Classes & Objects

A class is a blueprint for creating objects. Objects have data (attributes) and behavior (methods).

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Creating a Class

class User:
    def __init__(self, name: str, email: str) -> None:
        self.name = name
        self.email = email

    def display(self) -> str:
        return f"{self.name} <{self.email}>"

Creating Objects

user = User("Alice", "alice@example.com")
print(user.display())  # "Alice <alice@example.com>"
print(user.name)        # "Alice"

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The __init__ Method

__init__ is the constructor. It runs when you create a new object.

class ApiClient:
    def __init__(self, base_url: str, timeout: int = 30) -> None:
        self.base_url = base_url
        self.timeout = timeout

client = ApiClient("https://api.example.com")

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Instance vs Class Attributes

class TestConfig:
    max_retries: int = 3  # class attribute (shared)

    def __init__(self, env: str) -> None:
        self.env = env  # instance attribute (per object)

config1 = TestConfig("staging")
config2 = TestConfig("production")

print(config1.max_retries)  # 3 (from class)
print(config1.env)          # "staging" (from instance)

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Methods

Instance Methods

Regular methods that work with self:

class Calculator:
    def __init__(self) -> None:
        self.history: list[str] = []

    def add(self, a: int, b: int) -> int:
        result = a + b
        self.history.append(f"{a} + {b} = {result}")
        return result

Class Methods

Work with the class, not an instance:

class User:
    def __init__(self, name: str, role: str) -> None:
        self.name = name
        self.role = role

    @classmethod
    def create_admin(cls, name: str) -> "User":
        return cls(name, role="admin")

admin = User.create_admin("Alice")

Static Methods

Do not use self or cls:

class Validator:
    @staticmethod
    def is_valid_email(email: str) -> bool:
        return "@" in email and "." in email

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Properties

Use @property to control access to attributes:

class Circle:
    def __init__(self, radius: float) -> None:
        self._radius = radius

    @property
    def radius(self) -> float:
        return self._radius

    @radius.setter
    def radius(self, value: float) -> None:
        if value < 0:
            raise ValueError("Radius cannot be negative")
        self._radius = value

    @property
    def area(self) -> float:
        return 3.14159 * self._radius ** 2

circle = Circle(5.0)
print(circle.area)    # 78.53975
circle.radius = 10.0  # uses setter

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Naming Conventions

Convention	Meaning
name	Public attribute
_name	Protected (internal use)
__name	Private (name mangling)

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Best Practices

• Keep classes small and focused — one class, one responsibility
• Use __init__ to set up all attributes
• Use type hints for all attributes and methods
• Use @property instead of getter/setter methods
• Prefix internal attributes with _
• Avoid classes with too many methods (God objects)