Feature: Defer Default with attr.DEFERRED object?

[peteroconnor-bc]

I often, when writing demos, find myself doing something like this:

@attrs
class MyController:
    controller_param_1 = attrib(default=0.4)
    controller_param_2 = attrib(default=7)
    def update(self, x):
        ...
        return motor

def run_demo(dataset_name, n_samples = 100, controller_param_1=0.4, controller_param_2=7):
    controller = MyController(controller_param_1=controller_param_1, controller_param_2=controller_param_2)
    for x in get_data(dataset_name, n_samples): 
        print(controller.update(x))

Yes, I know I could just pass in MyController to run_demo(...) instead of the params - and that (dependency injection) is the right way to do things when designing scalable library code, but it's often easier to just pass in the args directly - run_demo(...) is easier to understand when you don't have to think about how to construct the arguments.

The trouble is that now we've defined defaults controller_param_1=0.4, controller_param_2=7 in multiple places, which violates "Don't Repeat Yourself" and can very easily lead to bugs.

Yes, I know I could just replace controller_param_1=0.4, controller_param_2=7 with **params, but that just obfuscates the interface of run_demo(...) which is exactly what we were trying to avoid in the first place.

My proposal is to just have an attr.DEFERRED object which, when passed as a constructor argument of an attr object, just looks up the default. i.e. change run_demo to:

def run_demo(dataset_name, n_samples = 100, controller_param_1=attr.DEFERRED, controller_param_2=attr.DEFERRED):
    controller = MyController(controller_param_1=controller_param_1, controller_param_2=controller_param_2)
    for x in get_data(dataset_name, n_samples): 
        print(controller.update(x))