Python Variables

A variable is a name used to store or refer to a value in a Python program.

Variables allow us to store information such as:

• Names
• Age
• Prices
• Email addresses
• Numbers
• Lists
• Dictionaries
• Objects
• Results of calculations

Instead of repeatedly writing the same value, we can store it in a variable and use the variable whenever we need that value.

Simple Example

name = "Raj"
age = 25

print(name)
print(age)

Output:

Raj
25

Here:

name → variable
"Raj" → value

age → variable
25 → value

1. Creating a Variable

Python does not require a special keyword to create a variable.

Simply assign a value using =.

name = "John"
age = 20
city = "Kathmandu"

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

The = operator assigns a value to a variable.

2. Understanding Variable Assignment

Consider:

x = 10

This means:

Store the value 10 and associate it with the name x.

We can then use x in our program:

x = 10

print(x)
print(x + 5)
print(x * 2)

Output:

10
15
20

The variable can therefore be used wherever we need the stored value.

3. Variables Can Store Different Types of Values

Python variables can refer to different types of data.

name = "Raj"
age = 25
height = 5.8
is_student = True

print(name)
print(age)
print(height)
print(is_student)

Here:

name        → string
age         → integer
height      → float
is_student  → boolean

Python automatically determines the type based on the assigned value.

4. No Need to Declare the Variable Type

In some programming languages, you may need to specify the type of a variable.

For example, another language might require something like:

int age = 25

Python does not require this.

You simply write:

age = 25

Python understands that age currently refers to an integer.

This is one reason Python is called a dynamically typed language.

5. Checking the Type of a Variable

Use the type() function to find the type of the value.

name = "Raj"
age = 25
height = 5.8
active = True

print(type(name))
print(type(age))
print(type(height))
print(type(active))

Output:

<class 'str'>
<class 'int'>
<class 'float'>
<class 'bool'>

6. Variable Names

A variable name should clearly describe the value it represents.

Good examples:

student_name = "Raj"
student_age = 22
course_name = "Python"
course_duration = 8

Poor examples:

x = "Raj"
a = 22
z = "Python"

The second approach may work, but meaningful names make programs much easier to understand.

7. Python Variable Naming Rules

Python has rules that variable names must follow.

Rule 1: Start with a letter or underscore

Valid:

name = "Raj"
_age = 25

Invalid:

1name = "Raj"

A variable name cannot start with a number.

Rule 2: Numbers can appear after the first character

Valid:

student1 = "Raj"
student2 = "John"
course2026 = "Python"

Invalid:

1student = "Raj"

Rule 3: Spaces are not allowed

Invalid:

student name = "Raj"

Use an underscore instead:

student_name = "Raj"

Rule 4: Special characters are generally not allowed

Avoid:

student-name = "Raj"
student@name = "Raj"

Use:

student_name = "Raj"

Rule 5: Python is case-sensitive

These are different variables:

name = "Raj"
Name = "John"
NAME = "Sita"

print(name)
print(Name)
print(NAME)

Output:

Raj
John
Sita

Python treats uppercase and lowercase letters as different.

8. Naming Convention - snake_case

Python commonly uses snake_case for variable names.

Example:

first_name = "Raj"
last_name = "Kandel"
student_age = 25
course_name = "Python Bootcamp"
total_price = 5000

Words are separated using _.

Instead of:

studentname = "Raj"

prefer:

student_name = "Raj"

9. Avoid Python Keywords

Python has reserved keywords that already have special meanings.

For example:

if
else
for
while
class
def
return
import
True
False
None

You should not use these as variable names.

Invalid:

class = "Python"

Invalid:

if = 10

Python will produce a syntax error.

10. Checking Python Keywords

Python provides a module that can show Python's keywords.

import keyword

print(keyword.kwlist)

This displays the keywords supported by your installed Python version.

You can also check whether a word is a keyword:

import keyword

print(keyword.iskeyword("class"))
print(keyword.iskeyword("student"))

Output:

True
False

11. Assigning Multiple Variables

Python allows multiple variables to be assigned in one statement.

name, age, city = "Raj", 25, "Kathmandu"

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

Output:

Raj
25
Kathmandu

This is called multiple assignment.

12. Assigning the Same Value to Multiple Variables

You can assign one value to several variables.

x = y = z = 100

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

Output:

100
100
100

This can be useful when several variables need the same initial value.

