Simplify Parameter Paths with `set_path`#

When fastcs-odin introspects an Odin server it walks the full JSON parameter tree and builds an OdinParameter for every leaf, using the complete path from the tree root as its URI. That URI becomes the attribute name on the fastcs controller (segments joined with _), which is often long and redundant once the parameter is already scoped inside a sub-controller.

set_path allows the URI-derived name to be replaced with a shorter, tidier path.

Background: how names are derived#

Field	Description
`uri`	List of path segments used to construct HTTP requests to the Odin server
`path`	List of strings representing position within the parameter tree (initially derived from `uri`; overridden by `set_path`)
`name`	`"_".join(path)` — becomes the fastcs attribute name

set_path(path) writes _path directly; the path property returns it in preference to uri.

When to call `set_path`#

set_path should be called inside initialise() of an OdinSubController subclass, before add_attribute(parameter.name, ...) is called. At that point the controller’s own position in the hierarchy is known, so the parts of the path already captured by that position can be removed.

Common patterns#

Strip a leading index added by `ControllerVector`#

OdinDataAdapterController is a ControllerVector. During initialise() it groups parameters by their leading integer index, creates a sub-controller for each index, and stores it at self[int(idx)]. ControllerVector automatically incorporates that index into the sub-controller’s path hierarchy, so the index in the parameter URI is now redundant. Strip it inside the sub-controller before creating attributes:

async def initialise(self):
    for parameter in self.parameters:
        parameter.uri = parameter.uri[1:]          # ["0", "status", "hdf", "frames"] → ["status", "hdf", "frames"]
        parameter.set_path(parameter.uri[1:])      # also strip "status"/"config"
        #                                          # path: ["hdf", "frames"]
        #                                          # name: "hdf_frames"
        self.add_attribute(
            parameter.name,
            create_attribute(parameter=parameter, api_prefix=self._api_prefix),
        )

Note: parameter.uri is also updated here so that any sub-controllers created afterwards receive a URI that no longer contains the stale index.

Strip a status/config wrapper#

Controllers sitting directly under a status or config node can remove that prefix. MetaWriterAdapterController drops both the leading index and the status/config segment in one step using a pattern match:

async def initialise(self):
    for parameter in self.parameters:
        match parameter.uri:
            case ["0", "status" | "config", *_]:
                parameter.set_path(parameter.path[2:])
                # uri:  ["0", "status", "timestamp"]
                # path: ["timestamp"]
                # name: "timestamp"

Strip the plugin name in a plugin sub-controller#

When a FrameProcessorPluginController is created it receives parameters that already have the leading status/config stripped, but still begin with the plugin name (hdf, blosc, …). Since the controller is that plugin, the name is redundant:

async def initialise(self):
    for parameter in self.parameters:
        parameter.set_path(parameter.path[1:])   # remove leading plugin name
        # path before: ["hdf", "frames_written"]
        # path after:  ["frames_written"]
        # name:        "frames_written"

Rename to resolve a clash#

Sometimes two URIs would produce the same name after stripping, or the auto-generated name conflicts with an existing hand-written attribute. set_path can assign a unique, descriptive name:

if parameter.uri == ["status", "hdf", "file_path"]:
    parameter.set_path(["current_file_path"])

elif parameter.uri == ["status", "hdf", "acquisition_id"]:
    parameter.set_path(["current_acquisition_id"])

Strip a fixed prefix for deeply nested parameters#

For parameters nested several levels deep where the first N segments are always the same, slicing by index removes the redundant prefix. FrameProcessorDatasetController only ever sees dataset parameters, so the status/hdf/dataset prefix carries no information:

async def initialise(self):
    for parameter in self.parameters:
        # uri: ["status", "hdf", "dataset", "raw", "compression"]
        parameter.set_path(parameter.uri[3:])
        # path: ["raw", "compression"]
        # name: "raw_compression"

What does not change#

set_path only affects OdinParameter.name, which is used as the Python attribute name on the controller and as the PV suffix in EPICS. It has no effect on how the parameter is read or written: HTTP request paths are always derived from parameter.uri (and the api_prefix passed to create_attribute), not from path.

Simplify Parameter Paths with set_path#