execution order of trait observers

[jgunstone]

its not clear to me in what order the trait observers get executed and why?
can anyone shed light on this?

class Test(tr.HasTraits):
    a = tr.Bool(default_value=True)
    b = tr.Bool(default_value=True)
    c = tr.Bool(default_value=True)

    @tr.observe("a")
    def _a(self, on_change):
        print(f"a = {self.a}")

    @tr.observe("b")
    def _b(self, on_change):
        print(f"b = {self.b}")

    @tr.observe("c")
    def _c(self, on_change):
        print(f"c = {self.c}")

    def __init__(self, **kwargs):
        super().__init__(**kwargs)


t1 = Test(a=False, b=False, c=False)
print("---")
t2 = Test(c=False, b=False, a=False)

# b = False
# c = False
# a = False
# ---
# b = False
# c = False
# a = False

many thanks

[jgunstone]

based on this:
#148

I implemented something like this for my use case:

class Test(tr.HasTraits):
    c = tr.Bool(default_value=True)
    b = tr.Bool(default_value=True)
    a = tr.Bool(default_value=True)

    @tr.observe("a")
    def _a(self, on_change):
        print(f"a = {self.a}")

    @tr.observe("b")
    def _b(self, on_change):
        print(f"b = {self.b}")

    @tr.observe("c")
    def _c(self, on_change):
        print(f"c = {self.c}")

    @classmethod
    def trait_order(cls):
        return [k for k, v in cls.__dict__.items() if isinstance(v, tr.TraitType)]

    def __init__(self, **kwargs):
        super().__init__()
        kwargs = self.get_ordered_kwargs(kwargs)
        {setattr(self, k, v) for k, v in kwargs.items()}

    def get_ordered_kwargs(self, kwargs):
        in_order = list(kwargs.keys())
        tr_order = self.trait_order()
        out_order = tr_order + [i for i in in_order if i not in tr_order]
        return {o: kwargs[o] for o in out_order}


t1 = Test(a=False, b=False, c=False)
print("---")
t2 = Test(c=False, b=False, a=False)

# c = False
# b = False
# a = False
# ---
# c = False
# b = False
# a = False

I'll leave the isse open for a while in case someone can suggest a "more traitlets native" way to approach this

[Paul-Aime]

Might not be directly related, but as a note, if multiple callbacks created with @observe listen on the same trait, then they will be fired in alphabetical order.

Would be better to follow the definition order though.

class Test(tr.HasTraits):
    trait = tr.Bool(default_value=True)

    @tr.observe("trait")
    def _b(self, on_change):
        print("From _b")

    @tr.observe("trait")
    def _c(self, on_change):
        print("From _c")

    @tr.observe("trait")
    def _a(self, on_change):
        print("From _a")

t = Test()

t.trait = False

# From _a
# From _b
# From _c

This is due to for _, v in getmembers(cls) in traitlets.py#L985 MetaHasDescriptors.setup_class(...) where getmembers uses python dir, which in turns returns a sorted dictionary.