Printing from within properties

Question

I'm trying to make a robotics kit. Its designed to be simple so I'm using properties so when the users change a parameter the property method sends the serial command which controls motors/ servos/whatever.

This is the code at the moment, directly from a previous question I asked on here.

class Servo(object):
def __init__(self, which_servo, angle = 0):
    self._angle = angle;
    self._servo_no = which_servo

def get_angle(self):
    return self._angle
def set_angle(self, value):
    self._angle = value
    print "replace this print statement with the code to set servo, notice that this method knows the servo number AND the desired value"

def del_angle(self):
    del self._angle
angle = property(get_angle, set_angle, del_angle, "I'm the 'angle' property.

this is then initialized as such:

class robot(object):
def __init___(self):
    self.servos = [Servo(0), Servo(1), Servo(2), Servo(3)]

Now, this works in the respect that it does change the variable through the getter and setter functions, however the prints in the getter and setter never is printed, thus if I replace it with a serial command I assume it won't do anything either, can anyone shed any light on this?

Thanks

Update: Thanks for the help using the servo file this is whats happened, there are three scenarios the first works and by extension I would have assumed the next two preferable scenarios would work but they don't any ideas?

This works

import servo

class Robot(object):
    def __init__(self):
        self.servos = [servo.Servo(0, 0), servo.Servo(1,0), servo.Servo(2,0)]

R = Robot()

R.servos[1].angle = 25

This does not:

import servo

class Robot(object):
    def __init__(self):
        self.servos = [servo.Servo(0, 0), servo.Servo(1,0), servo.Servo(2,0)]

R = Robot()

left_servo = R.servos[1].angle

left_servo = 25

Neither does this

import servo

class Robot(object):
    def __init__(self):
    self.servos = [servo.Servo(0, 0).angle, servo.Servo(1,0).angle,               servo.Servo(2,0).angle]

R = Robot()

R.servo[1] = 25

Answer 1

Using the preferred decorator syntax for properties, this works fine. It'll also help you avoid issues like this in the future

class Servo(object):
    def __init__(self, which_servo, angle = 0):
        self._angle = angle;
        self._servo_no = which_servo

    @property
    def angle(self):
        return self._angle

    @angle.setter
    def angle(self, value):
        self._angle = value
        print "replace this print statement with the code to set servo"

    @angle.deleter
    def angle(self):
        del self._angle

Seeing as your indentation is off here, I believe this is likely an issue of indentation in your source. This should work as well if you really want to use the old property function:

class Servo(object):
    def __init__(self, which_servo, angle = 0):
        self._angle = angle;
        self._servo_no = which_servo

    def get_angle(self):
        return self._angle

    def set_angle(self, value):
        self._angle = value
        print "replace this print statement with the code to set servo"

    def del_angle(self):
        del self._angle

    angle = property(get_angle, set_angle, del_angle,"I'm the 'angle' property.")

Both of these work successfully for me (inside a file called servo.py)

>>> import servo
>>> s = servo.Servo(1, 2)
>>> s.angle
2
>>> s.angle = 3
replace this print statement with the code to set servo

EDIT

To address your new issues. When you assign R.servos[1].angle to left_servo, its not creating a reference to the servos angle, it's just setting left_servo to whatever the angle is. When you reassign 25 to it, you're not assigning to the angle you're assigning to the left_servo.

On the second one, I'm assuming you meant R.servos and not R.servo which should be raising an AttributeError. But the real problem as I see it, is you should be saying R.servos[1].angle = 25 and you're omitting the .angle.

To (attempt to) put it simply: When you use the = operator, you are changing where a name refers to, not what it refers to.

>>> x = 1
>>> x = 2

the second assignment does not overwrite the 1 in memory with a 2, it just changes where x refers to. So if I did something like

>>> x = 1
>>> y = x
>>> y = 2
>>> print x
1

the output is 1 because your are telling y to refer to the same place that x refers. Changing y to 2 changes where y refers to, it does not change the 1 already in memory.