2.1 What Are Variables ?

Introduction

A variable in Python is a named container that stores data, allowing us to reference and manipulate it later. Variables are fundamental in programming because they help us store, retrieve, and modify data efficiently.

In this section, you will learn:

1. What a variable is
2. How to create and use variables
3. The concept of memory storage and how Python handles variables
4. Examples of using variables in Python

By the end of this lesson, you should understand how variables work and be able to use them in Python programs.

2.1.1 What is a Variable?

A variable is like a container or a label that holds a piece of data. You can think of a variable as a named box where we store information.

Think of a named box like a wooden box with a label on it, into which you can place something.

Example: Storing and Using Variables
The following shows how you assign values to variables

name = "Alice"
age = 25

print(name)  # Output: Alice
print(age)   # Output: 25

Here, "Alice" and 25 are stored in the variables called name and age.

When you do a print, using the print keyword, you can print out the content of a variable.

2.1.2 Why Use Variables?

1. Reusability: Instead of writing the same data multiple times, we store it in a variable and use it anywhere.
2. Code Readability: Using meaningful variable names makes code easier to understand.
3. Dynamic Values: You can change the value stored in a variable, making programs more flexible.

Without Variables (Hard to Maintain values)

print("Alice is 25 years old.")
print("In 5 years, Alice will be 30.")

With Variables (Easier to Maintain values)

name = "Alice"
age = 25
print(name, "is", age, "years old.")
print("In 5 years,", name, "will be", age + 5, ".")

If we change name = "Bob", the output automatically updates without modifying multiple lines!

2.1.3 Creating Variables in Python

Declaring a Variable

In Python, creating a variable is as simple as assigning a value using the = operator.

x = 10
y = 3.14
z = "Hello, Python!"

Explanation:

• x is assigned the integer 10.
• y is assigned the floating-point number 3.14.
• z is assigned the string "Hello, Python!".

Unlike other languages (such as C or Java), Python does not require the declaration of a variable type explicitly. Instead, it automatically detects the type based on the assigned value.

2.1.4 How Variables Work in Memory

When you create a variable, Python stores the value in memory and gives it a reference (name).

Example: Understanding Memory Allocation

a = 10
b = a  # b now refers to the same value as a
a = 20  # Changing a does not affect b

print(a)  # Output: 20
print(b)  # Output: 10

Here, b = a does not create a duplicate copy. Instead, both variables point to the same memory location until a is reassigned.

2.1.5 Changing the Value of a Variable

Variables are mutable, meaning their values can be changed.

name = "Alice"
print(name)  # Output: Alice

name = "Bob"  # Changing the value of name
print(name)  # Output: Bob

The value stored in name changed from "Alice" to "Bob".

2.1.6 Variable Naming Rules

Python has specific rules for naming variables:

Allowed:

1. Variable names must start with a letter (A-Z, a-z) or an underscore _.
2. After the first character, variable names can contain letters, numbers (0-9), or underscores.
3. Python is case-sensitive, meaning age, Age, and AGE are different variables.

Not Allowed:

1. Variable names cannot start with a number.
2. No special characters like @, $, % are allowed.
3. Variables cannot use Python keywords (if, while, def, class, etc.).

Examples of Valid and Invalid Variables

# Valid Variables
student_name = "Alice"
_age = 20
PI = 3.14159  # Constants are usually written in uppercase

# Invalid Variables (will cause an error)
2name = "John"   # Cannot start with a number
student-name = "Alice"  # Cannot contain a hyphen
class = "Python"  # 'class' is a reserved keyword

Best Practice: In order to create clear code, start to use descriptive variable names like user_name instead of x.

2.1.7 Assigning Multiple Variables in One Line

Python allows multiple assignments in a single line.

# Assigning multiple variables in one line
x, y, z = 10, 20, "Hello"

print(x)  # Output: 10
print(y)  # Output: 20
print(z)  # Output: Hello

Assigning the same value to multiple variables:

a = b = c = 50
print(a, b, c)  # Output: 50 50 50

2.1.8 Printing Variables with Different Data Types

You can print multiple variables using commas (,) or f-strings.

Note the preceding ‘f’ before the string in double quotes.

name = "Alice"
age = 25

# Using commas (adds spaces automatically)
print("My name is", name, "and I am", age, "years old.")

# Using f-strings (recommended)
print(f"My name is {name} and I am {age} years old.")

Output:

My name is Alice and I am 25 years old.

f-strings (f"...") make printing variables cleaner and more readable.

2.1.9 Deleting a Variable

To remove a variable from memory, use del.

Internally, this will release the memory space that was allocated to the variable.

x = 100
del x  # Deletes the variable x

print(x)  # This will raise an error since x no longer exists

Warning: After deletion, trying to access the variable will cause an error.

2.1.10 Summary

In this lesson, we have covered the following concepts:

1. Variables store data and allow dynamic updates
2. Python automatically determines variable types (no need for explicit declaration)
3. Variable names must follow naming rules (no special characters, no reserved words)
4. Multiple variables can be assigned in a single line
5. Printing variables using f-strings makes output cleaner
6. Variables can be deleted using del

What’s Next?

In the next lesson, we will explore best practices for naming variables in Python and how to write clean and readable code.