Expose Run.is_default in facade

[glatterf42]

Working on iiasa/ixmp#552, I realized that ixmp4.core.Run should expose its is_default value, which is already present in run._model.
This PR exposes the value by using a function (and tests that calling set_as_default() and unset_as_default() update is_default()). It could also be a property, though it's more dynamic than e.g. Run.id or Run.model, that's why I opted for a function: to indicate that this value needs to be retrieved when it's requested.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.9%. Comparing base (c8b231c) to head (64b32ca).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #153   +/-   ##
=====================================
  Coverage   88.9%   88.9%           
=====================================
  Files        236     236           
  Lines       8656    8661    +5     
=====================================
+ Hits        7698    7703    +5     
  Misses       958     958           

Files with missing lines	Coverage Δ
ixmp4/core/run.py	100.0% <100.0%> (ø)

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[meksor]

It's probably best to make this a @property like the rest of the attributes?

[meksor]

Suggested change

	def is_default(self) -> bool:
	@property
	def is_default(self) -> bool:

[glatterf42]

Sure, we can do that :)
Would we want this to be a read-only property or should this get a setter and deleter (that would then presumably call Run.set_as_default() and Run.unset_as_default())?

[glatterf42]

I've not written setter and deleter for now since they are not required for the message_ix tutorials. If we want them at a later point, I propose a follow-up issue/PR.

We would likely want something of this sort:

@is_default.setter
def is_default(self, value: bool) -> None:
    if value:
        self.set_as_default()
    else:
        self.unset_as_default()

If it bothers us that mypy deems one test case as unreachable, we might want to set up an attribute _is_default and use is_default.setter to set that directly. Though even if is_default.getter simply returns the state of _is_default, mypy might not recognize this properly because it's hidden behind this extra layer.

[meksor]

Yeah, I agree lets leave that for another time...

[glatterf42]

Maybe @danielhuppmann can clarify whether we actually want that :)

[danielhuppmann]

I think doing is_default as a property is a good idea, but I would stick with the more "functional" set_as_default() to change the status, to avoid confusion or making changes by accident.