TangoAttrIORController ReadOne returns `None` on invalid position

[marceloalcocer]

The TangoAttrIORController.ReadOne method returns None on invalid (out-of-bounds) values of the underlying Tango attribute. This return value does not conform to the Sardana API for Readable controllers (see e.g. SEP6).

More specifically, TangoAttrIORController.ReadOne asserts the underlying Tango attribute value is within the calibrated bounds, and raises an exception on assertion failure. This exception is subsequently caught, but no value is returned or exception raised in the handler.

The ultimate result of this is that the pool raises a ValueError (with generic message) when attempting to read the value of an IOR device whose underlying Tango attribute is our of bounds, e.g.;

>>> ior = DeviceProxy("my/tango/ior")
>>> dev = DeviceProxy("my/tango/device")

>>> ior.tangoattribute
'my/tango/device/attribute'
>>> dev.attribute
100.0

>>> ior.calibration
'[[0.0, 1.0, 2.0], [10.0, 20.0, 30.0]]'
>>> ior.value
PyDs_PythonError: ValueError: my_tango_ior_ctrl.ReadOne(my_tango_ior[1]) return error: Expected value(s), got None instead

As the original exception is not re-raised, its relevant explanatory message ("Invalid position.") is suppressed. Although currently suppressed, this message itself could also be improved, e.g.;

• Explain that the value is invalid due to being out of bounds
• Suggest corrective actions (review IOR calibration, write valid value to underlying Tango attribute, etc.)

[ovallcorba]

Hi @marceloalcocer

Thanks for creating the issue. You are right that the error handling and the output messages can be improved to reflect better why the values are not valid. I created a PR (https://github.com/ALBA-Synchrotron/sardana-tango/pull/17) following your suggestions to improve them.

Best regards.

[marceloalcocer]

Hej @ovallcorba

Thanks for taking the time to look into this and crafting a PR :+1:

17 LGTM and should fix the issue