Variables and Data Types

Creating variables, Python's core data types, and type conversion.

Overview

A variable in Python is a name that refers to a value stored in memory. Unlike many languages, you do not declare a type for a variable Python infers it from the value you assign. You can even reassign the same variable to a completely different type later (though doing so without good reason is generally considered bad practice). Python has four fundamental built-in data types you will use in almost every program: - **int** whole numbers: 0, 42, -7, 1_000_000 (underscores for readability) - **float** decimal numbers: 3.14, -0.5, 1.0e10 (scientific notation) - **str** text: "hello", 'world', """multi-line""" - **bool** True or False (capital T and F) Beyond these primitives, Python also has complex collection types (list, tuple, dict, set) which you will learn in later topics. The built-in functions type() and isinstance() let you inspect what type a value has at runtime.

Key Points

•Assignment uses = : `x = 10` stores 10 and binds it to the name x
•Python is dynamically typed: the type is attached to the value, not the variable name
•int can be arbitrarily large in Python no overflow as in many other languages
•float uses IEEE 754 double precision; be aware of rounding: 0.1 + 0.2 != 0.3 exactly
•Strings can use single quotes, double quotes, or triple quotes (for multi-line text)
•type() returns the type of a value; isinstance(x, int) checks if x is an int
•int(), float(), str(), bool() are type-conversion functions

Examples

Creating variables of different types

age = 25               # int
price = 9.99           # float
name = "Alice"         # str
is_active = True       # bool

print(type(age))       # <class 'int'>
print(type(price))     # <class 'float'>
print(type(name))      # <class 'str'>
print(type(is_active)) # <class 'bool'>

Multiple assignment and swapping

# Assign multiple variables on one line
x, y, z = 1, 2, 3
print(x, y, z)    # 1 2 3

# Swap without a temp variable
a, b = 10, 20
a, b = b, a
print(a, b)       # 20 10

This tuple unpacking syntax is very Pythonic and widely used.

Type conversion (casting)

n = int("42")        # str → int
f = float("3.14")    # str → float
s = str(100)         # int → str
b = bool(0)          # 0 → False, any non-zero → True

print(n, type(n))    # 42 <class 'int'>
print(b)             # False
print(bool(""))      # False (empty string is falsy)
print(bool("hi"))    # True

Common Mistakes

Confusing = (assignment) with == (equality check): `if x = 5` is a SyntaxError in Python
Integer division surprise: `7 / 2` gives 3.5 in Python 3 (true division); use `7 // 2` for 3
Forgetting that bool is a subclass of int: True == 1 and False == 0 in Python

Note

Use descriptive variable names in snake_case (my_variable, total_price). Python style guide PEP 8 recommends this, and it makes your code far more readable than single-letter names.

Variables and Data Types

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What is the result of `type(True)` in Python?