Variables & Data Types
Beginner⏱55 min📖4 min readLesson 3 of 10

Variables & Data Types

Master Python variables, primitive data types (int, float, str, bool), type conversion, and naming conventions.

Variables are the foundation of every program. They store data that your program manipulates. In Python, working with variables is intuitive and flexible.

What is a Variable?

A variable is a named location in memory that stores a value. Think of it as a labeled box where you can put different items.

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Creating Variables in Python

python
# Variable assignment age = 25 name = "Alice" height = 1.75 is_student = True # Multiple assignment x, y, z = 10, 20, 30 a = b = c = 0 # Displaying variables print(f"Name: {name}") print(f"Age: {age}") print(f"Height: {height}m") print(f"Student: {is_student}")

Output:

Name: Alice Age: 25 Height: 1.75m Student: True
ℹ️Note

Unlike languages like C or Java, Python doesn't require you to declare the type of a variable. Python infers the type from the value you assign. This is called "dynamic typing."

Python's Primitive Data Types

Python has several built-in data types. Let's explore the four most fundamental ones.

Overview of Data Types

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Type
Python Name
Example
Description
Integer
int
42, -7, 0
Whole numbers (positive, negative, zero)
Float
float
3.14, -0.5, 2.0
Decimal numbers
String
str
"hello", 'world'
Text/characters
Boolean
bool
True, False
Truth values

Integers (int)

Integers represent whole numbers without a decimal point.

Integer Operations

python
# Creating integers positive = 100 negative = -50 zero = 0 large_number = 1_000_000 # Underscores for readability # Arithmetic with integers a = 15 b = 4 print(f"Addition: {a} + {b} = {a + b}") # 19 print(f"Subtraction: {a} - {b} = {a - b}") # 11 print(f"Multiplication: {a} * {b} = {a * b}") # 60 print(f"Division: {a} / {b} = {a / b}") # 3.75 (returns float!) print(f"Floor Division: {a} // {b} = {a // b}") # 3 print(f"Modulus: {a} % {b} = {a % b}") # 3 print(f"Exponent: {a} ** {b} = {a ** b}") # 50625

Output:

Addition: 15 + 4 = 19 Subtraction: 15 - 4 = 11 Multiplication: 15 * 4 = 60 Division: 15 / 4 = 3.75 Floor Division: 15 // 4 = 3 Modulus: 15 % 4 = 3 Exponent: 15 ** 4 = 50625
💡Tip

Python integers have unlimited precision! You can work with numbers as large as your memory allows:

python
>>> 2 ** 1000 107150860718626732094842504906000181056140481170553360744375038837...

Floats (float)

Floats represent numbers with decimal points.

Float Operations and Precision

python
# Creating floats pi = 3.14159 temperature = -5.5 scientific = 6.022e23 # Scientific notation: 6.022 × 10²³ # Float operations x = 10.5 y = 3.2 print(f"Addition: {x + y}") # 13.7 print(f"Multiplication: {x * y}") # 33.6 # Float precision caveat print(f"\n0.1 + 0.2 = {0.1 + 0.2}") # 0.30000000000000004 (!) print(f"0.1 + 0.2 == 0.3: {0.1 + 0.2 == 0.3}") # False!

Output:

Addition: 13.7 Multiplication: 33.6 0.1 + 0.2 = 0.30000000000000004 0.1 + 0.2 == 0.3: False
⚠️Warning

Floating-point arithmetic can have precision issues due to how computers represent decimals in binary. For financial calculations, use the decimal module instead of floats.

python
# Using decimal for precise calculations from decimal import Decimal a = Decimal('0.1') b = Decimal('0.2') print(f"Decimal: {a + b}") # 0.3 (exact!)

Strings (str)

Strings represent text and are enclosed in quotes.

Creating and Using Strings

python
# Different ways to create strings single_quotes = 'Hello' double_quotes = "World" triple_quotes = """This is a multi-line string""" # String operations first_name = "Alice" last_name = "Smith" # Concatenation full_name = first_name + " " + last_name print(f"Full name: {full_name}") # Repetition separator = "-" * 30 print(separator) # String length message = "Python Programming" print(f"Length: {len(message)}") # 18 # Accessing characters (indexing) word = "Python" print(f"First character: {word[0]}") # P print(f"Last character: {word[-1]}") # n print(f"Third character: {word[2]}") # t

Output:

Full name: Alice Smith ------------------------------ Length: 18 First character: P Last character: n Third character: t

String Methods

python
text = " Hello, World! " print(f"Original: '{text}'") print(f"Upper: '{text.upper()}'") print(f"Lower: '{text.lower()}'") print(f"Title: '{text.title()}'") print(f"Strip: '{text.strip()}'") print(f"Replace: '{text.replace('World', 'Python')}'") print(f"Split: {text.strip().split(',')}")

Output:

Original: ' Hello, World! ' Upper: ' HELLO, WORLD! ' Lower: ' hello, world! ' Title: ' Hello, World! ' Strip: 'Hello, World!' Replace: ' Hello, Python! ' Split: ['Hello', 'World!']

