cattr.structure isn't working for classes uncluding dicts

[Natanela]

• cattrs version: 1.7.1 (attrs-21.2.0 cattrs-1.7.1)
• Python version: 3.8.1
• Operating System: both Windows 10 and Linux

Description

I'm trying to structure a JSON file into a class defined with attrs.

Here is my class definition:

@attr.s(frozen=True, auto_attribs=True)
class SchedulingDefinition:
    schedule: str = attr.ib()
    job_name: str = attr.ib(default=attr.Factory(lambda self: self.notebook_name, takes_self=True))
    job_id: str = attr.ib(default=attr.Factory(lambda: hashlib.sha224(uuid.uuid4().bytes).hexdigest()[:10]))
    timeout_in_min: int = 60
    retry_count: int = 1
    parameters: Optional[Dict] = attr.ib(default=None)

I'm using the following line to destructure the JSON file:

    with open(os.path.join(scheduling_definitions_dir, scheduling_definition_file), 'rb') as f:
            definition = cattr.structure(json.load(f), SchedulingDefinition)

JSON files that contain a parameters property fail with the following error message:
Unsupported type: ~KT. Register a structure hook for it.

here is the full trace:

File "./scheduling.py", line 170, in load_scheduling_definitions
  definition = cattr.structure(json.load(f), SchedulingDefinition)
File "/opt/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/site-packages/cattr/converters.py", line 223, in structure
  return self._structure_func.dispatch(cl)(obj, cl)
File "", line 17, in structure_SchedulingDefinition
File "/opt/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/site-packages/cattr/converters.py", line 439, in _structure_union
  return self._structure_func.dispatch(other)(obj, other)
File "", line 2, in structure_mapping
File "", line 2, in <dictcomp>
File "/opt/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/site-packages/cattr/converters.py", line 304, in _structure_default
  raise ValueError(msg)

ValueError: Unsupported type: ~KT. Register a structure hook for it.

Here is an example JSON input file which fails to structure:

{
    "schedule": "0 15 8 ? * * *",
    "job_name": "Backfill",
    "job_id": "602608e0c9",
    "timeout_in_min": 60,
    "retry_count": 1,
    "parameters": {
        "days_back": 90,
        "shard_size": "2D"
    }
}

This logic worked perfectly fine up until v1.6.0 (included) but started to fail on 1.7.0 and 1.7.1

[Tinche]

Hm, a regression. I will fix this! It looks like the issue is handling the parameters field.

In the meantime, your options are:

• keep using version 1.6 until this is fixed
• change the field definition to read: parameters: Optional[dict] = attr.ib(default=None) (so, using the built-in dict instead of the Dict from typing). This is the preferred way in 3.9 forward.
• change the field definition to read: parameters: Optional[Dict[Any, Any]] = attr.ib(default=None) (basically flesh out the type parameters)
• upgrade to Python 3.9, where this use case seems to work out of the box

[Tinche]

This should be fixed on master now!