Access instance variable in python

Question

I have a classes a and b as below:

class a(object):
    counter=10
    def __init__(self):        
        self.counter=1
    def process(self):
        self.counter+=1
        return self.counter

class b(a):
    def __init__(self,a_obj):
        self.counter=a_obj.counter
    def process(self):
        result=self.counter
        return (result)

My so called main function is as :

a_object=a()
b_object=b(a)

for x in range(1,6):
    print(a_object.process())

print(b_object.process())

My results for which are as below:

But I want to have access to the last updated value of counter in A i.e. 6 (referencing the instance variable) rather than 10 (which is the value of the class variable).

Please advise where am I going wrong with this.

no, I am looking to access the last updated value in class a (in the example it is last updated as 6) and use it in class b for some operation (as of this example , just print) — Abhi
– Abhi, Commented Jul 19, 2017 at 7:54
You are passing a to b constructor when you probably want to pass a_object — EsotericVoid
– EsotericVoid, Commented Jul 19, 2017 at 7:55
a_obj.counter is a immutable object. Keeping a reference to it is pointless, keep a reference to a_obj instead. — Martijn Pieters
– Martijn Pieters, Commented Jul 19, 2017 at 7:57

Martijn Pieters · Accepted Answer · 2017-07-19 08:17:47Z

2

There is no point in keeping a reference to a_obj.counter. All you are doing is creating a new reference to an immutable object. You are not referencing the attribute on a_obj, only the object that a_obj.counter also points to.

But when a.process() is run, the counter reference is replaced by one that points to a different integer object. b.counter still references the initial value.

You want to keep a reference to a_obj itself, so you can ask it what the counter attribute there points to whenever you need it:

class b(a):
    def __init__(self, a_obj):
        self.a_obj = a_obj
    def process(self):
        result = self.a_obj.counter
        return result

You may want to read up on how Python names and attributes work; they are merely little name tags tied to the actual object. See Ned Batchelder's excellent Facts and myths about Python names and values article.

edited Jul 19, 2017 at 8:17

answered Jul 19, 2017 at 8:06

Martijn Pieters

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2 Comments

Abhi Over a year ago

Thanks a lot, that totally made sense and your explanation also gave me much clarity on the subject. Cheers.

oxfn Over a year ago

It also worths noting that in original code counter=10 declares it as the class member, not the instance member, which is not necessary and even confusing in this case. Instance fields should be defined in constructor only

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