python-attrs - setting other attributes when one attribute is set

attrs_set_other_attributes.py

def _set_y(instance, attribute, value):
    object.__setattr_(instance, 'y', value)
    # returning a value is important, otherwise attribute will become None
    return value 

@define
class MyClass:
    x: str = field(on_settattr=[_set_y])
    y: str = field()