Is this just list(self._components.values())?

this may lead to duplicate indices. e.g. stress supports both "xy" and "yx" as self._components. self.components are the particular (say stress) components an observable "tracks" hence the need to access those particular values from the dict.

I've changed this so there is no allowed_components property, just a _allowed_components member to track this. Hopefully makes this clearer.

Don't like that this isn't StressShear to go with others. May also want StressHydrostatic rather than Diagonal?

those are both good ideas,StressHydrostatic sounds good, unless someone can think of better I'll go with it

Blindly applying this isn't right.

You no longer need to check the if, as that's done by the operation. component_names is now just undefined and need to refer to the components which are not defined.

if any(self.allowed_components.keys() - components.keys()):
    raise ValueError(
                    f"'{','.join(self.allowed_components.keys() - components.keys())}' invalid, must be in '{', '.join(self.allowed_components)}'"
    )

yes, unfortunately I did check the changes locally but then added them all together anyway... the code was odd due to the line length but this can be solved

It's my problem really, I click "suggestion" because I'm too lazy to match the formatting and then it renders with "add this" even when it's invalid code. I should change the type to Python

I think this is a bit messy and confusing, personally would prefer something more like:

        stresses = sliced_atoms.get_stress(
                include_ideal_gas=self.include_ideal_gas, voigt=True
            ) / units.GPa
        return stresses[self._indices]

I've split the index onto the return line as above, it is clearer.

It might be best to move the list case into slicelike_len_for so we can have a clean interface here. Maybe selector_len or something in that case?

done, but this also made me realise slicelike_len_for is not defined due to the import guarding (ultimately a circular import issue from before). @ElliottKasoar I have moved CorrelationKwargs into correlator.py so there is no longer an issue with Observable and SliceLike circular imports.

Is it worth checking if index is a StartStopStep (or any other valid input)?

do you mean throwing an error if it is not actually a SliceLike?

i.e. these cases are no good but "run"

>>> slicelike_to_startstopstep([1,2])
[1, 2]
>>> slicelike_to_startstopstep((None, None, None))
(None, None, None)

its probably a good move

We could do

def validate_slicelike(maybe_slicelike: SliceLike):
    # ...
    if isinstance(maybe_slicelike, (slice, range, int)):
        return
    if isinstance(maybe_slicelike, tuple) and len(maybe_slicelike) == 3:
        start, stop, step = maybe_slicelike
        if (
            isinstance(start, Optional[int])
            and isinstance(stop, Optional[int])
            and isinstance(step, int)
        ):
            return

    raise ValueError(f"{maybe_slicelike} is not a valid SliceLike")

?

Yes, that sort of thing would be great