Two instances of class share same vairable in Python

Question

So this is very wierd. The code below creates 2 instances of MyClass. One would expect that each has its own copy of the private variable __x which is a dictionary. However, my_class2 updates the value of __x in my_class1 which should not happen in my opinion?

What am I missing?

class MyClass:
    __x = dict()

    def __init__(self, name, init_value):
        self.__x[name] = init_value

    def get_value(self):
        return self.__x


if __name__ == '__main__':
    my_class1 = MyClass("one", 1)
    print("my_class1: {}".format(my_class1.get_value()))
    my_class2 = MyClass("two", 2)
    print("my_class2: {}".format(my_class2.get_value()))
    print("my_class1: {}".format(my_class1.get_value()))

The code above leads to the following output:

my_class1: {'one': 1}
my_class2: {'one': 1, 'two': 2}
my_class1: {'one': 1, 'two': 2}

Yes. Of course. Missed that one. Instance variables are defined in functions. — Paul, Nov 23 '18 at 13:04
Does this answer your question? [How to avoid having class data shared among instances?](https://stackoverflow.com/questions/1680528/how-to-avoid-having-class-data-shared-among-instances) — Karl Knechtel, Mar 27 '23 at 06:27

score 3 · Accepted Answer · answered Nov 23 '18 at 13:04

3

You have to declare __x inside the __init__ function, like this:

class MyClass:

    def __init__(self, name, init_value):
        self.__x = dict()
        self.__x[name] = init_value

    def get_value(self):
        return self.__x

answered Nov 23 '18 at 13:04

user9849588

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1
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Yes, I got that just after I posted. Thanks. – Paul Nov 23 '18 at 13:05

score 1 · Answer 2 · answered Nov 23 '18 at 13:16

In your case, the way you were defining __x - it was a class variable - i.e. a variable that is shared by all instances of this class.

You can see this if you run a dir on your instances:

dir(my_class1)
['_MyClass__x', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'get_value']

Note the first entry - it is same above and below.

dir(my_class2) 
['_MyClass__x', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'get_value']

Two instances of class share same vairable in Python

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