There's two things that people keep asking for:

1. validation on setting attributes
2. freezing single attributes

Those two features have something in common: they require attrs to write a __setattr__ method.

I actually had 1 done when I implemented validators but I took it out again, because I didn't want to tamper with __setattr__ too. But it totally makes sense to expect that validators run there too.

Now that argument has gone away thanks to frozen classes and attrs is in the __setattr__ business. So it feels like the right thing to do, to implement it and make it default for Operation import attrs (I hope this is legit the last part of the puzzle).

To allow for 2 too, I would suggest to add a hook called on_setattr (better names welcome) that takes a callable that is called with the instance, the attribute definition, and the new value.

To solve 2, the implementation would look like

def frozen(_, __, ___):
    raise FrozenInstanceError

Open questions:

• what to do about on_setattr attributes in a frozen class (incl inheritance)
• what about converters? Maybe it should take a list/and like validator/converter do? They would need to work as a chain, returning values for the next one.