Changing an instance variable from another class

Question

I'm trying to change self.var_1 in the code below from another class, but I receive the error test1 has no attribute 'var_1'. I feel like I'm making a simple mistake but can't seem to find the issue.

class test1:
    def __init__(self):
        self.var_1 = "bob"
        instance2 = test2()

    def change_var(self, new_var):
        print(self.var_1) #should print "bob"
        self.var_1 = new_var #should change "bob" to "john"
        print(self.var_1) #should print "john"

class test2:
    def __init__(self):
        test1.change_var(test1, "john")

instance = test1()

@SunnyPatel, doesn't work, yields error change_var() missing 1 required positional argument: 'new_var' — Matt
– Matt, Commented Jul 17, 2018 at 21:39
No. That would not follow proper OOD. You could instantiate test1 inside test2 and call instance.change_var there. — Sunny Patel
– Sunny Patel, Commented Jul 17, 2018 at 21:39
@SunnyPatel that sounds like a messy solution, I'm using this in a GUI and instantiating Test1 twice would open a bunch of random frames. — Matt
– Matt, Commented Jul 17, 2018 at 21:42
You should probably implement a Singleton Pattern if you are wanting to have only one instance of it. — Sunny Patel
– Sunny Patel, Commented Jul 17, 2018 at 21:44

cdarke · Accepted Answer · 2018-07-17 21:45:01Z

3

var_1 is an instance variable, so you need to use the instance:

class test1:
    def __init__(self):
        self.var_1 = "bob"
        instance2 = test2(self)    # <<<<<< pass test1 instance

    def change_var(self, new_var):
        print(self.var_1) #should print "bob"
        self.var_1 = new_var #should change "bob" to "john"
        print(self.var_1) #should print "john"

class test2:
    def __init__(self, t1obj):      # <<<< take test1 instance
        t1obj.change_var("john")    # <<<< use test1 instance

instance = test1()

Gives:

bob
john

answered Jul 17, 2018 at 21:45

cdarke

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Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

Based on your comments, I feel like you are looking to create a singleton pattern for your class.

There are multiple ways to accomplish this, most of which can be found in the above SO question.

I made one that uses the first method outlined in the top answer by creating a decorator, and applying it to your class.

def singleton(class_):
    instances = {}
    def getinstance(*args, **kwargs):
        if class_ not in instances:
            instances[class_] = class_(*args, **kwargs)
        return instances[class_]
    return getinstance

@singleton
class Test1(object):
    def __init__(self):
        self.var_1 = "bob"

    def change_var(self, new_var):
        print(self.var_1) #should print "bob"
        self.var_1 = new_var #should change "bob" to "john"
        print(self.var_1) #should print "john"

class Test2(object):
    def __init__(self):
        test1 = Test1()
        test1.change_var("john")

Test2()

You can view this on repl.it and see the results match your output.

bob

john

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