intercepting attribute assignment

value.py

class InterceptableAttribute:
    def __init__(self, name, *twiddles):
        self.twiddles = twiddles

        # NOTE: this call and the 'name' arg will no longer be necessary in
        # Python 3.6
        self.__set_name__(None, name)

    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, instance, owner):
        if instance is None:
            return self

        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        if self.name in instance.__dict__:
            old_value = instance.__dict__[self.name]
            for twiddle in self.twiddles:
                value = twiddle(old_value, value)

        instance.__dict__[self.name] = value


class SomeClass:
    can_only_shrink = InterceptableAttribute('can_only_shrink', min)
    can_only_grow = InterceptableAttribute('can_only_grow', max)

    def __init__(self, start):
        self.can_only_shrink = start
        self.can_only_grow = start


foo = SomeClass(5)
print(foo.can_only_shrink, foo.can_only_grow)  # 5, 5
foo.can_only_shrink += 20
foo.can_only_grow -= 20
print(foo.can_only_shrink, foo.can_only_grow)  # 5, 5
foo.can_only_shrink -= 20
foo.can_only_grow += 20
print(foo.can_only_shrink, foo.can_only_grow)  # -15, 25
foo.can_only_shrink += 20
foo.can_only_grow -= 20
print(foo.can_only_shrink, foo.can_only_grow)  # -15, 25