Python Variables Data Types

Python Variables

Python comments explain why code exists, what a tricky line means, or which decision a future reader should not miss.

Python variables are names that point to values. A variable can point to a number now, a string later, or a list after that, so clear names matter more than type declarations.

On this page, focus on writing code that another beginner can read: helpful comments, meaningful variable names, and simple assignments.

Comments

Comments are lines that Python ignores during execution. They are used to explain code, make it more readable, or temporarily disable code.

Use the # symbol. Everything after # on that line is a comment.

Python doesn't have a dedicated multi-line comment syntax. Use multiple # lines, or use triple-quoted strings (which are technically string literals, not comments).

Single-Line Comments

# This is a single-line comment
print("Hello")  # inline comment

# Comments are ignored by Python
# print("This line won't run")

Multi-Line Comments

# Consecutive comments explain nearby code.
# Use them for context that is not obvious from the statement itself.

def greet(name: str) -> str:
    """Return a greeting for one person.

    Args:
        name: Person to greet.
    """
    return f"Hello, {name}!"

print(greet("Maya"))

Output

Hello, Maya!

Variables

A variable is a named container that stores a value in memory. In Python, you don't need to declare a variable's type - it's inferred automatically.

Creating Variables

name = "Alice"        # str
age = 25              # int
height = 5.7          # float
is_student = True     # bool

print(name)           # Alice
print(type(name))     # <class 'str'>
print(type(age))      # <class 'int'>

# Multiple assignment
x = y = z = 0         # all three equal 0

# Tuple unpacking
a, b, c = 1, 2, 3
print(a, b, c)        # 1 2 3

# Swap variables (Python style)
a, b = b, a
print(a, b)           # 2 1

Variable Naming Rules

Must start with a letter or underscore (_)
Can contain letters, digits, and underscores
Cannot start with a digit
Cannot use Python keywords (if, for, class, etc.)
Case-sensitive: name and Name are different variables

Valid vs Invalid Names

# Valid variable names
my_name = "Alice"
_private = 42
camelCase = "ok but not Pythonic"
snake_case = "preferred in Python"
value2 = 100

# Invalid variable names (will cause SyntaxError)
# 2value = 10       # starts with digit
# my-name = "Bob"   # hyphen not allowed
# class = "Python"  # reserved keyword

# Python naming conventions (PEP 8)
user_name = "alice"          # variables: snake_case
MAX_SIZE = 100               # constants: UPPER_CASE
class MyClass:               # classes: PascalCase
    pass

Dynamic Typing

Python is dynamically typed - a variable can hold different types at different times. Use type() to check the current type.

Reassign One Name to Different Types

x = 10
print(type(x))   # <class 'int'>

x = "hello"
print(type(x))   # <class 'str'>

x = 3.14
print(type(x))   # <class 'float'>

x = [1, 2, 3]
print(type(x))   # <class 'list'>

# Type hints (Python 3.5+) - optional but recommended
def add(a: int, b: int) -> int:
    return a + b

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Python Variables Data Types