Instance variable as function of other instance variables

Question

Is it possible to define an instance variable in a class as a function of another? I haven't gotten it to work unless you redefine the "function instance variable" all the time.

Basically you could have a scenario where you have one instance variable that is a list of integers, and want to have the sum of these as an instance variable, that automatically redefines every time the list is updated.

Is this possible?

class Example:
    list_variable = []
    sum_variable = sum(list_variable)

    def __init__(self, list_variable):
        self.list_variable = list_variable
        return

This will result in sum_variable = 0 unless you change it.

I understand that this is far from a major issue, you could either define sum_variable as a method or redefine it every time you change list_variable, I'm just wondering if it's possible to skip those things/steps.

Possible duplicate of Real world example about how to use property feature in python? — mkrieger1
– mkrieger1, Commented Nov 26, 2019 at 15:57
sum_variable could be a property that is calculated from list_variable (possibly also cached) when it is accessed. — mkrieger1
– mkrieger1, Commented Nov 26, 2019 at 15:57

jfaccioni · Accepted Answer · 2019-11-26 15:58:24Z

4

Python offers the property decorator for a syntatically identical use of your example:

class Example:
    list_variable = []

    def __init__(self, list_variable):
        self.list_variable = list_variable
        return

    @property
    def sum_variable(self):
        return sum(self.list_variable)

e = Example(list_variable=[10, 20, 30])
e.sum_variable  # returns 60

answered Nov 26, 2019 at 15:58

jfaccioni

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3 Comments

David Buck Over a year ago

I was looking at this earlier. The property isn't quite equivalent to the OP's question as Example.list_variable would be [10,20,30] but Example.sum_variable gives a property object. Is there any way of returning Example.sum_variable that would return 3, rather than having to create an instance of Example?

jfaccioni Over a year ago

You'd probably have to use something like this: using property on classmethods. That being said, in my code above Example.list_variable would still be [], not [10, 20, 30], because the creation of my instance e does not affect the class variable list_variable - it just overwrites the self.list_variable namespace for that particular instance, and not for the class as a whole.

David Buck Over a year ago

Good point. Then again, just realised that the post does talk about instance variables, even though the example defines sum_variable as a class variable, so I guess it should answer the OP.

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