Run attribute validator only after __attrs_post_init__ ends

Question

I have:

@attr.s
class Example:
    number = attr.ib(validator=attr.validators.instance_of(int), init=False)

    def __attrs_post_init__(self):
        self.number = 'string'
        print('It seems, validation was running before:(')


t = Example()

How properly setup validation here? I want to validate self.number after instantiation.

Answer 1

There was a bit of a discussion when we implemented __attrs_post_init__ whether to run validators before or after __init__.

We decided for before, because their main raison d'être is to protect the class from wrong instantiation and giving you the confidence about what's in your attributes.

---

That said, you can always run validators manually using attr.validate():

@attr.s
class Example:
    number = attr.ib(validator=attr.validators.instance_of(int), init=False)

    def __attrs_post_init__(self):
        self.number = 'string'
        # ...
        attr.validate(self)

We do plan on making validation – and when it's performed – more flexible but nothing concrete came out of it yet.