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Exceptions

Instro-specific errors inherit from InstroError. 
from instro.lib.exceptions import InstroError

try:
    ...
except InstroError:
    ...
When a capability is unavailable, the library raises one of two distinct exceptions. They differ in why the capability is missing and, just as importantly, in what they inherit from.

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FeatureNotSupportedError

FeatureNotSupportedError (a subclass of InstroError) is raised by a concrete driver to declare that the connected instrument genuinely lacks a capability. For example, the BK9115 power supply has no overcurrent protection or remote sense, so its driver raises FeatureNotSupportedError for those methods. 
from instro.lib.exceptions import FeatureNotSupportedError

try:
    psu.set_remote_sense_enabled(True, channel=1)
except FeatureNotSupportedError:
    ...
Because it inherits from InstroError, except InstroError catches it.

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NotImplementedError

NotImplementedError is the Python builtin, not an InstroError. Each <Category>DriverBase declares its optional methods to raise NotImplementedError by default, and a driver overrides only the ones its instrument supports. Calling an optional method on a driver that has not overridden it therefore raises NotImplementedError: the capability is part of the category contract, but this driver does not implement it. Because NotImplementedError is a builtin, except InstroError does not catch it.

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Which one will I catch?

It depends on the driver behind the HAL:
  • A driver that explicitly declares a feature absent raises FeatureNotSupportedError.
  • A driver that has simply not overridden an optional base method raises NotImplementedError.
To handle both in one place, catch them together: 
from instro.lib.exceptions import FeatureNotSupportedError

try:
    psu.set_overcurrent_protection_level(5.0, channel=1)
except (FeatureNotSupportedError, NotImplementedError):
    ...
When writing a driver, raise FeatureNotSupportedError when the hardware lacks the feature, and leave the inherited NotImplementedError in place for optional methods that are not implemented yet.