python member variable of instance works like member variable, and some works like static variable

Question

i'm python newbie, and member variable of class works weird in my python code. some works like normal variable, but some works like static variable!

class Chaos:
    list_value = []
    value = "default"

    def set_value(self, word):
        self.list_value.append(word)
        self.value = word 

    def show(self, num):
        print(str(num) + "====")
        print("value : " + self.value)
        for st in self.list_value:
            sys.stdout.write(st)
        print("\n=====\n")

a = Chaos()
a.show(0)
a.set_value("A")
a.show(1)
b = Chaos()
a.show(2)
b.show(3)

result

0====
value : default

=====

1====
value : A
A
=====

2====
value : A
A
=====

3====
value : default
A
=====

but the last result of the test is different from what i expected in last test. There should be no "A" in the 'list_value' of the instance of 'b'. It was just created, and never have been added 'A' before. I added 'A' to the instance of 'a', not 'b'. But the result show me that there are also 'A' in 'b' More over, the 'list_value' and the 'value' in the class works differently. It looks like the both have same syntax. why do they work differently?

Answer 1

Those are, in fact, class variables. To create instance variables, initialize them in the __init__ function:

class Chaos:
    def __init__(self):
        self.list_value = []
        self.value = "default"

The reason value is behaving like instance variables is because you're setting it using self.value. When Python sees self.X it looks if there's a property X in your object, and if there is none, it looks at its class. Since you never set self.list_value, it's accessing the class variable, that is shared among all instances, so any modifiations will reflect in every other object.