How to get object attributes to update dynamically in Python

Question

I'd like to create a class that has 2 input attributes and 1 output attribute such that whenever one of the input attributes are modified the output attribute is modified automatically

I've tried defining the attributes as instance variables within and outside the constructor function but in either case, after instantiating the object, the output attribute remains fixed at the value set at the moment of instantiation

class Example():

    def __init__(self,n):
        self.name=n

    inA=1
    inB=1

    if inA==1 and inB==1:
        outA=1
    else:
        outA=0

when instantiated outA is set to 1 as expected but if I try to update: object.inA=0

object.outA remains 1 whereas I need it to be updated to 0

Trying to avoid the use of functions if possible. New to python and OOP so sorry if this question is nonsensical or has an obvious answer

Code defined at class level (outside of a method) is run only once at class initialization in the context of the class (not the instance). — Klaus D.
– Klaus D., Commented May 14, 2019 at 3:17

kosayoda · Accepted Answer · 2019-05-14 03:24:37Z

5

If you want instance attributes that depend on other instance attributes, properties are the way to go.

class Example:
    def __init__(self, n):
        self.name = n
        self.inA = 1
        self.inB = 1

    @property
    def outA(self):
        return self.inA and self.inB

You access outA like a regular instance attribute, obj.outA.

>>> my_obj = Example("example")
>>> my_obj.outA
1

Changing the attributes inA and inB affect outA.

>>> my_obj.inA = 0
>>> my_obj.outA
0

answered May 14, 2019 at 3:24

kosayoda

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Comments

U13-Forward · Accepted Answer · 2019-05-14 03:19:30Z

0

You can create a function in the class and some other minor changes:

class Example():

    def __init__(self,n):
        self.name=n
        self.inA=1
        self.inB=1

    def f(self):
        if self.inA==1 and self.inB==1:
            self.outA=1
        else:
            self.outA=0

To call it:

a = Example('foo')
a.inA = 0
a.f()
print(a.outA)

Output:

As you can see, taking out:

a.f()

line would make it give an error:

AttributeError: 'Example' object has no attribute 'outA'

answered May 14, 2019 at 3:19

U13-Forward

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1 Comment

plowcow Over a year ago

sorry new to stackexchange didn't see the comment/know about accepting until now

Artemis F · Accepted Answer · 2019-05-14 03:30:54Z

0

Do you want it to return your output?

Expanding on U9-Forward's answer:

class Example():

    def __init__(self,n):
        self.name = n
        self.inA = 1
        self.inB = 1

    def f(self):
        return self.inA and self.inB

edited May 14, 2019 at 3:30

answered May 14, 2019 at 3:24

Artemis F

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