t.py

import attr
from attr import fields
from attr.exceptions import AttrsAttributeNotFoundError
from attr.validators import instance_of, optional


@attr.s
class A:
    """A trivial class."""
    a = attr.ib()


@attr.s
class B:
    """A less trivial class."""
    b = attr.ib(validator=instance_of(str))
    c = attr.ib(convert=int)
    d = attr.ib(validator=optional(instance_of(float)))
    e = attr.ib(default=1)


@attr.s
class C(B, A):
    """A big class."""
    f = attr.ib(convert=str, default='f')
    g = attr.ib(default=7)
    h = attr.ib(validator=instance_of(int), default=8)
    i = attr.ib(default=attr.Factory(lambda: b'11'))


def assoc(inst, **changes):
    """An optimized assoc."""
    for a in fields(inst.__class__):
        name = a.name
        if name not in changes:
            changes[name] = getattr(inst, name)
    try:
        return inst.__class__(**changes)
    except TypeError as exc:
        k = exc.args[0].split("'")[1]
        raise AttrsAttributeNotFoundError(
            "{k} is not an attrs attribute on {cl}."
            .format(k=k, cl=inst.__class__))