Proposal: nice way to write ad-hoc validators

[bluetech]

Hopefully this wasn't proposed before.

@attr.s
class C(object):
    x = attr.ib()
    y = attr.ib()

    @x.validator                                               
    def x_smaller_than_y(self, attribute, value):
        if value >= self.y:
            raise ValueError("'x' has to be smaller than 'y'!")

If you managed to guess what this does it means the syntax is intuitive enough :)

Similar syntax familiar from property.deleter and friends can also be useful for other parts, but for validators it feels like a natural fit (especially with the recent change to allow multiple validators).

[hynek]

That looks interesting indeed!

[hynek]

fixed in 37d38c4

[xguse]

My question is: why is attribute included? It is not used in the validator function. What purpose does this serve? How would it be used. The docs at this opint make it seem like this is required for the api to work but it is just ignored in the example given.

Can someone provide an example where attribute is used to illustrate the logic to me?

[bluetech]

The validator argument to attr.ib, which preceded the decorator, takes the same arguments. So the decorator takes them for consistency.

[hynek]

Yep that's the reason. Internally they're treated the same. We could add some magic to make it optional I guess but no one bothered so far.