当列表用作Python中的属性时，当您更改列表中的单个值时，实际上不会调用设置方法

Question

Essentially what I'm trying to do is create a class that represents a robot arm.

Inside of the arm there's a list that represents the joints, and my hope was that whenever those joints are 'set', it would transmit the values to a secondary microcontroller that controls the PWM signals that physically set the servos.

My problem is that when I only want to move 1 joint, but leave the others to be the same, I can't just say arm.joints[0] = 5 , because it sets the value but doesn't actually call the setter.

Is there a better way to structure this?

class RobotArm(object):

    """An object that resembles a robotic arm with n joints"""
    def __init__(self):

        # Empty list of zeros representing each joint   
        self.joints = [0]*5

    @property
    def joints(self):
        return self.__joints

    @joints.setter
    def joints(self, vals):
        print "setting values"
        self.__joints = vals
        self.serial_transmission(vals)

Answer 1

One solution is to use a custom list :

class Joints(list):
    """A custom list owned by another object. The owner should have
    a before_set_joints() method and an after_set_joints() method.
    """
    def __init__(self, owner, *args):
        self.owner = owner
        list.__init__(self, *args)

    def __setitem__(self, key, value):
        self.owner.before_set_joints()
        list.__setitem__(self, key, value)
        self.owner.after_set_joints()


# Define the onwer's methods cooperatively
class RobotArm(object):

    def __init__(self):
        self.__joints = Joints(self, [0] * 5)  # make self the owner

    def before_set_joints(self):
        print 'setting values'

    def after_set_joints(self):
        self.serial_transmission(self.joints)  # note a slight change here

The property definition can be just as before, but the setter needs to be modified:

    @joints.setter
    def joints(self, vals):
        self.before_set_joints()
        self.__joints = Joints(self, vals)
        self.after_set_joints()

Demo (I defined serial_transmission to simply print "serial_transmission" and the list content):

>>> arm = RobotArm()
>>> print(arm.joints)
[0, 0, 0, 0, 0]
>>> arm.joints = [1] * 5
setting joints
serial_transmission [1, 1, 1, 1, 1]
>>> arm.joints[0] = 5
setting joints
serial_transmission [5, 1, 1, 1, 1]