grammar rules (sentences start with a capital letter, end with a full stop, etc.), Python has syntax rules. Syntax means the correct way to write code so that Python can understand it.

Think of syntax as the grammar of your programming language. Break the rules, and Python gives you an error · just like how a teacher might mark a sentence wrong if you break grammar rules.

Rule 1: Python Executes Code Line by Line

Python reads your code from the very first line to the very last line, one line at a time. Each line is a new instruction.

print("First line runs first")
print("Second line runs second")
print("Third line runs third")

Expected Output:

First line runs first
Second line runs second
Third line runs third

This is called sequential execution · instructions run in the order you write them. This is very important: if you want something to happen first, you must write it first.

Rule 2: Python Uses Indentation (Spaces) to Define Blocks

This is one of Python's most important and unique rules.

What is indentation? Indentation means the spaces at the beginning of a line. Look at this:

This line has no spaces at the start.
    This line has 4 spaces at the start.

In many other programming languages, indentation is optional · just for humans to read more easily. But in Python, indentation is mandatory. It is how Python knows which lines of code belong together.

Let's see this with a simple example. You will not fully understand the if statement yet (we cover that in a later lesson), but focus on the indentation:

Correct · with indentation:

if 5 > 2:
    print("Five is greater than two!")

Expected Output:

Five is greater than two!

Incorrect · without indentation:

if 5 > 2:
print("Five is greater than two!")

Expected Output (Error!):

IndentationError: expected an indented block after 'if' statement on line 1

Do you see the difference? The second example is missing the 4 spaces before print(...). Python sees this as a syntax error because the if statement needs at least one indented line after it.

How many spaces?

You can use any number of spaces, as long as you are consistent within a block. The standard is 4 spaces, which most professional programmers and style guides recommend.

if 5 > 2:
    print("Using 4 spaces — this works!")

if 5 > 2:
        print("Using 8 spaces — this also works!")

Expected Output:

Using 4 spaces — this works!
Using 8 spaces — this also works!

Mixing spaces in the same block causes an error:

if 5 > 2:
    print("Four spaces here")
        print("Eight spaces here — ERROR!")

Expected Output (Error!):

IndentationError: unexpected indent

Golden Rule: Always use the same number of spaces for each level of indentation. Using 4 spaces consistently is the best practice.

Thinking Prompt: What do you think would happen if you used a tab (Tab key) instead of spaces? In Python, mixing tabs and spaces causes errors. Always pick one and stick with it. The recommended choice is spaces.

Rule 3: Variables Are Created by Assignment

A variable is a named storage box in your computer's memory. You put a value into it, and you can use that value later by calling its name.

In Python, you create a variable simply by writing its name, the = sign, and the value:

x = 5
y = "Hello, World!"

Let's break this down:

• x is the variable name (you choose this name).
• = is the assignment operator · it means "store this value into this variable."
• 5 is the value being stored. Since it has no quotes, Python knows it is a number.
• y is another variable name.
• "Hello, World!" is the value stored in y. Since it has quotes, Python knows it is text (a string).

Unlike some other languages, you do not need to tell Python in advance what type of value a variable will hold. Python figures it out automatically.

age = 25
name = "Adaora"
height = 1.72
is_student = True

You will explore variables much more deeply in Lesson 02. For now, just know they exist and how to create them.

Rule 4: Comments Explain Your Code

A comment is a note written in your code for humans to read. Python completely ignores comments when running the program.

Comments start with the # (hash) symbol:

# This is a comment. Python ignores this line.
print("Hello, World!")  # This comment is at the end of a line

Expected Output:

Hello, World!

Notice that the comment on line 1 produces no output · Python just skips it.

Why are comments important?

• They help you (and others) remember why you wrote a piece of code.
• They are essential for collaboration with other programmers.
• They help when you return to your code weeks or months later.

# Calculate the area of a rectangle
length = 10   # length in centimetres
width = 5     # width in centimetres
area = length * width
print(area)

Expected Output:

50

What is a Statement?

A statement is a single instruction in a Python program. It is the most basic unit of a program · one action the computer performs.

Here is a very simple statement:

print("Python is fun!")

Expected Output:

Python is fun!

