Python Variables

What are Variables?

A variable is a container for storing data values. In Python:
✔ Variables do not need explicit declaration.
✔ The type of the variable is automatically determined.
✔ Variable names are case-sensitive.

Example:

name = "Alice"  # String
age = 25        # Integer
height = 5.6    # Float
is_student = True  # Boolean

print(name, age, height, is_student)

1. Variable Naming Rules

✅ Valid variable names:
✔ Must start with a letter (a-z, A-Z) or an underscore (_).
✔ Can contain letters, numbers, and underscores.
✔ Cannot use Python keywords (like if, while, class).

❌ Invalid variable names:

2name = "John"   # ❌ Cannot start with a number
my-name = "Doe"  # ❌ Hyphens are not allowed
class = "Python" # ❌ "class" is a reserved keyword

✅ Correct Examples:

_name = "Alice"
myVar = 10
age_25 = 25

2. Assigning Values to Variables

Python allows dynamic typing, meaning you don’t need to declare the type explicitly.

x = 10      # Integer
y = "Hello" # String
z = 3.14    # Float

print(x, y, z)

3. Multiple Variable Assignments

✔ Assigning the same value to multiple variables:

a = b = c = 100
print(a, b, c)  # Output: 100 100 100

✔ Assigning different values in one line:

x, y, z = 5, "Python", 3.14
print(x, y, z)  # Output: 5 Python 3.14

4. Type Checking & Conversion

✔ Check Data Type:

x = 10
print(type(x))  # Output: <class 'int'>

✔ Convert Data Types:

x = str(10)   # Convert integer to string
y = int("20") # Convert string to integer
z = float(5)  # Convert integer to float

print(x, y, z)

5. Variable Scope

✔ Local Variable (Defined inside a function):

def my_function():
    local_var = "I am local"
    print(local_var)

my_function()
# print(local_var)  # ❌ This will cause an error (not accessible outside the function)

✔ Global Variable (Accessible throughout the program):

global_var = "I am global"

def my_function():
    print(global_var)  # Accessible inside function

my_function()
print(global_var)  # Accessible outside function too

✔ Modifying a Global Variable Inside a Function:

count = 0

def update_count():
    global count  # Use global keyword
    count += 1

update_count()
print(count)  # Output: 1

6. Deleting a Variable

✔ Use del keyword to remove a variable:

x = 100
del x  # Deletes x
# print(x)  # ❌ This will cause an error (x is deleted)

7. Constants in Python

Python does not have built-in constants, but you can use uppercase variable names to indicate they should not be changed.

PI = 3.14159  # Constant (By convention)
GRAVITY = 9.8

8. Best Practices for Variables

✅ Use descriptive variable names (avoid x, y, z).
✅ Follow snake_case for variable names (user_name, total_price).
✅ Avoid using reserved keywords.
✅ Use global variables sparingly.