13. Swapping Variables

Python makes it easy to swap two variables.

a = 10
b = 20

a, b = b, a

print(a)
print(b)

Output:

20
10

No temporary variable is required.

This is a useful Python feature.

14. Changing a Variable's Value

A variable can be assigned a new value.

age = 20

print(age)

age = 21

print(age)

Output:

20
21

The variable now refers to the new value.

15. A Variable Can Refer to Different Types

Python allows a variable to refer to different types of values at different times.

x = 100

print(x)
print(type(x))

x = "Hello"

print(x)
print(type(x))

Initially:

100
<class 'int'>

Later:

Hello
<class 'str'>

This behavior is related to Python's dynamic typing.

16. Variables and Objects

A useful way to understand Python variables is:

A variable is a name that refers to an object.

For example:

x = 100

Conceptually:

x ───────→ 100

If we do:

y = x

then:

x ───────→ 100
y ───────→ 100

Both names refer to the same integer object.

This concept becomes especially important when working with lists, dictionaries, and other mutable objects.

17. Assigning One Variable to Another

x = 100
y = x

print(x)
print(y)

Output:

100
100

Changing y later:

x = 100
y = x

y = 200

print(x)
print(y)

Output:

100
200

The assignment y = 200 makes y refer to another value.

18. Variables with Calculations

Variables can store calculation results.

price = 500
quantity = 3

total = price * quantity

print(total)

Output:

1500

This is one of the main reasons variables are useful in programming.

19. Updating a Variable

Suppose:

score = 50

We can update it:

score = score + 10

print(score)

Output:

60

Python also provides a shorter form:

score += 10

print(score)

20. Assignment Operators

Common assignment operators include:

Example:

x = 10

x += 5
print(x)

x *= 2
print(x)

x -= 5
print(x)

21. Variables with Strings

Variables are commonly used to store text.

first_name = "Raj"
last_name = "Kandel"

full_name = first_name + " " + last_name

print(full_name)

Output:

Raj Kandel

22. Variables with Lists

A variable can store a list.

fruits = ["apple", "mango", "banana"]

print(fruits)
print(fruits[0])

Output:

['apple', 'mango', 'banana']
apple

23. Variables with Dictionaries

A variable can also refer to a dictionary.

student = {
    "name": "Raj",
    "age": 25,
    "course": "Python"
}

print(student)
print(student["name"])

Output:

{'name': 'Raj', 'age': 25, 'course': 'Python'}
Raj

24. Variables with Tuples and Sets

Tuple:

coordinates = (27.7, 85.3)

print(coordinates)

Set:

skills = {"Python", "Django", "SQL"}

print(skills)

A variable can refer to almost any Python object.

25. Constants in Python

Python does not have a special constant keyword.

However, programmers commonly use UPPERCASE names to indicate that a value should be treated as a constant.

Example:

PI = 3.14159
MAX_LOGIN_ATTEMPTS = 5
COMPANY_NAME = "SkillMantra"

Python will not prevent you from changing these values:

PI = 4

But uppercase naming communicates:

This value is intended to remain unchanged.

26. Local and Global Variables - Introduction

Variable scope determines where a variable can be accessed.

For example:

name = "Raj"

def welcome():
    print(name)

welcome()

Here name is defined outside the function, so it is a global variable.

We will study local and global scope in detail when we learn functions.

27. Checking Whether a Variable Exists

Python does not provide a simple exists() function for variables.

However, you can inspect the current namespace using:

x = 10

print("x" in globals())

Output:

True

This is more advanced and is generally not needed in beginner programs.

28. Deleting a Variable

The del statement can remove a variable.

x = 100

print(x)

del x

After deleting it:

print(x)

will produce a NameError because the variable name no longer exists.

29. None as a Variable Value

Python provides a special value called None.

None represents the absence of a value.

Example:

result = None

print(result)
print(type(result))

Output:

None
<class 'NoneType'>

This is useful when a value is not available yet.

For example:

student_email = None

Later:

student_email = "student@example.com"

30. User Input Stored in Variables

Variables become especially useful when combined with input().

name = input("Enter your name: ")

print("Welcome", name)

If the user enters:

Raj

the output becomes:

Welcome Raj

The value entered by the user is stored in the name variable.

31. Important: input() Returns a String

Consider:

age = input("Enter your age: ")

print(type(age))

Even if the user enters:

25

the type is:

<class 'str'>

If we need an integer, convert it:

age = int(input("Enter your age: "))

print(type(age))

Now the type is:

<class 'int'>

We will cover type conversion in more detail in the next lessons.