This is one statement. It does one thing: print the text "Python is fun!" to the screen.

Programs Contain Many Statements

Real programs contain many statements. Each statement runs in order, from top to bottom:

print("Hello World!")
print("Have a good day.")
print("Learning Python is fun!")

Expected Output:

Hello World!
Have a good day.
Learning Python is fun!

Let's trace through this step by step:

1. Python reads line 1: print("Hello World!") → It prints "Hello World!" → moves on.
2. Python reads line 2: print("Have a good day.") → It prints "Have a good day." → moves on.
3. Python reads line 3: print("Learning Python is fun!") → It prints "Learning Python is fun!" → program ends.

Thinking Prompt: What would happen if you swapped line 1 and line 3? Try it and see!

In Python, Each Statement Goes on Its Own Line

Unlike many other languages (like Java or C++), Python does not require a semicolon (;) at the end of each statement. Just pressing Enter (a new line) is enough to end a statement:

Python (correct · no semicolons needed):

print("Statement one")
print("Statement two")

Java (requires semicolons):

System.out.println("Statement one");
System.out.println("Statement two");

This is one reason Python is much cleaner and easier to read.

Optional: Multiple Statements on One Line with Semicolons

Python does allow you to put multiple statements on one line by separating them with semicolons, but this is considered bad practice and is rarely done. It makes code hard to read:

print("Hello"); print("How are you?"); print("Bye bye!")

Expected Output:

Hello
How are you?
Bye bye!

This works, but the best practice is to put each statement on its own line:

print("Hello")
print("How are you?")
print("Bye bye!")

Expected Output:

Hello
How are you?
Bye bye!

The second version is much cleaner. Follow this rule: one statement per line.

What Happens if You Put Two Statements on One Line Without a Separator?

print("Python is fun!") print("Really!")

Expected Output (Error!):

SyntaxError: invalid syntax

Python sees the second print and does not know what to do with it because there is no separator (newline or semicolon). This is a SyntaxError · you have broken a syntax rule.

What is Output?

In computing, output means information that a program sends out to be seen or used. The most common form of output in Python for beginners is text or numbers displayed in the terminal (the black Command Prompt or Terminal window).

Think of output as your program talking to you.

The print() Function

The print() function is the most fundamental output tool in Python. It displays whatever you give it on the screen.

Syntax:

print(something_to_display)

The word print is the function name. The parentheses () contain what you want to display. That content inside the parentheses is called the argument.

Printing Text

Text in Python must always be wrapped in quotes. You can use either double quotes ("...") or single quotes ('...') · both work:

print("This will work!")
print('This will also work!')

Expected Output:

This will work!
This will also work!

What happens without quotes?

print(This will cause an error)

Expected Output (Error!):

SyntaxError: invalid syntax

Python sees This will cause an error without quotes and tries to treat each word as a variable name or function. Since these variables do not exist, it throws an error.

Golden Rule: Text (strings) must always be wrapped in quotes.

Printing Multiple Lines of Text

You can call print() multiple times to display multiple lines:

print("Hello World!")
print("I am learning Python.")
print("It is awesome!")

Expected Output:

Hello World!
I am learning Python.
It is awesome!

Each print() call automatically moves to a new line before printing the next item. This is the default behaviour · it happens automatically.

Printing on the Same Line (Removing the New Line)

Sometimes you want multiple prints to appear on the same line. You can do this using the end parameter:

print("Hello World!", end=" ")
print("I will print on the same line.")

Expected Output:

Hello World! I will print on the same line.

Let's break this down:

• By default, print() adds a newline character (\n) at the end of its output, which moves the cursor to the next line.
• When you write end=" ", you are telling Python: "Instead of ending with a newline, end with a space." So the next print() call continues on the same line.

Thinking Prompt: What do you think would happen if you wrote end="" (with nothing inside the quotes)?

Numbers vs. Text: A Key Difference

In Python, there is an important distinction between text (strings) and numbers:

• Text must be surrounded by quotes: "Hello" or '42'
• Numbers must NOT have quotes: 42 or 3.14

If you put a number inside quotes, Python treats it as text, not as a number. This matters a lot when you want to do maths with it.

print(42)        # This is the number 42
print("42")      # This is the TEXT "42" — not a number!

