PYTHON

Variables & Constants in Python

Variables & Constants in Python

Variables and constants are fundamental concepts in Python programming. Whenever you write a program, you need to store information such as names, numbers, prices, or configuration values.

In this tutorial, we will learn what variables and constants are, how to create them, how Python handles data types, and how to follow proper naming conventions.

This guide is designed for beginners who are starting their Python programming journey.

What You Will Learn

  • What a variable is in Python
  • How to create and assign variables
  • How to use variables in programs
  • Python variable naming rules
  • Different data types used with variables
  • How to reassign variables
  • What constants are in Python
  • The difference between variables and constants
  • Best practices for naming variables and constants
  • Common mistakes and practice questions

1. What Is a Variable in Python?

A variable is a name that refers to a value stored in a program.

For example, if you want to store a person’s name, you can create a variable called name.

name = "Himanshu"

Here:

  • name is the variable.
  • "Himanshu" is the value.
  • = is the assignment operator.

You can use the variable later in your program:

name = "Himanshu"

print(name)

Output:

Himanshu

How Variable Assignment Works

age = 25

Python creates the integer value 25 and makes the name age refer to it.

You can think of a variable as a label attached to a value.

age  ─────►  25

In Python, variables are names that refer to objects. They do not need to be declared with a separate data type.

2. Creating Variables in Python

Python does not require a special keyword to declare a variable. You simply assign a value to a name.

name = "Himanshu"
age = 25
salary = 45000.50
is_developer = True

Python automatically determines the type of each value.

Example

name = "Himanshu"
age = 25

print(name)
print(age)

Output:

Himanshu
25

3. Variables Can Store Different Data Types

Python variables can refer to values of different data types.

name = "Himanshu"       # String
age = 25                # Integer
price = 99.99           # Float
is_active = True        # Boolean

Common Data Types

Data TypeExampleDescription
str"Hello"Text
int25Whole number
float99.99Decimal number
boolTrueTrue or false
list[1, 2, 3]Ordered collection
tuple(1, 2, 3)Immutable collection
dict{"name": "Himanshu"}Key-value collection
NoneTypeNoneRepresents no value

Check the Type of a Variable

Use the type() function:

age = 25
name = "Himanshu"

print(type(age))
print(type(name))

Output:

<class 'int'>
<class 'str'>

4. Multiple Variable Assignment

Python allows you to assign values to multiple variables in one line.

name, age, city = "Himanshu", 25, "Gurugram"

print(name)
print(age)
print(city)

Output:

Himanshu
25
Gurugram

You can also assign the same value to multiple variables:

x = y = z = 100

print(x)
print(y)
print(z)

Output:

100
100
100

5. Variable Naming Rules in Python

Python has specific rules for naming variables.

Valid Rules

  1. A variable name can contain letters, numbers, and underscores.
  2. A variable name cannot start with a number.
  3. A variable name cannot contain spaces.
  4. Python variable names are case-sensitive.
  5. A variable name cannot be a Python keyword.

Valid Examples

name = "Himanshu"
age25 = 25
user_name = "Himanshu"
_total = 1000

Invalid Examples

2name = "Himanshu"       # Invalid
user name = "Himanshu"   # Invalid
class = "Python"         # Invalid

The invalid examples will produce a syntax error.

Case Sensitivity

Python treats uppercase and lowercase names as different.

name = "Himanshu"
Name = "Rahul"

print(name)
print(Name)

Output:

Himanshu
Rahul

name and Name are two different variables.

6. Python Variable Naming Convention

The recommended naming style for variables in Python is snake_case.

first_name = "Himanshu"
last_name = "Sharma"
total_price = 500

Avoid unclear names:

x = 500
a = "Himanshu"

Prefer meaningful names:

total_price = 500
customer_name = "Himanshu"

Meaningful names make your code easier to read and maintain.

7. Reassigning Variables

Python variables can be reassigned to a new value.

age = 25
print(age)

age = 26
print(age)

Output:

25
26

You can also assign a different data type to the same variable:

value = 100
print(value)

value = "Hello"
print(value)

Output:

100
Hello

Python is dynamically typed, so the same variable name can refer to values of different types at different times.

8. Using Variables in Expressions

Variables can be used in calculations and expressions.

price = 100
quantity = 3

total = price * quantity

print(total)

Output:

300

Example: Calculate a Discount

price = 1000
discount = 10

discount_amount = price * discount / 100
final_price = price - discount_amount

print("Discount:", discount_amount)
print("Final Price:", final_price)

Output:

Discount: 100.0
Final Price: 900.0

9. Variables and String Formatting

You can use variables inside strings with f-strings.

name = "Himanshu"
age = 25

print(f"My name is {name} and I am {age} years old.")

