Variables and Data Types

Introduction

A variable is a named reference to a value stored in memory. Python variables are labels/pointers attached to objects, not containers.

Variables

# Simple assignment
x = 10
name = "Alice"
pi = 3.14159

# Multiple assignment
a = b = c = 0
x, y = 10, 20

# Swap variables (Pythonic!)
x, y = y, x

# Extended unpacking
first, *rest = [1, 2, 3, 4, 5]
print(first)   # 1
print(rest)    # [2, 3, 4, 5]

Naming Conventions (PEP 8)

Type	Convention	Example
Variables	`snake_case`	`user_name`
Constants	`UPPER_SNAKE_CASE`	`MAX_SIZE`
Classes	`PascalCase`	`UserAccount`
Functions	`snake_case`	`calculate_tax()`

Data Types Overview

x = 42           # int
y = 3.14         # float
s = "hello"      # str
lst = [1, 2, 3]  # list
t = (1, 2, 3)    # tuple
d = {"key": "value"}  # dict
st = {1, 2, 3}    # set
flag = True      # bool
nothing = None   # NoneType

flowchart TB
    subgraph Mutable["Mutable Objects (can change in place)"]
        direction TB
        L["list"]
        D["dict"]
        S["set"]
        BA["bytearray"]
    end

    subgraph Immutable["Immutable Objects (cannot change in place)"]
        direction TB
        I["int"]
        F["float"]
        ST["str"]
        T["tuple"]
        BO["bool"]
        FR["frozenset"]
    end

    style Mutable fill:#1e40af,color:#fff
    style Immutable fill:#7c3aed,color:#fff
    style L fill:#2563eb,color:#fff
    style D fill:#2563eb,color:#fff
    style S fill:#2563eb,color:#fff
    style BA fill:#2563eb,color:#fff
    style I fill:#8b5cf6,color:#fff
    style F fill:#8b5cf6,color:#fff
    style ST fill:#8b5cf6,color:#fff
    style T fill:#8b5cf6,color:#fff
    style BO fill:#8b5cf6,color:#fff
    style FR fill:#8b5cf6,color:#fff

Dynamic Typing

x = 5            # int
x = "hello"      # now str
x = [1, 2, 3]   # now list

Type Checking

# Preferred: isinstance()
print(isinstance(x, int))          # True
print(isinstance(x, (int, float))) # True — multiple types

Why It Matters

Understanding Python’s reference model and dynamic typing is fundamental to writing correct code and avoiding bugs.

Common Mistakes

Confusing = (assignment) with == (comparison)
Not understanding that variables reference objects
Thinking variables are boxes (they’re labels!)

Interview Questions

Q1: What is dynamic typing in Python?

A: Variable types are determined at runtime. A variable can reference any type of object, and you can change it at any time.

Q2: Why does a = [1,2,3]; b = a; b.append(4) modify a?

A: Because b and a reference the same list object. Variables are labels, not containers.

Practice Exercises

Swap two variables without using a third variable.
Check the type of different values using type() and isinstance().
Demonstrate Python’s reference model with lists.