Add a default attribute to properties

[rwb27]

This PR adds a default attribute to properties, where it can be defined. Currently, this applies only to DataProperty and DataSetting as we don't have a neat way to define a default for functional properties. The default may be accessed in two ways:

1. As thing.properties['name'].default (from Python)
2. As the default key in the PropertyAffordance in the Thing Description.

I have also added a reset method, which resets the property to that value, if it exists.

If you attempt to reset or get the default of a FunctionalProperty we raise a new FeatureNotAvailable exception which I hope makes it clear.

The last step is to add a way to reset properties over HTTP. I am minded to make that a separate PR, because:

• The current (standard compliant) PR already makes it possible to reset, because the default value is exposed in the TD.
• There's no WoT standard way to reset properties to default, as far as I know.

I can see three sensible ways to expose resetting over HTTP:

• Add an extra POST endpoint: POST http://my.server:5000/my_thing/my_property/reset
• Add a DELETE endpoint at the property URL DELETE http://my.server:5000/my_thing/my_property
• Add an action that takes the property name as a parameter: POST http://my.server:5000/my_thing/reset_property with payload my_property.

Of these, I think we discussed DELETE before and nobody liked it. I think the first one is clearest if we ignore WoT standards. The action is fine, but feels ugly, and currently it involves a lot more HTTP requests than it really needs, as the action would need polling to completion to find out whether it was successful.

I'd be happy to merge this in its present state (i.e. without extra endpoints to reset) and we can discuss what to do about resetting. If we get a consensus quickly, I have also written the code to do resets, but not yet tested it.

Closes #257

[barecheck]

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Total: 96.52%

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src/labthings_fastapi/properties.py	504, 794, 798, 821-824, 896, 915, 1166

[julianstirling]

I've tried to go through it. I think it is going to be annoying to have some property resetable and others not so I would really prefer FunctionalProperties to be resettable as quote a few of our properties have side effects and are still functional.

This being said it isn't too clear to me how the extra methods for a descriptor are defined hence some of the questions that go deeper.

[julianstirling]

It is generally recommended for all exceptions to end with Error. On the OFM there is a Ruff rule that enforces this.

[julianstirling]

It would be nice if FunctionalProperty could allow setting a reset method as:

@myprop.reset
def _reset_myprop(self):
    """Resetting my property."""
    self._myprop = 1066

or even

myprop.default = 1066

[julianstirling]

Actually would it be possible to pass an an argument to the decorator?

_my_property=1066

@lt.property(default=1066):
def my_property(self):
    ...

I suppose could even then be:

_my_property=1066

@lt.property(default=_my_property):
def my_property(self):
    ...

[rwb27]

Having a reset function for a functional property makes sense to me, and I've deliberately left it so that this should be quite simple to implement. I was trying to keep this PR, so I've not added that feature in here, though if I'm adding more commits to add a reset endpoint, I could also add resetting for functional properties.

That said, if you've already reviewed this, it would be not unreasonable to merge it, and then add reset for functional properties in a future one.

Having a default for functional properties feels slightly odd to me, though as I write it I can't fully explain why. I think it's because the default for a DataProperty is the value it starts at - the same isn't necessarily true for a FunctionalProperty.

I really need to add side-effects! But that is an entirely separate PR...

[julianstirling]

Sorry, quite deep into the weed trying to follow this PR. Is this implemented? I don't see where it is defined, but I find eveything about these descriptors really really confusing

[julianstirling]

Ok I have checked it, and it works, I don't think it has a test. And I don't actually see how it is defined.

[rwb27]

I'm not quite sure what you mean by "it" here... do you mean setting myprop.readonly? If so, I was fairly sure there was at least one test for that. It's implemented by exactly the same code that implements

myotherprop: int = lt.property(default=whatever, readonly=True)

because in both cases, the code that generates the Thing Description and the endpoints is looking at the readonly attribute of the descriptor.

[rwb27]

In test_property.py you can search for ro_functional_property_with_setter or localonlysetting, both of which use this form of read-only.

I realise having test_property and test_properties is confusing. test_property is the more recently written test module, which is rather more bottom-up than the older module.