Variables and Data Types

Variables are the building blocks of every Python program. A variable is a name that refers to a value stored in your computer's memory. In this lesson, you will learn how to create variables, understand Python's core data types, and convert between them.

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Creating Variables

In Python, you create a variable simply by assigning a value to a name using the = operator:

age = 25
name = "Alice"
pi = 3.14159
is_student = True

print(age)         # 25
print(name)        # Alice
print(pi)          # 3.14159
print(is_student)  # True

There is no need to declare the type — Python figures it out automatically. This is called dynamic typing.

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Variable Naming Rules

• Must start with a letter or underscore _
• Can contain letters, numbers, and underscores
• Cannot start with a number
• Cannot use Python keywords (like if, for, class, return)
• Are case-sensitive (age and Age are different)

# Valid names
user_name = "Alice"
_count = 10
total2 = 50
MAX_SIZE = 100

# Invalid names
2nd_place = "Bob"     # starts with a number
my-name = "Charlie"   # hyphens not allowed
class = "Math"        # 'class' is a reserved keyword

Naming Conventions

Python developers follow PEP 8 style guidelines:

# Variables and functions: snake_case
user_age = 25
first_name = "Alice"

# Constants: UPPER_SNAKE_CASE
MAX_RETRIES = 3
PI = 3.14159

# Classes: PascalCase (you'll learn about these later)
# class MyClass:

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Core Data Types

Python has several built-in data types. The four most fundamental are:

Integers (int)

Whole numbers — positive, negative, or zero:

age = 25
temperature = -10
count = 0
big_number = 1_000_000  # underscores for readability

print(type(age))         # <class 'int'>
print(type(big_number))  # <class 'int'>

Python integers have unlimited precision — they can be as large as your memory allows:

huge = 10 ** 100  # 10 to the power of 100
print(huge)       # a 1 followed by 100 zeros

Floating-Point Numbers (float)

Numbers with decimal points:

price = 19.99
pi = 3.14159
negative = -0.5
scientific = 2.5e6  # 2.5 × 10^6 = 2500000.0

print(type(price))       # <class 'float'>
print(type(scientific))  # <class 'float'>

Be aware of floating-point precision:

print(0.1 + 0.2)        # 0.30000000000000004
print(0.1 + 0.2 == 0.3) # False!

# For precise decimal arithmetic, use the decimal module
from decimal import Decimal
print(Decimal('0.1') + Decimal('0.2'))  # 0.3

Strings (str)

Text data — sequences of characters:

greeting = "Hello, World!"
name = 'Alice'
multiline = """This is a
multi-line string."""

print(type(greeting))  # <class 'str'>

Booleans (bool)

Logical values — either True or False:

is_active = True
is_logged_in = False

print(type(is_active))  # <class 'bool'>

# Booleans are also integers: True = 1, False = 0
print(True + True)   # 2
print(False + 1)     # 1

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The type() Function

Use type() to check the type of any value:

print(type(42))        # <class 'int'>
print(type(3.14))      # <class 'float'>
print(type("hello"))   # <class 'str'>
print(type(True))      # <class 'bool'>
print(type(None))      # <class 'NoneType'>

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None — The Absence of a Value

None is a special value that means "nothing" or "no value":

result = None
print(result)        # None
print(type(result))  # <class 'NoneType'>