32. Variable Naming Best Practices

Good variable names make code easier to understand.

Good

student_name = "Raj"
student_age = 25
course_price = 15000
total_students = 50

Avoid vague names

x = "Raj"
a = 25
p = 15000

Unless the variable has a very small and obvious scope, descriptive names are better.

33. Avoid Overly Long Names

A variable name should be descriptive but not unnecessarily complicated.

Instead of:

the_total_number_of_students_registered_for_python_bootcamp = 50

use:

python_students = 50

The goal is:

Clear enough to understand, short enough to use comfortably.

34. Avoid Naming Variables After Built-in Functions

Python already has built-in functions such as:

print
len
list
str
int
sum
max
min
input
type

Avoid:

list = [1, 2, 3]

because now list no longer refers to the built-in list() constructor in that scope.

Similarly, avoid:

str = "Hello"

Prefer:

message = "Hello"

35. Practical Example : Student Information

student_name = "Raj"
student_age = 25
course_name = "Python Bootcamp"
course_duration = 8
course_price = 15000

print("Student:", student_name)
print("Age:", student_age)
print("Course:", course_name)
print("Duration:", course_duration, "weeks")
print("Price:", course_price)

This demonstrates how variables can represent real-world information.

36. Practical Example : Shopping Calculation

item_name = "Keyboard"
price = 1500
quantity = 2

total = price * quantity

print("Item:", item_name)
print("Price:", price)
print("Quantity:", quantity)
print("Total:", total)

Output:

Item: Keyboard
Price: 1500
Quantity: 2
Total: 3000

37. Practical Example : Student Result

student_name = "Raj"

python_marks = 85
database_marks = 78
web_marks = 90

total = python_marks + database_marks + web_marks
average = total / 3

print("Student:", student_name)
print("Total:", total)
print("Average:", average)

Here, variables make the calculation much easier to understand.

38. Copy Code

# ==========================================
# Python Variables
# ==========================================

# Creating variables

name = "Raj"
age = 25
height = 5.8
is_student = True

print(name)
print(age)
print(height)
print(is_student)


# Checking data types

print(type(name))
print(type(age))
print(type(height))
print(type(is_student))


# Changing a variable

age = 26

print("Updated age:", age)


# Multiple assignment

first_name, last_name, city = "Raj", "Kandel", "Kathmandu"

print(first_name)
print(last_name)
print(city)


# Same value assigned to multiple variables

x = y = z = 100

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


# Swapping variables

a = 10
b = 20

a, b = b, a

print("a:", a)
print("b:", b)


# Calculation using variables

price = 500
quantity = 3

total = price * quantity

print("Total:", total)


# Updating a variable

score = 50

score += 10

print("Updated score:", score)


# String variables

first_name = "Raj"
last_name = "Kandel"

full_name = first_name + " " + last_name

print(full_name)


# List variable

fruits = ["apple", "mango", "banana"]

print(fruits)


# Dictionary variable

student = {
    "name": "Raj",
    "age": 25,
    "course": "Python"
}

print(student)


# None

result = None

print(result)
print(type(result))

39. Practice Exercise 1 : Personal Information

Create variables for:

Name
Age
City
Country
Profession

Print all the information.

40. Practice Exercise 2 : Shopping Cart

Create:

product_name
price
quantity

Calculate:

total = price × quantity

Print the result.

41. Practice Exercise 3 : Student Marks

Create variables for three subjects:

python_marks
database_marks
web_marks

Calculate:

• Total
• Average

Print all results.

42. Practice Exercise 4 : Variable Swapping

Create:

a = 100
b = 200

Swap the values so that:

a = 200
b = 100

Do it without creating a third variable.

43. Practice Exercise 5 : User Input

Create a program that asks the user for:

• Name
• Age
• Course

Then print:

Welcome Raj!
You are 25 years old.
You are learning Python.

Hint:

name = input("Enter your name: ")

44. Challenge : Simple Invoice

Create a small invoice program.

Store:

customer_name
product_name
price
quantity
discount

Calculate:

subtotal = price × quantity
discount_amount = subtotal × discount / 100
final_amount = subtotal - discount_amount

Then display:

Customer: Raj
Product: Laptop
Price: 80000
Quantity: 1
Subtotal: 80000
Discount: 10%
Final Amount: 72000

Try solving it without copying the solution.