PEP-563 (Postponed evaluation of annotations) breaks loading attr classes #80

• cattrs version: 0.9.0
• Python version: 3.8
• Operating System: windows

Description

It appears that when you have attr classes that are in a file with from __future__ import annotations, cattrs does not work when trying to structure a simple type to your attrs class. This is because attrs uses __annotations__ to get the type annotations and with PEP-563, these are now strings instead of the actual type, see python-attrs/attrs#288 (comment) for a description

This little script below demonstrates the problem. If you comment out the first line, the from __future__ import annotations, it works, but if you put it back, you get a ValueError because the type that cattr passed to functools.singledispatch is a string of the type, not the type itself.

from __future__ import annotations

import attr
import cattr

@attr.s
class Item:
    desc:str = attr.ib()

@attr.s
class Box:
    itemone:Item = attr.ib()
    itemtwo:Item = attr.ib()

x = Item("pencil")
y = Item("pen")
box = Box(x,y)

c = cattr.Converter()
c.register_structure_hook(Box, c.structure_attrs_fromdict)
c.register_structure_hook(Item, c.structure_attrs_fromdict)

unstructured = c.unstructure(box)

print(unstructured)

structured = c.structure(unstructured, Box)

print(structured)