Variables

What is a Variable?

A variable is a name that refers to a value stored in memory. In Python, you don’t need to declare the type of a variable explicitly—Python determines it automatically.

1. Declaring a Variable

x = 10        # Integer
y = 3.14      # Float
name = "Alice"  # String
is_valid = True  # Boolean

Here,

• x stores an integer (int).
• y stores a floating-point number (float).
• name stores a string (str).
• is_valid stores a boolean (bool).

2. Variable Naming Rules

When naming variables, follow these rules: ✅ Allowed:

• Must start with a letter (a-z, A-Z) or an underscore _
• Can contain letters, digits (0-9), and underscores
• Case-sensitive (name and Name are different)

❌ Not Allowed:

• Cannot start with a digit (1name ❌)
• Cannot contain special characters (name@ ❌)
• Cannot use reserved keywords like class, def, if, etc.

✔ Valid Examples:

my_var = 10
_name = "Python"
user_age = 25

❌ Invalid Examples:

1name = "Alice"  # ❌ Starts with a number
user@name = "Bob"  # ❌ Contains @
class = "Math"  # ❌ Uses a reserved keyword

3. Dynamic Typing

Python allows reassignment of variables with different types.

x = 5       # Integer
x = "Hello" # Now x is a string
print(x)    # Output: Hello

4. Assigning Multiple Variables

You can assign multiple variables in one line.

a, b, c = 1, 2, 3
print(a, b, c)  # Output: 1 2 3

Or assign the same value to multiple variables.

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

5. Type Checking and Type Conversion

To check a variable’s type, use type().

x = 5
print(type(x))  # Output: <class 'int'>

You can also convert data types:

x = "100"
y = int(x)  # Converts string to integer
print(y, type(y))  # Output: 100 <class 'int'>

6. Global and Local Variables

• Local variables are declared inside a function and used only there.
• Global variables are declared outside functions and can be accessed anywhere.

global_var = "I am global"

def my_function():
    local_var = "I am local"
    print(local_var)  # Accessible inside function

my_function()
print(global_var)  # Accessible anywhere

To modify a global variable inside a function, use the global keyword:

x = 10

def update():
    global x
    x = 20

update()
print(x)  # Output: 20

7. Constants in Python

Python does not have built-in constants, but by convention, constants are written in uppercase.

PI = 3.14159
GRAVITY = 9.8

Even though Python allows changing them, it’s a best practice not to modify constants.

8. Deleting Variables

You can delete a variable using del.

x = 100
del x
print(x)  # ❌ Error: x is not defined