Python Variables

Variables in Python are used to store data values. Unlike statically typed languages, Python is dynamically typed, which means you do not need to declare a variable type explicitly — Python determines it at runtime.

1. Creating Variables

Assign a value using the = operator:

x = 10        # Integer
        name = "Talha"   # String
        pi = 3.14        # Float
        

2. Rules for Variable Names

• Must start with a letter or underscore (_)
• Can only contain letters, numbers, and underscores
• Case-sensitive (myVar and myvar are different)
• Cannot use Python keywords (like if, for, class)

3. Multiple Assignments

Python allows assigning multiple variables in one line:

a, b, c = 10, 20, 30
        x = y = z = 5
        

4. Variable Types

Python automatically assigns types based on the value:

• int → whole numbers
• float → decimal numbers
• str → strings
• bool → True / False
• list, tuple, set, dict → collections

5. Changing Variable Values

In Python, variables act as labels pointing to objects in memory. You can always reassign a variable to a new value. This is why we say Python variables are mutable by default.

However, the object itself may be mutable or immutable:

• Mutable objects – their content can be changed without creating a new object (e.g., list, dict, set).
• Immutable objects – their content cannot be changed. To “change” them, a new object is created (e.g., int, float, str, tuple).

Examples:

# Changing an integer (immutable object)
        x = 10
        x = x + 5   # x now points to a new integer object 15

        # Changing a string (immutable object)
        name = "Talha"
        name = name + " Abbas"  # Creates a new string object "Talha Abbas"

        # Changing a list (mutable object)
        nums = [1, 2, 3]
        nums[0] = 10   # Modifies the list in-place: nums = [10, 2, 3]
        nums.append(4) # Adds 4 to the same list object: nums = [10, 2, 3, 4]

        # Reassigning a list variable
        nums = [100, 200]  # nums now points to a new list object
        

Key Points:

• Variable names can always be reassigned to point to new objects.
• Immutable objects cannot be changed in-place; any “change” creates a new object.
• Mutable objects can be updated directly without creating a new object.

6. Dynamic Typing

Python determines the variable type at runtime and can change it:

var = 10       # int
        var = "hello"   # now str
        var = 3.14      # now float
        

7. Type Conversion

You can explicitly convert variables from one type to another:

x = int(3.14)      # 3
        y = float(10)         # 10.0
        z = str(100)          # "100"
        

8. Constants

Python doesn’t have true constants, but by convention, uppercase names are used to indicate a value that should not change:

PI = 3.14159
        MAX_USERS = 100
        

9. Variable Scope

Scope defines where a variable can be accessed:

9.1 Local Variables

Declared inside a function and accessible only within it:

def my_func():
            x = 10
            print(x)   # 10
        

9.2 Global Variables

Declared outside functions and accessible anywhere:

x = 100

        def my_func():
            print(x)  # 100
        

9.3 Global Keyword

Used to modify a global variable inside a function:

x = 5

        def update():
            global x
            x = 10

        update()
        print(x)  # 10
        

10. Best Practices

• Use meaningful variable names
• Follow snake_case naming convention
• Avoid using single-character names except in loops or small functions
• Use constants for fixed values (uppercase)

11. Interview Questions

Q1. Can Python variables change type?

Yes, Python is dynamically typed.

Q2. What is the difference between local and global variables?

Local variables are defined in a function and accessible only there. Global variables are accessible anywhere.

Q3. How do you create a constant in Python?

Python has no true constants. By convention, uppercase variable names indicate constants.

Q4. Can you assign multiple variables in one line?

Yes, using comma separation:

a, b, c = 1, 2, 3