Variables, Data Types and Operators

Variables and Naming Conventions

What is a Variable?

A variable is a name that stores a value. Think of it as a labeled box where you can put data and use it later in your code. For a broader overview of Python basics, see Introduction to Python.

x = 5
name = “Alice”
is_logged_in = True

• You use the = symbol (assignment operator) to assign a value to a variable.
• Variables do not need a type declaration. Python figures it out automatically.

Why Use Variables?

• To store user input
• To reuse values without repeating them
• To make your code more readable and maintainable

To see variables in action within practical examples, explore Loops and Iteration.

Examples:

age = 25
greeting = “Hello”
pi = 3.14159
is_admin = False

You can also reassign values:

age = 30

Valid Variable Names

• Must start with a letter (a–z, A–Z) or underscore (_)
• Can include letters, digits (0–9), and underscores
• Case-sensitive (age and Age are different)

my_name = “Sarah”
_age = 20
score2 = 99

Invalid Variable Names

2cool = “nope” # starts with a number
user-name = “error” # contains a dash
class = “reserved” # ‘class’ is a keyword

Best Practices (PEP8 Guidelines)

Element	Style	Example
Variable names	snake_case	user_name, score_total
Constants	ALL_CAPS	MAX_USERS = 10
Avoid single letters	Unless in loops	Prefer count over c
Descriptive is better	Be clear	email_address > e

Avoid These Mistakes

• Don’t name variables after built-in functions like list, str, input

input = “Hello” # Bad: overrides built-in input() function

• Don’t use spaces or special characters

user name = “Bob” # invalid

Python Keywords (Can’t be Variable Names)

Here are a few reserved keywords you can’t use as variable names:

False, True, None, if, else, elif, for, while, class, def, try, except, with, return, import, as, pass, break, continue, and, or, not, in, is

Quick Exercise:

Which of these are valid variable names?

1. user_age = 20
2. $salary = 50000
3. full name = “Amy”
4. _secret = “shhh”
5. def = “define”

Answers: 1 and 4 are valid.

Data types: int, float, str, bool

Python is a dynamically typed language, meaning you don’t need to specify the data type — Python figures it out automatically. For a broader overview, see Introduction to Python.

You can use the type() function to check the data type of any value or variable.

x = 10
print(type(x)) # <class ‘int’>

Integer (int)

• Whole numbers (positive or negative) without decimals
• Unlimited size (up to your machine’s memory)

age = 30
year = 2026
temperature = -5

type(age) # <class ‘int’>

You can do math with integers:

a = 10
b = 3
print(a + b) # 13
print(a – b) # 7
print(a * b) # 30
print(a // b) # 3 (floor division)
print(a % b) # 1 (modulus)
print(a ** b) # 1000 (exponent)

Float (float)

• Numbers with decimal points
• Used for measurements, precise calculations, etc.

pi = 3.14159
weight = 65.5
gpa = 3.75

type(pi) # <class ‘float’>

Be careful with floating point precision:

print(0.1 + 0.2) # 0.30000000000000004

You can convert between int and float:

int(3.9) # 3
float(4) # 4.0

String (str)

• Text values wrapped in quotes
• Can use single ‘, double “, or triple “”” for multiline

name = “Alice”
greeting = ‘Hello’
quote = “””Python is fun!”””

type(name) # <class ‘str’>

String operations:

full_name = “John” + ” ” + “Doe” # Concatenation
print(len(full_name)) # Length of the string
print(name.upper()) # “ALICE”
print(name.lower()) # “alice”
print(name[0]) # “A”

You can use f-strings for formatting:

age = 25
print(f”My age is {age}”)

For more on string operations, see Working with Strings.

Boolean (bool)

• Only two values: True or False (capitalized)
• Used for conditions and logic

is_active = True
is_admin = False

type(is_active) # <class ‘bool’>

Booleans often come from comparisons:

print(5 > 3) # True
print(10 == 5) # False

Used in if statements:

logged_in = True

if logged_in:
print(“Welcome!”)
else:
print(“Please log in.”)

To see how booleans drive decisions in code, check Control Flow.

Type Conversion Summary:

Function	Converts to	Example
int()	Integer	int("10") → 10
float()	Float	float("3.14") → 3.14
str()	String	str(123) → "123"
bool()	Boolean	bool(0) → False

Empty values like 0, 0.0, “”, None, and [] are all considered False.

Type casting and input()

What is Type Casting?

Type casting means converting one data type into another — for example, changing a string into an integer. For a quick refresher, see Introduction to Python.

Python has built-in functions to do this:

Function	Converts to	Example
int()	Integer	int("10") → 10
float()	Float	float("3.14") → 3.14
str()	String	str(100) → "100"
bool()	Boolean	bool("hi") → True

Examples of Type Casting:

x = “25”
y = int(x) # y becomes 25 (an int)
print(x, type(x)) # ’25’ <class ‘str’>
print(y, type(y)) # 25 <class ‘int’>

pi = “3.14”
print(float(pi)) # 3.14

number = 99
print(str(number)) # ’99’

input() Function

What is input()?

The input() function asks the user for input from the keyboard and always returns a string. For more on Python basics, see Introduction to Python.

name = input(“What is your name? “)
print(“Hello, ” + name)

Even if the user enters numbers, Python treats them as strings:

age = input(“Enter your age: “)
print(age, type(age)) # ’25’ <class ‘str’>

Using input() with Type Casting

To use the input as a number, you must cast it manually:

# Get input and cast to int
age = int(input(“Enter your age: “))
print(age + 5) # Adds 5 to age
# Get input and cast to float
height = float(input(“Enter your height in meters: “))
print(“Your height + 0.1m =”, height + 0.1)

Be careful! If the user enters non-numeric text when casting to int() or float(), it will cause an error.

Pro Tip: Add Input Validation (Optional Advanced)

age_input = input(“Enter your age: “)

if age_input.isdigit():
age = int(age_input)
print(f”You will be {age + 1} next year.”)
else:
print(“Please enter a valid number.”)

Summary:

Concept	Converts to	Example
int()	Integer	int("10") → 10
float()	Float	float("3.14") → 3.14
str()	String	str(100) → "100"
bool()	Boolean	bool("hi") → True