dataclasses — Data Classes

Data classes automatically generate __init__(), __repr__(), __eq__() and other special methods.

from dataclasses import dataclass, field

Basic Usage

from dataclasses import dataclass

@dataclass
class Point:
    x: float
    y: float

p = Point(3.0, 4.0)
print(p)        # Point(x=3.0, y=4.0)   — auto __repr__
print(p.x)      # 3.0
print(p == Point(3.0, 4.0))  # True     — auto __eq__

Default Values

from dataclasses import dataclass, field

@dataclass
class User:
    name: str
    email: str
    age: int = 0                              # Simple default
    tags: list = field(default_factory=list)   # Mutable default

u = User("Alice", "alice@example.com")
print(u)  # User(name='Alice', email='alice@example.com', age=0, tags=[])

Frozen (Immutable)

@dataclass(frozen=True)
class Coordinate:
    lat: float
    lon: float

c = Coordinate(48.8566, 2.3522)
c.lat = 0  # FrozenInstanceError! Can't modify

Post-Init

@dataclass
class Rectangle:
    width: float
    height: float
    area: float = field(init=False)  # Not in __init__

    def __post_init__(self):
        self.area = self.width * self.height

r = Rectangle(5, 3)
print(r.area)  # 15.0

Ordering

@dataclass(order=True)
class Student:
    sort_index: float = field(init=False, repr=False)
    name: str
    grade: float

    def __post_init__(self):
        self.sort_index = self.grade

students = [Student("Alice", 95), Student("Bob", 87)]
sorted(students)  # Sorted by grade

Inheritance

@dataclass
class Animal:
    name: str
    sound: str

@dataclass
class Dog(Animal):
    breed: str

d = Dog("Rex", "Woof", "Labrador")

Common Pitfalls

• Mutable defaults — use field(default_factory=list) NOT tags: list = []
• Fields with defaults must come after fields without — like function args
• frozen=True uses __hash__ — allows use as dict keys / set members

Official Documentation

dataclasses — Data Classes

API Reference

Decorators & Functions