@dataclass writes __init__, __repr__ and __eq__ from a list of annotated fields. Use it for objects that mainly hold data.

from dataclasses import dataclass, field

@dataclass
class Order:
    item: str
    qty: int = 1
    notes: list[str] = field(
        default_factory=list)

Each Order gets its own notes list. A plain notes: list = [] is rejected with an error, because all instances would share one list.

• frozen=True makes instances immutable and hashable, so they can be dict keys.
• order=True adds <, > and friends, comparing fields in order.
• slots=True uses less memory and blocks stray attributes.

__post_init__ runs right after the generated __init__:

@dataclass
class Age:
    years: int

    def __post_init__(self):
        if self.years < 0:
            raise ValueError("negative")

Write a dataclass Task with title: str, priority: int = 0 and tags: list[str] defaulting to a new empty list, raising ValueError for an empty title. Write a frozen dataclass Point with x: int and y: int.

The explanation opens here once every check passes.