Data Types & Variables

Python has simple and powerful data types. This page covers the basics you use every day.

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Basic Data Types

Type	Example	Description
int	42	Whole numbers
float	3.14	Decimal numbers
str	"hello"	Text (strings)
bool	True, False	Logical values
NoneType	None	No value / empty

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Variables

A variable stores a value. You do not need to declare the type — Python figures it out.

name = "Alice"
age = 30
is_active = True
salary = 50000.50
nothing = None

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Dynamic Typing

In Python, a variable can change its type at any time:

value = 10       # int
value = "hello"  # now it is str

Be careful with dynamic typing

Changing types can cause bugs. Use type hints to make your code clear.

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Type Hints

Type hints tell other developers (and tools) what type a variable should have:

name: str = "Alice"
age: int = 30
is_active: bool = True
scores: list[int] = [90, 85, 92]

Type hints do not stop the code from running. They are for readability and tools like mypy.

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Type Checking with type() and isinstance()

x = 42
print(type(x))           # <class 'int'>
print(isinstance(x, int)) # True

Use isinstance() instead of type() — it works with inheritance.

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Type Conversion

Convert between types when needed:

number = int("42")       # str -> int
text = str(42)           # int -> str
decimal = float("3.14")  # str -> float
flag = bool(1)           # int -> bool (True)

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String Basics

greeting = "Hello"
name = "World"

# Concatenation
message = greeting + " " + name

# f-strings (recommended)
message = f"{greeting} {name}"

# Useful methods
print(name.lower())    # "world"
print(name.upper())    # "WORLD"
print(name.strip())    # remove whitespace
print(name.replace("o", "0"))  # "W0rld"

Use f-strings

f-strings are the cleanest way to format strings in modern Python.

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Numbers

a = 10
b = 3

print(a + b)   # 13 — addition
print(a - b)   # 7  — subtraction
print(a * b)   # 30 — multiplication
print(a / b)   # 3.333... — division (float)
print(a // b)  # 3  — integer division
print(a % b)   # 1  — remainder
print(a ** b)  # 1000 — power

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Boolean Logic

x = True
y = False

print(x and y)  # False
print(x or y)   # True
print(not x)     # False

Falsy values in Python: 0, 0.0, "", [], {}, None, False.

Everything else is truthy.

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Duck Typing

In Python, you do not need to check a type — you just call the method. If the object has that method, it works.

class Dog:
    def speak(self) -> str:
        return "Woof!"

class Cat:
    def speak(self) -> str:
        return "Meow!"

def make_sound(animal) -> None:
    print(animal.speak())  # works for Dog, Cat, or any object with speak()

make_sound(Dog())  # "Woof!"
make_sound(Cat())  # "Meow!"

Duck typing means: if it walks like a duck and quacks like a duck, it is a duck.

Python does not care about the class — it cares about the behavior.

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Comments & Docstrings

Single-line Comments

# This is a comment — Python ignores this line
x = 42  # inline comment

Multi-line Comments

# First line
# Second line
# Third line

Docstrings

A docstring is a string at the top of a function, class, or module. It explains what the code does.

def calculate_discount(price: float, percent: float) -> float:
    """Calculate the final price after applying a discount.

    Args:
        price: Original price in USD.
        percent: Discount as a decimal (e.g. 0.2 for 20%).

    Returns:
        Final price after discount.
    """
    return price * (1 - percent)

Access a docstring with help() or .__doc__:

print(calculate_discount.__doc__)

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Indentation Rules

Python uses indentation (spaces) to define blocks of code. This is not optional — it is required.

# Correct — 4 spaces
if True:
    print("inside block")
    if True:
        print("nested block")

# Wrong — inconsistent indentation causes IndentationError
if True:
  print("2 spaces — will work but not PEP 8")
    print("4 spaces after 2 spaces — IndentationError!")

Rule	Detail
Use 4 spaces per level	Standard across Python community (PEP 8)
Never mix tabs and spaces	Will cause TabError
Configure your editor	Set "indent = 4 spaces" and "insert spaces"

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Best Practices

• Use meaningful names: user_name not x
• Follow snake_case for variables and functions
• Use UPPER_CASE for constants: MAX_RETRIES = 3
• Add type hints to function parameters and return values
• Avoid magic values — use named constants instead
• Write docstrings for every function, class, and module
• Use comments to explain why, not what the code does