spec_scan lookup not working

[callumforrester]

When running the spec_scan plan, blueapi's device reference deserialization mechanism cannot extract motors from a scanspec. The axes of the scan are still strings by the time the scanspec is passed to the plan, leading to an error message when blueapi tries to lookup the name of the "device":

2025-01-16 08:23:17,187 - 'str' object has no attribute 'name'
Traceback (most recent call last):
  File "/workspaces/blueapi/src/blueapi/worker/task_worker.py", line 340, in _cycle
    self._current.task.do_task(self._ctx)
  File "/workspaces/blueapi/src/blueapi/worker/task.py", line 33, in do_task
    ctx.run_engine(func(**prepared_params))
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 973, in __call__
    plan_return = self._resume_task(init_func=_build_task)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 1109, in _resume_task
    raise exc
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 1751, in _run
    raise err
  File "/venv/lib/python3.11/site-packages/bluesky/run_engine.py", line 1605, in _run
    msg = self._plan_stack[-1].send(resp)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/utils/__init__.py", line 1260, in dec_inner
    return (yield from plan)
            ^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/dodal/plan_stubs/data_session.py", line 40, in attach_data_session_metadata_wrapper
    yield from bpp.inject_md_wrapper(plan, md={DATA_SESSION: data_session})
  File "/venv/lib/python3.11/site-packages/bluesky/preprocessors.py", line 755, in inject_md_wrapper
    return (yield from msg_mutator(plan, _inject_md))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/preprocessors.py", line 246, in msg_mutator
    msg = plan.send(None)
          ^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/dodal/plans/scanspec.py", line 47, in spec_scan
    yield from bp.scan_nd(tuple(detectors), _as_cycler(spec), md=_md)
  File "/venv/lib/python3.11/site-packages/bluesky/plans.py", line 1063, in scan_nd
    "motors": [motor.name for motor in cycler.keys],
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/bluesky/plans.py", line 1063, in <listcomp>
    "motors": [motor.name for motor in cycler.keys],
               ^^^^^^^^^^
AttributeError: 'str' object has no attribute 'name'

Steps To Reproduce

Run the failing spec_scan system test in #783

Acceptance Criteria

• Running the plan (via, for example, `blueapi controller run spec_scan '{...}') performs a scan with no errors

[callumforrester]

Tagging @huw-dls for awareness

[tpoliaw]

I think this is down to scanspec's discriminated_union_of_subclasses decorator not supporting generic types. For instance removing all references to ophyd types and models

❯ uv tool run --with scanspec --with pydantic ptpython                                                                                                                                                   15:58:51
>>> import scanspec
... import pydantic
... from typing import Generic, Any, TypeVar
... T = TypeVar('T')
... @scanspec.core.discriminated_union_of_subclasses
... class Foo(Generic[T]):
...     pass
... @pydantic.dataclasses.dataclass
... class Bar(Foo[T]):
...     a: T
... print(pydantic.TypeAdapter(Foo[int]).validate_python({'a': '23', 'type': 'Bar'}))
... print(pydantic.TypeAdapter(Bar[int]).validate_python({'a': '23', 'type': 'Bar'}))
Bar(a='23')
Bar(a=23)

I'll keep looking but I think this might be better as a bug there instead of here.

[callumforrester]

I think I am misunderstanding but I thought bluesky/scanspec#132 was supposed to cover this. Tagging @DiamondJoseph and @ZohebShaikh.

[tpoliaw]

I have opened a new ticket in scanspec for this but will leave this open until it no longer affects blueapi.

[tpoliaw]

Potential fix in scanspec: bluesky/scanspec#157