Python; class instances

Question

Suppose I have a class, call it class1, with 3 class variables var1,var2,var3, and __init__ method, which assigns passed arguments to the class variables:

class class1(object):
    var1 = 0
    var2 = 0
    var3 = 0

    def __init__(self,a,b,c):
       class1.var1 = a
       class1.var2 = b
       class1.var3 = c

Now I'm going to make two instances of the same class:

obj1 = class1(1,2,3)
obj2 = class1(4,5,6)

And now, let's take a look at variables values:

print (obj1.var1, obj1.var2,obj1.var3)
4 5 6
print (obj2.var1, obj2.var2,obj2.var3)
4 5 6

Shouldn't obj1 have values 1,2,3 ? Why __init__ method of the second instance changes vaues in the first instance(obj1)? And how to make two independent separate instances of a class?

You have specifically made these class, not instance, attributes. If you want instance attributes, use e.g. self.var1 = a, and remove the assignments outside __init__. — jonrsharpe
– jonrsharpe, Commented Mar 19, 2014 at 15:47

Christian Tapia · Accepted Answer · 2014-03-19 15:56:00Z

4

Variables declared in the class definition, but not inside a method, will be class variables. In other words, they will be the same for the whole class.

To solve this you could declare them in the __init__ method, so they become instance variables:

class class1():
    def __init__(self,a,b,c):
        self.var1 = a
        self.var2 = b
        self.var3 = c

edited Mar 19, 2014 at 15:56

answered Mar 19, 2014 at 15:46

Christian Tapia

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falsetru · Accepted Answer · 2014-03-19 15:46:43Z

2

Class variables are shared by all instances of the class (and the class itself). You need to use instance variables instead.

>>> class class1(object):
...     def __init__(self,a,b,c):
...        self.var1 = a
...        self.var2 = b
...        self.var3 = c
...
>>> obj1 = class1(1,2,3)
>>> obj2 = class1(4,5,6)
>>> print (obj1.var1, obj1.var2,obj1.var3)
(1, 2, 3)
>>> print (obj2.var1, obj2.var2,obj2.var3)
(4, 5, 6)

answered Mar 19, 2014 at 15:46

falsetru

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