Self-referential type specification issues

[python-attrs]

Self-referential type specification issues

[dcczi]

For those of you who found your way to this issue when attempting to structure self-referential types, here is a demonstration of how to make it work:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from typing import List, Optional, ForwardRef

import attr
from cattr import structure, unstructure, register_structure_hook


@attr.s
class Subattr(object):
    a: int = attr.ib()


@attr.s
class Node(object):
    value: int = attr.ib()
    subattr: Optional[Subattr] = attr.ib(default=None)
    children: List['Node'] = attr.ib(default=[])


def main():
    child_with_attr_field = Node(value=1, children=[Node(value=2, subattr=Subattr(a=2))])
    print(child_with_attr_field)
    print(unstructure(child_with_attr_field))
    print(structure(unstructure(child_with_attr_field), Node))


register_structure_hook(
    ForwardRef("Node"), lambda d, t: structure(d, Node),
)

Notably, if you use the structure hook from the sample code in the issue description:

register_structure_hook(
    ForwardRef("Node"), lambda d, t: Node(**d),  # wrong
)

... then attributes that are themselves attr types will not structure as their attr classes.

Edited: The previous incorrect comment has been replaced with a useful one.

[Tinche]

Hi! Thanks for the detailed write up. I'll brood on this and get back to you.

[johnwlockwood]

This script demos the issue:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from typing import List, Optional
from cattr import structure, unstructure
import attr


@attr.s
class Node(object):
    value: int = attr.ib()
    children: Optional[List['Node']] = attr.ib(default=None)


def main():
    g = {
        'value': 1,
        'children': [
            {'value': 3},
            {'value': 4, 'children': [{'value':44}]},
            {'value': 5, 'children': [{'value':55}]}
        ]
    }
    n = structure(g, Node)
    print(n)


if __name__ == "__main__":
    main()

[Tinche]

Apart from this issue being almost 6 years old, this actually works out of the box nowadays!

from __future__ import annotations

from attrs import Factory, define

from cattrs import structure, unstructure


@define
class Subattr:
    a: int


@define
class Node:
    value: int
    subattr: Subattr | None = None
    children: list[Node] = Factory(list)


child_with_attr_field = Node(value=1, children=[Node(value=2, subattr=Subattr(a=2))])
print(child_with_attr_field)
print(unstructure(child_with_attr_field))
print(structure(unstructure(child_with_attr_field), Node))

Also super cool to see how attrs, cattrs and Python typing in general have come a long way!