Maybe, converters and validators can (or event should) be merged (similarly to click callbacks)?

def int_validator(self, attrib, value):
    return int(value)  # Validates and converts at the same time :-)

I guess that would be very backwards incompatible, but maybe this can (still) be added to the new attrs API?

To support one, two and three-argument converters there should be either inspect-magic (in py2 - getargspec, in py3 - signature) or mandatory Converter wrapper for two and three-argument converters.

This is something I've been thinking about in the past days. I think a marker-wrapper for three-argument converters would be most solid and in line with our other APIs?

There's also #146 which still bothers me that it's unresolved but to which I still don't have a good answer, 3.5 years later. :(

Thank you for the discussion!

@sscherfke I think this is kind of thing that happened to on_setattr already. The only big difference here is that converters should work with half-initialized self, but @default shows that it can be tackled by users (and #404 shows that some of them are actually craving that possibility in a converter).

Personally I like separate validators, as they are running after all the conversions (hence - guaranteed to work with fully-initialized self!) and imply that the field value doesn't change (unless some reckless guy will do object.__setattr__(self, attr.name, new_value) inside, but it seems like a dirty hack and wouldn't pass any code-review. Or at least I hope so...)

@hynek #146 is pretty informative, thank you for mentioning it! :) I actually missed the stacktrace point when I was thinking about it (and maybe some performance issues, as there is a level of indirection here...). Though, I think that having on_setattr on board actually introduces some common API for a pipeline here.

I do like the marker-wrapper idea (e.g. Converter(fun, takes_self=True, takes_attr=True)), though it may be a bit unexpected for a newcomer that attr.ib(converter=c) expects one-argument function or wrapper, while attr.ib(validator=v, on_seattr=s) expect three-argument function (and no wrappers provided). But - considering possible issues with half-initialized self - it may be a good practice, actually.

UPD: Just saw the comment in the PR. Actually, if converter is kind of shortcut to something like on_init - it sounds like a great way to get away from this unexpected behavior :)