structure raises a KeyError when optional attribute is missing

• cattrs version: 22.1.0
• Python version: 3.9.10
• Operating System: macOS Big Sur (11.5.2)

Description

Passing a dictionary to structure that has missing entries for Optional values results in an error. Instead I would expect the instance to get built successfully with the optional property unset.

What I Did

Example code:

from cattrs import structure
from attrs import define
from typing import Optional


@define
class Dog:
    name: str
    nickname: Optional[str]


if __name__ == "__main__":
    # This works
    spike = structure({"name": "Spike", "nickname": "Carl"}, Dog)
    # This raises an error
    rex = structure({"name": "Rex"}, Dog)

The result:

  + Exception Group Traceback (most recent call last):
  |   File "/Users/cmadd/tractable/graphs/cattrs_bug.py", line 13, in <module>
  |     rex = structure({"name": "Rex"}, Dog)
  |   File "/Users/cmadd/tractable/graphs/venv/lib/python3.9/site-packages/cattrs/converters.py", line 281, in structure
  |     return self._structure_func.dispatch(cl)(obj, cl)
  |   File "<cattrs generated structure __main__.Dog>", line 14, in structure_Dog
  |     if errors: raise __c_cve('While structuring Dog', errors, __cl)
  | cattrs.errors.ClassValidationError: While structuring Dog
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "<cattrs generated structure __main__.Dog>", line 10, in structure_Dog
    |     res['nickname'] = __c_structure_nickname(o['nickname'], __c_type_nickname)
    | KeyError: 'nickname'
    | Structuring class Dog @ attribute nickname
    +------------------------------------

Start by running the reported Python example with cattrs 22.1.0 and inspect the structure entry point used by the Dog example. Done means structuring the dictionary without nickname succeeds and produces a Dog instance with the optional attribute unset, while the existing case with nickname continues to work.