F-Strings (Formatted String Literals)

python
name = "Alice" age = 30 score = 95.678 # Basic f-string print(f"My name is {name} and I am {age} years old.") # Expressions in f-strings print(f"Next year I will be {age + 1}.") # Formatting numbers print(f"Score: {score:.2f}") # 2 decimal places: 95.68 print(f"Score as percent: {score:.1%}") # Not applicable here, but useful print(f"Right aligned: {name:>10}") # Right aligned in 10 chars print(f"Center aligned: {name:^10}") # Center aligned

Output:

My name is Alice and I am 30 years old. Next year I will be 31. Score: 95.68 Right aligned: Alice Center aligned: Alice

Booleans (bool)

Booleans represent truth values: True or False.

Boolean Basics

python
# Boolean values is_active = True is_admin = False print(f"Active: {is_active}") # True print(f"Admin: {is_admin}") # False print(f"Type: {type(is_active)}") # <class 'bool'> # Boolean from comparisons x = 10 y = 20 print(f"\n{x} > {y}: {x > y}") # False print(f"{x} < {y}: {x < y}") # True print(f"{x} == {y}: {x == y}") # False print(f"{x} != {y}: {x != y}") # True print(f"{x} >= 10: {x >= 10}") # True

Truthiness in Python

python
# Python evaluates many values as True or False in boolean context print(f"bool(1): {bool(1)}") # True print(f"bool(0): {bool(0)}") # False print(f"bool('hello'): {bool('hello')}") # True print(f"bool(''): {bool('')}") # False print(f"bool([]): {bool([])}") # False (empty list) print(f"bool([1, 2]): {bool([1, 2])}") # True (non-empty list) print(f"bool(None): {bool(None)}") # False
ℹ️Note

In Python, the following are considered "falsy": False, None, 0, 0.0, "", [], {}, set(). Everything else is "truthy".

Type Conversion (Casting)

You often need to convert between types. Python provides built-in functions for this.

Converting Between Types

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python
# String to int age_str = "25" age_int = int(age_str) print(f"int('25') = {age_int}") # 25 print(f"Type: {type(age_int)}") # <class 'int'> # String to float price_str = "19.99" price_float = float(price_str) print(f"float('19.99') = {price_float}") # 19.99 # Int/float to string number = 42 number_str = str(number) print(f"str(42) = '{number_str}'") # '42' # Float to int (truncates, doesn't round!) pi = 3.14159 pi_int = int(pi) print(f"int(3.14159) = {pi_int}") # 3 # Boolean to int/float print(f"int(True) = {int(True)}") # 1 print(f"int(False) = {int(False)}") # 0 print(f"float(True) = {float(True)}") # 1.0

Practical Example: Input Conversion

python
# calculator_input.py # Getting numeric input from user print("=== Rectangle Area Calculator ===") # input() always returns a string, so we must convert width_str = input("Enter width: ") height_str = input("Enter height: ") # Convert strings to floats width = float(width_str) height = float(height_str) # Calculate area area = width * height print(f"\nRectangle Dimensions:") print(f" Width: {width}") print(f" Height: {height}") print(f" Area: {area}")

Sample output:

=== Rectangle Area Calculator === Enter width: 5.5 Enter height: 3.2 Rectangle Dimensions: Width: 5.5 Height: 3.2 Area: 17.6

Variable Naming Conventions

Python has specific rules and conventions for naming variables.

