A pattern for "required" arguments in dataclasses, allowing for fields without defaults to follow fields with defaults.

dataclass_required_field.py

from dataclasses import dataclass, field
from datetime import datetime
from typing import Optional, TypeVar

T = TypeVar("T")


def required() -> T:
    f: T

    def factory() -> T:
        # mypy treats a Field as a T, even though it has attributes like .name, .default, etc
        field_name = f.name  # type: ignore[attr-defined]
        raise ValueError(f"field '{field_name}' required")

    f = field(default_factory=factory)
    return f


@dataclass
class Event:
    id: str
    created_at: datetime
    updated_at: Optional[datetime] = None


@dataclass
class NamedEvent(Event):
    name: str = required()


event = NamedEvent(name="Some Event", id="ab13c1a", created_at=datetime.now())
print("created event:", event)


event2 = NamedEvent("ab13c1a", datetime.now(), name="Some Other Event")
print("created event:", event2)

event3 = NamedEvent("ab13c1a", datetime.now())

Example output:

created event: NamedEvent(id='ab13c1a', created_at=datetime.datetime(2022, 7, 23, 19, 22, 17, 944550), updated_at=None, name='Some Event')
created event: NamedEvent(id='ab13c1a', created_at=datetime.datetime(2022, 7, 23, 19, 22, 17, 944588), updated_at=None, name='Some Other Event')
Traceback (most recent call last):
  File ".../gist.py", line 39, in <module>
    event3 = NamedEvent("ab13c1a", datetime.now())
  File "<string>", line 6, in __init__
  File ".../gist.py", line 14, in factory
    raise ValueError(f"field '{field_name}' required")
ValueError: field 'name' required