How can each dataclass instance get its own list field?

A mutable field must be constructed per instance rather than reused as a class-level default. Dataclasses provide field(default_factory=list) for that purpose. Declare tags: list[str] = field(default_factory=list) after importing field. Create two instances, append a tag to one, and assert the other remains empty and the lists are not identical. The factory is called without arguments, so initialization requiring another field belongs in explicit initialization logic. A frozen dataclass prevents rebinding its fields, but it does not make an embedded list immutable; use a tuple when the contents must also stay fixed. Minimal example:

from dataclasses import dataclass, field

@dataclass
class Item:
    tags: list[str] = field(default_factory=list)

a, b = Item(), Item()
a.tags.append("new")
assert b.tags == [] and a.tags is not b.tags