Output:

My name is Himanshu and I am 25 years old.

F-strings are one of the easiest ways to combine text and variables in Python.

10. What Is a Constant in Python?

A constant is a value that is intended to remain unchanged throughout a program.

Unlike some programming languages, Python does not have a built-in keyword that makes a variable truly constant.

Instead, Python uses a naming convention:

Constants are written in UPPERCASE letters, with words separated by underscores.

Example

PI = 3.14159
MAX_USERS = 100
APP_NAME = "My Python App"

These names indicate that their values should not be changed.

Important Note

Python does not prevent reassignment of constants.

PI = 3.14159

PI = 3.14

print(PI)

Output:

3.14

The reassignment works because Python does not enforce constants by default.

The uppercase naming convention is a signal to developers: “Do not change this value.”

11. Variables vs Constants

FeatureVariableConstant
PurposeStores values that may changeStores values intended to remain unchanged
Naming stylesnake_caseUPPER_SNAKE_CASE
Can be reassigned?YesYes, technically
Python enforcementNo restrictionNo built-in enforcement
Exampleuser_age = 25MAX_USERS = 100

Example

user_age = 25          # Variable
MAX_LOGIN_ATTEMPTS = 3 # Constant

The user’s age may change, but the maximum login attempts is intended to remain fixed.

12. Constants in a Real Python Project

Constants are useful for values such as:

  • Application names
  • API URLs
  • Maximum limits
  • Mathematical values
  • Configuration defaults
  • File paths
  • Status codes

Example

APP_NAME = "CodeWithCoffee"
MAX_LOGIN_ATTEMPTS = 3
DEFAULT_LANGUAGE = "en"

print(APP_NAME)
print(MAX_LOGIN_ATTEMPTS)
print(DEFAULT_LANGUAGE)

In larger projects, constants are often placed in a separate file such as constants.py.

Example Project Structure

python-project/
│
├── app.py
├── constants.py
└── requirements.txt

constants.py

APP_NAME = "CodeWithCoffee"
MAX_LOGIN_ATTEMPTS = 3

app.py

from constants import APP_NAME, MAX_LOGIN_ATTEMPTS

print(APP_NAME)
print(MAX_LOGIN_ATTEMPTS)

This keeps reusable constant values organized.

13. Variables vs Constants in PHP

If you know PHP, the concept may look familiar.

PHP Constant

define("APP_NAME", "CodeWithCoffee");

Python Constant

APP_NAME = "CodeWithCoffee"

The difference is that Python’s uppercase naming convention does not enforce immutability.

Python relies on developer discipline and conventions.

14. Common Mistakes

Mistake 1: Starting a Variable Name with a Number

1name = "Himanshu"

Why it is wrong: Variable names cannot start with a number.

Correct:

name1 = "Himanshu"

Mistake 2: Using Spaces in Variable Names

first name = "Himanshu"

Correct:

first_name = "Himanshu"

Mistake 3: Using a Python Keyword

class = "Python"

Correct:

class_name = "Python"

Mistake 4: Assuming Uppercase Makes a Value Immutable

MAX_USERS = 100
MAX_USERS = 200

Python allows this. Uppercase is only a convention.

Mistake 5: Using Unclear Variable Names

x = 45000

Prefer:

monthly_salary = 45000

15. Best Practices

  • Use meaningful variable names.
  • Follow snake_case for variables and functions.
  • Use UPPER_SNAKE_CASE for constants.
  • Avoid single-letter names unless they are used for simple counters.
  • Keep variable names descriptive but concise.
  • Do not use Python keywords as variable names.
  • Avoid unnecessary reassignment.
  • Keep constants in a separate module in larger projects.
  • Remember that Python constants are conventions, not enforced restrictions.

16. Practice Questions

Try solving these questions yourself:

  1. Create a variable named student_name and assign your name.
  2. Create variables for age, height, and is_student.
  3. Print the data type of each variable.
  4. Create three variables in one line.
  5. Calculate the total price using price and quantity.
  6. Create a constant named MAX_SCORE.
  7. Explain the difference between a variable and a constant.
  8. Write a program that calculates the area of a rectangle.

Practice Example

length = 10
width = 5

area = length * width

print("Area of rectangle:", area)

Output:

Area of rectangle: 50

Conclusion

In this tutorial, we learned how variables and constants work in Python. We covered variable creation, data types, naming rules, reassignment, expressions, and string formatting.

We also learned that Python constants are written in uppercase, but Python does not enforce them as immutable values.

Understanding variables and constants is essential because almost every Python program uses them to store and manage information.

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