Can't arbitrarily deserialize a Spec

[callumforrester]

Spec provides a deserialize() method which uses an internal pydantic type adapter to deserialize a dict into any Spec.

As a developer I would like to deserailize a dict into an arbitrary type that I do not know at compile time so that I can include scanspecs in blueapi plans.

In the last version of scanspec I could do this with:

def deserialize(thing: T, type_of_thing: type[T]) -> T:
    return pydantic.parse_obj_as(type_of_thing, thing)

my_spec = deserialize(Spec, {"type": "Line", "start": 0.0, "stop": 1.0, "num": 10})
my_other_thing = deserialize(Foo, {"bar": 1})

This is now deprecated and has been replaced with TypeAdapter, so the equivalent would be:

def deserialize(thing: T, type_of_thing: type[T]) -> T:
    adapter = pydantic.TypeAdapter(type_of_thing)
    return adapter.validate_python(type_of_thing, thing)

my_spec = deserialize(Spec, {"type": "Line", "start": 0.0, "stop": 1.0, "num": 10})
my_other_thing = deserialize(Foo, {"bar": 1})

Using both of the above I get the same error:

Traceback (most recent call last):
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/type_adapter.py", line 277, in _init_core_attrs
    self._core_schema = _getattr_no_parents(self._type, '__pydantic_core_schema__')
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/type_adapter.py", line 119, in _getattr_no_parents
    raise AttributeError(attribute)
AttributeError: __pydantic_core_schema__

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/type_adapter.py", line 264, in __init__
    self._init_core_attrs(rebuild_mocks=False)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/type_adapter.py", line 142, in wrapped
    return func(self, *args, **kwargs)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/type_adapter.py", line 284, in _init_core_attrs
    self._core_schema = _get_schema(self._type, config_wrapper, parent_depth=self._parent_depth)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/type_adapter.py", line 102, in _get_schema
    schema = gen.generate_schema(type_)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/_internal/_generate_schema.py", line 507, in generate_schema
    from_property = self._generate_schema_from_property(obj, obj)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/_internal/_generate_schema.py", line 679, in _generate_schema_from_property
    schema = get_schema(
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/scanspec/core.py", line 234, in __get_pydantic_core_schema__
    return handler(source_type)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/_internal/_schema_generation_shared.py", line 83, in __call__
    schema = self._handler(source_type)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/_internal/_generate_schema.py", line 789, in _generate_schema_inner
    return self.match_type(obj)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/_internal/_generate_schema.py", line 875, in match_type
    return self._unknown_type_schema(obj)
  File "/home/vid18871/projects/bluesky/blueapi/venv/lib64/python3.10/site-packages/pydantic/_internal/_generate_schema.py", line 415, in _unknown_type_schema
    raise PydanticSchemaGenerationError(
pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'scanspec.specs.Spec'>. Set `arbitrary_types_allowed=True` in the model_config to ignore this error or implement `__get_pydantic_core_schema__` on your type to fully support it.

If you got this error by calling handler(<some type>) within `__get_pydantic_core_schema__` then you likely need to call `handler.generate_schema(<some type>)` since we do not call `__get_pydantic_core_schema__` on `<some type>` otherwise to avoid infinite recursion.

For further information visit https://errors.pydantic.dev/2.8/u/schema-for-unknown-type

[callumforrester]

I suspect, but have not confirmed, that this is the same bug as #130

[callumforrester]

Fixed by #132