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An alternative API for creating attrs fields

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Elysia is an addon for attrs that provides an alternative API for creating fields on @defined classes.


pip install elysia


Elysia's sole export is the Fields class, which wraps attrs.field, attrs.setters, and attrs.validators to provide a more concise API for defining instance attributes.

Here's a brief example of a class created with attrs and Elysia:

from datetime import datetime

from attrs import define
from elysia import Fields

class User:
    name: str = Fields.field()
    password: str = Fields.field(

    created_at: datetime = Fields.attr(factory=datetime.utcnow, frozen=True)

The User class has two __init__ arguments: name and password. Whenever set, password is validated to ensure it's at least 8 characters long.

User also has a created_at attribute that can't set via an __init__ argument. When a User object is instantiated, created_at is set to the current time and cannot be changed afterwards. does all that work, exactly?

Glad you asked.

There are two ways to define an attribute with Elysia: Fields.field() and Fields.attr(). Fields.field() defines attributes that map to __init__ arguments; Fields.attr() defines attributes that do not. Both are wrappers around attrs.field and accept all the same arguments. Like attrs.field, all arguments to Fields.field() and Fields.attr() are keyword-only.

Both methods also accept an optional, boolean, frozen argument. Setting it to True is a shortcut for on_setattr=attrs.setters.frozen — that is, it freezes the attribute, raising an exception if you try to set it after initialization.


Elysia is happy to combine frozen=True with anything else you pass to on_setattr, but attrs.setters.frozen will be applied first, which may not be what you expect.

Fields also provides access to attrs' setters and validators via Fields.setters and Fields.validators, respectively. It makes no difference whether setters and validators are accessed through Fields or attrs. Do what you like.


Elysia is licensed under the MIT License.

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