Plans do not function with *args #1450

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1. oliwenmandiamond commented on Mar 23, 2026

   

   oliwenmandiamond

   on Mar 23, 2026

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   So there seems to be a deliberate choice to not support *args in plans according to those docs. I don't understand why this is the case because this means any generic scan command will now not be possible in the new system even though we already have scan plans that we could use.
2. oliwenmandiamond commented on Mar 26, 2026

   

   oliwenmandiamond

   on Mar 26, 2026

   Last edited by oliwenmandiamond

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   So from your logs, we can see it is using your second value, the move position and putting it in the groups argument. I've tested to see if providing a list or args=[...] fixes it but same result
   >>> plans.test_move([devs.fsj1, "Out"])
>>> plans.test_move(args=[devs.fsj1, "Out"])

   2026-03-26 11:30:54,591     INFO blueapi.worker.task Asked to run plan test_move with {'args': ['fsj1', 'Out']} and metadata {'user': 'xol73553', 'instrument_session': 'cm12345-1'} for all runs
   2026-03-26 11:30:54,592    ERROR blueapi.worker.task_worker Task failed
   2026-03-26 11:30:54,592    ERROR blueapi.worker.task_worker test_move() got an unexpected keyword argument 'args'
   Traceback (most recent call last):
     File "/app/.venv/lib/python3.11/site-packages/blueapi/worker/task_worker.py", line 441, in process_task
       result = self._current.task.do_task(self._ctx)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     File "/app/.venv/lib/python3.11/site-packages/blueapi/worker/task.py", line 41, in do_task
       result = ctx.run_engine(func(**prepared_params))
                               ^^^^^^^^^^^^^^^^^^^^^^^
     File "/app/.venv/lib/python3.11/site-packages/bluesky/utils/__init__.py", line 2015, in wrapper
       return Plan(bs_plan, *args, **kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     File "/app/.venv/lib/python3.11/site-packages/bluesky/utils/__init__.py", line 1966, in __init__
       self._iter = f(*args, **kwargs)
                    ^^^^^^^^^^^^^^^^^^
   TypeError: test_move() got an unexpected keyword argument 'args'
   Exception ignored in: <function Plan.__del__ at 0x7f5ef9fda480>
   Traceback (most recent call last):
     File "/app/.venv/lib/python3.11/site-packages/bluesky/utils/__init__.py", line 1982, in __del__
       if self._stack:
   

   We need to make it so if *args is used, blueapi must enforce anything after must be keyword argument and for it to even accepts args as valid.
3. oliwenmandiamond commented on Mar 26, 2026

   

   oliwenmandiamond

   on Mar 26, 2026

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   This is a more fundamental framework issue as blueapi is only designed to work with kwargs rather than args and kwargs

   blueapi/worker/task.py

       def do_task(self, ctx: BlueskyContext) -> None:
           ...
           func = ctx.plan_functions[self.name]
           prepared_params = self.prepare_params(ctx)
           ctx.run_engine.md.update(self.metadata)
           result = ctx.run_engine(func(**prepared_params))
           ...

   the prepared_params runs the plan with every argument as kwargs so *args would never work. You would have to redesign task and TaskRequest to support

       args: list[Any] = []
       kwargs: dict[str, Any] = {}

   rather than just parameters

       parameters: dict[str, Any] = {}

   Then, we can support this by giving these arguments to the plan

           func = ctx.plan_functions[self.name]
           prepared_args, prepared_kwargs = self.prepare_params(ctx)
           ctx.run_engine.md.update(self.metadata)
           result = ctx.run_engine(func(*prepared_args, **prepared_kwargs))

   In the meantime, the only alternative we can do is

   @plan
   def test_move(
       args: list[Movable[Any] | Any], group: str | None = None, wait: bool = True
   ) -> MsgGenerator:

   which becomes

   >>> plans.test_move([devs.fsj1.open, False])

   rather than

   @plan
   def test_move(
       *args: Movable[Any], group: str | None = None, wait: bool = True
   ) -> MsgGenerator:
4. tpoliaw

   linked a pull request that will close this issuefeat: Support *args and **kwargs in plan definitions #1715on Oct 22, 2026
5. tpoliaw

   closed this as completedin #1715on Oct 22, 2026