SubsetEnum behaviour is unexpected and/or not entirely documented

[shihab-dls]

It seems that setting a signal with type SubsetEnum allows strings that are not attributes of the enum. Passing other types, like an int, will raise a ValueError
ex: ValueError: 1 is not a valid TestEnums

Is this expected behaviour? And if so, should this be better documented in the SubsetEnum definition?

Steps To Reproduce

Steps to reproduce the behavior:

class TestEnums(SubsetEnum):
    TEST = "str"


class TestDevice(StandardReadable):
    def __init__(self, prefix: str, name: str = ""):
        with self.add_children_as_readables():
            self.signal = epics_signal_rw(TestEnums, f"{prefix}TEST")
        super().__init__(name=name)


async def test_string(RE):
    with DeviceCollector(mock=True):
        my_filter = TestDevice("test-prefix")
    await my_filter.signal.set("test_str")

[coretl]

This is expected behaviour. EPICS enums can be address by string value or integer index, but the integer index is generally more liable to change than the string value. For this reason we make SubsetEnum a str, Enum, and allow any other str to be put. The only exception to this is 2 member enums that are effectively True/False. For these you can use bool rather than SubsetEnum.

I'm a little confused about the code above, it looks like it should work, does it fail?

[shihab-dls]

Quick Note: Sorry, I've just edited the issue, as I mistakenly put signal.set("str") instead of signal.set("test_str") to show any str is accepted; although, I think you addressed this in your comment anyways @coretl

[olliesilvester]

As discussed, setting a SubsetEnum signal to an arbitrary string will fail at runtime on real devices, but not in unit tests due to how mock mode works. We should include in a docstring that this behaviour is different in unit tests