define validators/converters for attrs classes

[mikewilli]

See the thread at the link below, but here's the main comment:

+1 to "let's do it later, not necessary for this PR"

As I understand it, default defines the default argument value while converter applies a function to the argument. Converters allow you to differentiate between the types of the parameters and the types of the attributes

So

class MyClass:
 x = attr.field(default = None, type = str | None)

will define a class with a constructor like:

MyClass.__init__(self, x: str | None = None):
 self.x = x # type will be str | None

If you add a converter:

class MyClass: 
 x = attr.field(default = None, type = str, converter=attr.converter.default_if_none("default"))

you'll get a class with a constructor like:

MyClass.__init__(self, x: str | None = None):
  self.x = x if x is not None else "default" # type will be str, no longer optional

I think it comes down to a question of 1) what parameters do we want callers to be able to pass to the constructor and 2) what type do we want the class attributes to be.

Originally posted by @danielkberry in #553 (comment)