old_contents: from .dask import DaskPipelineExecutor
  from .prefect import PrefectPipelineExecutor
  from .python import PythonPipelineExecutor
  
  __all__ = ["PythonPipelineExecutor", "DaskPipelineExecutor", "PrefectPipelineExecutor"]
  
new_contents: from .python import PythonPipelineExecutor
  
  __all__ = ["PythonPipelineExecutor"]
  
  try:
      from .dask import DaskPipelineExecutor
  
      __all__.append("DaskPipelineExecutor")
  except ImportError:
      pass
  
  try:
      from .prefect import PrefectPipelineExecutor
  
      __all__.append("PrefectPipelineExecutor")
  except ImportError:
      pass
  
subject: make executor imports optional (#85)