Expected Output:

42
42

They look the same when printed! But internally Python treats them very differently. The first can be used in maths; the second cannot.

Printing Numbers Directly

You can print numbers without any quotes:

print(3)
print(358)
print(50000)

Expected Output:

3
358
50000

Doing Mathematics Inside print()

One of the most useful beginner tricks: you can perform calculations directly inside print():

print(3 + 3)
print(2 * 5)

Expected Output:

6
10

Let's trace through:

• Line 1: Python sees 3 + 3, evaluates it (calculates it), gets 6, then prints 6.
• Line 2: Python sees 2 * 5, evaluates it, gets 10, then prints 10.

Python supports all the basic arithmetic operations:

Let's try a few:

print(10 + 5)
print(10 - 5)
print(10 * 5)
print(10 / 5)
print(2 ** 4)
print(10 // 3)
print(10 % 3)

Expected Output:

15
5
50
2.0
16
3
1

Notice: 10 / 5 gives 2.0 (with a decimal point), not just 2. Python's / operator always returns a decimal (float). If you want a whole number result, use //.

Mixing Text and Numbers in One Output

You can combine text and numbers in a single print() by separating them with a comma:

print("I am", 35, "years old.")

Expected Output:

I am 35 years old.

Let's break this down:

• "I am" · this is a text string.
• 35 · this is a number.
• "years old." · this is another text string.
• The commas , inside print() separate multiple items to display. Python automatically puts a space between each item.

Another example:

print("The answer is", 5 + 7, "and that is correct!")

Expected Output:

The answer is 12 and that is correct!

Python first evaluates 5 + 7 to get 12, then combines everything and prints it.

Let's look at the most common mistakes new Python learners make · and how to fix them.

Mistake 1: Forgetting Quotes Around Text

Wrong:

print(Hello World)

Error:

SyntaxError: invalid syntax

Fixed:

print("Hello World")

Why: Text that is not a number must be wrapped in quotes. Without quotes, Python thinks you are referencing a variable called Hello, which does not exist.

Mistake 2: Using the Wrong Quote Style (Mixing Quotes)

Wrong:

print("Hello World')

Error:

SyntaxError: EOL while scanning string literal

Fixed:

print("Hello World")
# OR
print('Hello World')

Why: You must open and close with the same type of quote. If you open with ", you must close with ". If you open with ', you must close with '.

Mistake 3: Missing Indentation

Wrong:

if 5 > 2:
print("Five is greater!")

Error:

IndentationError: expected an indented block

Fixed:

if 5 > 2:
    print("Five is greater!")

Why: Python requires at least one indented line inside blocks like if statements. The indentation tells Python which code belongs inside the block.

Mistake 4: Inconsistent Indentation

Wrong:

if 5 > 2:
    print("Line one")
      print("Line two — extra spaces!")

Error:

IndentationError: unexpected indent

Fixed:

if 5 > 2:
    print("Line one")
    print("Line two — consistent now!")

Why: All lines inside the same block must have the exact same indentation level.

Mistake 5: Putting Numbers Inside Quotes When You Want to Do Maths

Wrong (if you want to add numbers):

print("5" + "3")

Unexpected Output:

53

What you probably wanted:

print(5 + 3)

Expected Output:

8

Why: When numbers are inside quotes, Python treats them as text (strings). Adding two text strings with + joins them together (called concatenation), not adds them mathematically. So "5" + "3" becomes "53", not 8.

Mistake 6: Running Two Statements on the Same Line Without a Separator

Wrong:

print("Hello") print("World")

Error:

SyntaxError: invalid syntax

Fixed:

print("Hello")
print("World")

Why: Python expects one statement per line. If you must put two on one line, separate them with a semicolon (but the recommended practice is always one statement per line).

Mistake 7: Case Sensitivity · Print Is Not print

Wrong:

Print("Hello World!")

Error:

NameError: name 'Print' is not defined

Fixed:

print("Hello World!")

Why: Python is case-sensitive. print, Print, and PRINT are three completely different names in Python. The built-in function is all lowercase: print.