Rules (Must Follow)

Rule
Valid
Invalid
Start with letter or underscore
name, _age
1name, 2nd
Only letters, numbers, underscores
user_name, var2
user-name, var@2
Case-sensitive
age, Age, AGE are different
-
Not a reserved keyword
-
if, for, class, def

Conventions (Should Follow)

python
# Snake case for variables (recommended) user_name = "Alice" total_amount = 100.50 is_active = True # Constants (all uppercase) PI = 3.14159 MAX_RETRIES = 3 API_KEY = "secret123" # Private variables (leading underscore - convention only) _internal_value = 42 _temp_data = [] # Bad names - avoid these! x = 10 # Too vague temp123 = "hi" # Not descriptive myVariableName = "Alice" # camelCase not standard in Python
💡Tip

Choose descriptive names! student_age is much better than sa or x. Your future self (and other developers) will thank you.

Checking Types with type() and isinstance()

python
# Using type() value1 = 42 value2 = 3.14 value3 = "hello" value4 = True print(f"type(42) = {type(value1)}") # <class 'int'> print(f"type(3.14) = {type(value2)}") # <class 'float'> print(f"type('hello') = {type(value3)}") # <class 'str'> print(f"type(True) = {type(value4)}") # <class 'bool'> # Using isinstance() for type checking print(f"\nisinstance(42, int): {isinstance(42, int)}") # True print(f"isinstance(3.14, int): {isinstance(3.14, int)}") # False print(f"isinstance(3.14, float): {isinstance(3.14, float)}") # True print(f"isinstance(True, int): {isinstance(True, int)}") # True (bool is subclass of int!)

Real-World Example: Student Record System

python
# student_record.py # A simple student record using different data types # Student information student_id = 1001 # int name = "Maria Santos" # str age = 22 # int gpa = 3.85 # float is_enrolled = True # bool courses = ["Math", "Physics", "CS"] # list # Display student record print("=" * 40) print(" STUDENT RECORD") print("=" * 40) print(f"Student ID: {student_id}") print(f"Name: {name}") print(f"Age: {age}") print(f"GPA: {gpa:.2f}") print(f"Enrolled: {'Yes' if is_enrolled else 'No'}") print(f"Courses: {', '.join(courses)}") print("=" * 40) # Type summary print("\nData Types Used:") print(f" student_id: {type(student_id).__name__}") print(f" name: {type(name).__name__}") print(f" age: {type(age).__name__}") print(f" gpa: {type(gpa).__name__}") print(f" is_enrolled: {type(is_enrolled).__name__}")

Output:

======================================== STUDENT RECORD ======================================== Student ID: 1001 Name: Maria Santos Age: 22 GPA: 3.85 Enrolled: Yes Courses: Math, Physics, CS ======================================== Data Types Used: student_id: int name: str age: int gpa: float is_enrolled: bool

Practice Exercises

Exercise 1: Type Identification

Identify the type of each value:

  • 42
  • "42"
  • 42.0
  • True
  • "True"
  • 0

Exercise 2: Variable Creation

Create variables for a product: name (str), price (float), quantity (int), and in_stock (bool). Print a summary.

Exercise 3: Type Conversion

Convert the string "3.14159" to a float, then to an int. What happens to the decimal part?

Exercise 4: String Operations

Given text = "Python Programming", write code to:

  • Get the length
  • Convert to uppercase
  • Replace "Python" with "Java"
  • Get the first 6 characters

Exercise 5: F-String Formatting

Create an f-string that displays a price of $19.99 with 2 decimal places, right-aligned in 10 characters.

Exercise 6: Naming Convention Fix

Fix these variable names to follow Python conventions:

  • myVariableName
  • 2ndPlace
  • user-email
  • TOTALTAX

Exercise 7: Temperature Conversion Program

Write a program that converts Celsius to Fahrenheit and Kelvin:

  • F = C × 9/5 + 32
  • K = C + 273.15

Exercise 8: Type Checker Function

Write a function that takes any value and prints its type and whether it's truthy or falsy.

Summary

In this lesson, you learned:

  • How to create and use variables in Python
  • The four primitive data types: int, float, str, bool
  • Arithmetic operations and their behavior with different types
  • String operations, methods, and f-string formatting
  • Boolean values and truthiness in Python
  • Type conversion between different data types
  • Variable naming rules and conventions
  • How to check types using type() and isinstance()

Variables and data types are the building blocks of every Python program. Master these fundamentals before moving on to operators and expressions.

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Python Setup & First Program
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Operators & Expressions

Course Content

1. How Computers Work2. Python Setup & First Program3. Variables & Data Types4. Operators & Expressions5. Control Flow: Conditionals6. Control Flow: Loops7. Functions8. Data Structures: Lists & Tuples9. Data Structures: Dictionaries & Sets10. Imperative Algorithm Projects