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.