Variables and Data Types

Variables and Data Types in Python

In Python, variables are used to store data values. Python is dynamically typed, meaning you don't need to declare the type of a variable explicitly. Variables can hold different types of data, such as numbers, strings, booleans, lists, tuples, and dictionaries.

Declaring Variables

Variables in Python are created when you assign a value to them. You can reassign variables to different types of data.


x = 10  # Integer
name = "Alice"  # String
is_student = True  # Boolean

Common Data Types

int: Whole numbers (e.g., 10, -5).
float: Decimal numbers (e.g., 3.14, -0.001).
str: Text data (e.g., "Hello", 'Python').
bool: True or False values.
list: Ordered collection of elements (e.g., [1, 2, 3]).
tuple: Immutable ordered collection (e.g., (1, 2, 3)).
dict: Key-value pairs (e.g., {"name": "Alice", "age": 25}).
set: Unordered collection of unique elements (e.g., {1, 2, 3}).

Example:


age = 25
name = "Alice"
is_student = True
scores = [90, 85, 95]
person = {"name": "Alice", "age": 25}

print("Name:", name)
print("Age:", age)
print("Is Student:", is_student)

Type Conversion

Python allows you to convert one data type to another using built-in functions like int(), float(), str(), and bool().


# Converting between types
num_str = "10"
num_int = int(num_str)  # Convert string to integer
num_float = float(num_int)  # Convert integer to float
bool_val = bool(num_float)  # Convert float to boolean

print(num_int, num_float, bool_val)

Dynamic Typing

Python is dynamically typed, meaning the type of a variable is determined at runtime and can change.


x = 10  # x is an integer
x = "Hello"  # x is now a string
x = [1, 2, 3]  # x is now a list

Variable Naming Rules

Variable names in Python must follow these rules:

Must start with a letter or underscore (_).
Can only contain letters, numbers, and underscores.
Cannot be a reserved keyword (e.g., if, else, for).
Are case-sensitive (e.g., name and Name are different).


# Valid variable names
my_var = 10
_var2 = "Hello"
myVar3 = 3.14

# Invalid variable names
# 2var = 10  # Error: Cannot start with a number
# my-var = 20  # Error: Cannot contain hyphens

Constants

Although Python doesn't have built-in support for constants, it's a convention to use uppercase names for variables that should not be changed.


PI = 3.14159
GRAVITY = 9.81

# Attempting to change a constant (not enforced)
PI = 3.14  # This is allowed but not recommended

Multiple Assignment

Python allows you to assign multiple variables in a single line.


x, y, z = 10, 20, 30
print(x, y, z)  # Output: 10 20 30

# Swapping variables
x, y = y, x
print(x, y)  # Output: 20 10

Next: Control Structures