How do you change the value of one attribute by changing the value of another? (dependent attributes)

Question

When assigning attributes to objects, I often find that I have to change attributes that are dependent upon others, say, light and darkness. Here's an example:

class shade:
    def __init__(self, light):
        self.light=light
        self.darkness=100-light

    def __str__(self):
        return (str(self.light) +  ',' + str(self.darkness))



>>> shade1=shade(30,70)
>>> shade1.light
30
>>> shade1.darkness
70

Now, while that is cool and all, what I would like is the same process to occur, but within the same object if a change in one attribute occurs. If I reset the property of light, I want darkness to incr/decr accordingly. I can do this with a function if it works to change the property of light, returning the new value of light/darkness, but I'd like a way to do this if I change the property of light simply by re-assigning its value, without the use of functions. Like this:

>>> shade1.light=50
>>> shade1.light
50
>>> shade1.darkness
50

Answer 1

Define darkness as a property

class shade:
    def __init__(self, light):
        self.light = light

    @property
    def darkness(self):
        return 100 - self.light

    def __str__(self):
        return '{},{}'.format(self.light, self.darkness)

Properties outwardly appear as attributes, but internally act as function calls. When you say s.darkness it will call the function you've provided for its property. This allows you to only maintain one variable internally.

If you want to be able to modify it by assigning to darkness, add a setter for the property

class shade:
    def __init__(self, light):
        self.light = light

    @property
    def darkness(self):
        return 100 - self.light

    @darkness.setter
    def darkness(self, value):
        self.light = 100 - value

Thereby actually modifying light. If you've never seen properties before, I'd recommend throwing in some print()s to the bodies of the functions so you can see when they are called.

>>> s = shade(70)
>>> s.light
70
>>> s.darkness
30
>>> s.light = 10
>>> s.darkness
90
>>> s.darkness = 20
>>> s.light
80