Python by Dr.

Sadaf Zeeshan

Lecture 3

Common Concepts in Python: Variables in Python

A variable stores a value that can change during program execution.

Example: Assigning and Using Variables

name = "Alice" # String (text)

age = 25 # Integer (whole number)
height = 5.6 # Float (decimal number)
is_student = True # Boolean (True/False)

print("Name:", name)
print("Age:", age)
print("Height:", height, "feet")
print("Student:", is_student)

Common Variable Types in Python

Type Example Description

String (str) "Hello" Text data

Integer (int) 42 Whole numbers

Float (float) 3.14 Decimal numbers

Boolean (bool) True / False Logical values

List (list) [1, 2, 3] Collection of items

Tuple (tuple) (4, 5, 6) Immutable (unchangeable) collection

Dictionary (dict) {"name": "Alice", "age": 25} Key-value pairs

 Python by Dr. Sadaf Zeeshan

Types of Variables in Python (with Examples)

A variable is used to store data in Python. Different types of variables hold different types of
data. Below are the most common types:

1. String (str)

A string is a sequence of characters, enclosed in quotes (" " or ' ').

Example:

name = "Alice"
greeting = 'Hello, how are you?'
print(name) # Output: Alice
print(greeting) # Output: Hello, how are you?

 Strings can contain letters, numbers, and special characters.

 We can perform operations like concatenation (+) and repetition (*).

full_name = name + " Johnson" # Concatenation

print(full_name) # Output: Alice Johnson

repeat_text = "Python " * 3

print(repeat_text) # Output: Python Python Python

2. Integer (int)

An integer is a whole number, positive or negative, without decimals.

Example:

age = 25
temperature = -10
count = 1000
 Python by Dr. Sadaf Zeeshan

print(age, temperature, count) # Output: 25 -10 1000

 Integers are used for counting and indexing.

 We can perform arithmetic operations like addition, subtraction, multiplication, etc.

x = 10
y=3
print(x + y) # Output: 13
print(x * y) # Output: 30
print(x // y) # Output: 3 (integer division)
print(x % y) # Output: 1 (remainder)

3. Float (float)

A float is a number with a decimal point.

Example:

height = 5.8
price = 99.99
pi = 3.14159
print(height, price, pi) # Output: 5.8 99.99 3.14159

 Used for measurements and precise calculations.

 Supports arithmetic operations similar to integers.

a = 10.5
b = 2.3
print(a + b) # Output: 12.8
print(a * b) # Output: 24.15

4. Boolean (bool)

A boolean holds True or False values.

 Python by Dr. Sadaf Zeeshan

Example:

is_raining = True
is_summer = False
print(is_raining) # Output: True
print(is_summer) # Output: False

 Used in conditions and decision-making.

x = 10
y = 20
print(x > y) # Output: False
print(x < y) # Output: True
print(x == y) # Output: False

5. List (list)

A list is an ordered, changeable collection of items. It can contain different data types.

Example:

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

numbers = [1, 2, 3, 4, 5]
mixed = [10, "hello", 3.5, True]

print(fruits) # Output: ['apple', 'banana', 'cherry']

print(mixed) # Output: [10, 'hello', 3.5, True]

 Lists are mutable (changeable).

 Supports indexing and slicing.

print(fruits[0]) # Output: apple

fruits.append("orange") # Adds an item
print(fruits) # Output: ['apple', 'banana', 'cherry', 'orange']
 Python by Dr. Sadaf Zeeshan

List Methods

Method Description Example

append(x) Adds item x to end lst.append(4)

insert(i, x) Inserts x at index i lst.insert(2, "hello")

remove(x) Removes first occurrence of x lst.remove(3)

pop(i) Removes item at index i (default last) lst.pop(2)

clear() Removes all elements lst.clear()

sort() Sorts list in ascending order lst.sort()

reverse() Reverses the list order lst.reverse()

count(x) Counts occurrences of x lst.count(5)

index(x) Returns index of x lst.index(7)

6. Tuple (tuple)

A tuple is like a list but immutable (cannot be changed after creation). Tuples are faster and
use less memory than lists.

Example:

coordinates = (10, 20)

colors = ("red", "green", "blue")

print(coordinates) # Output: (10, 20)

print(colors[1]) # Output: green

 Tuples use () instead of [].

 Faster and safer than lists.
 Python by Dr. Sadaf Zeeshan

# Attempting to change a tuple will cause an error

# colors[0] = "yellow" # TypeError: 'tuple' object does not support item assignment

List vs. Tuple – Key Differences

Feature List (list) Tuple (tuple)

Syntax [item1, item2] (item1, item2)

Mutable? Yes (modifiable) No (fixed)

Ordered? Yes Yes

Performance Slower Faster

Duplicates Allowed? Yes Yes

Memory Usage Higher (due to flexibility) Lower (due to immutability)

Best Used For Collections that change frequently Fixed data sets

Real-World Example

Storing Student Records

student1 = ("Alice", 20, "A") # Tuple (fixed)

students_list = [("Alice", 20, "A"), ("Bob", 21, "B")] # List of tuples

 Tuple → Used for a single student (data doesn’t change).

 List → Used for multiple students (data can be updated).
 Python by Dr. Sadaf Zeeshan

7. Dictionary (dict)

A dictionary stores data in key-value pairs.

A dictionary in Python is a data structure that stores information in key-value pairs. Unlike
lists and tuples, where data is stored in an ordered sequence, dictionaries allow quick access
to values based on unique keys.

A dictionary is created using curly braces {} or the dict() function.

Dictionaries are one of the most powerful and flexible data structures in Python. They allow
fast lookups, efficient storage, and easy data organization, making them essential for real-
world applications like:

 Storing user profiles

 Managing databases
 Configuring settings in applications
 Creating machine learning models with structured data

Example:

student = {"name": "Alice", "age": 20, "grade": "A"}

car = {"brand": "Toyota", "year": 2022, "color": "red"}

print(student["name"]) # Output: Alice

print(car["brand"]) # Output: Toyota

 Uses {key: value} format.

 Keys must be unique and immutable (strings, numbers, or tuples).

student["age"] = 21 # Updating a value

print(student) # Output: {'name': 'Alice', 'age': 21, 'grade': 'A'}

student["city"] = "New York" # Adding a new key-value pair

print(student) # Output: {'name': 'Alice', 'age': 21, 'grade': 'A', 'city': 'New York'}
 Python by Dr. Sadaf Zeeshan

Summary Table

Variable Type Description Example

String (str) Text data "Hello", 'Python'

Integer (int) Whole numbers 10, -5, 1000

Float (float) Decimal numbers 3.14, 99.99, -0.5

Boolean (bool) True/False values True, False

List (list) Ordered, changeable collection ["apple", "banana"], [1, 2, 3]

Tuple (tuple) Ordered, unchangeable collection ("red", "blue"), (10, 20)

Dictionary (dict) Key-value pairs {"name": "Alice", "age": 20}

By understanding these variables, you can store and manipulate different types of data in
Python effectively! 🚀