Question

This should be simple…

class Object:
    x = 0
    y = []

a = Object()
b = Object()

a.x = 1
b.x = 2
print a.x, b.x
# output: 1 2
# works as expected

a.y.append(3)
b.y.append(4)
print a.y, b.y
# output: [3, 4] [3, 4]
# same list is referenced  how to fix?
# desired output: [3] [4]

As far as I can tell, a.y and b.y reference the same list. How can I get them to be separate? Preferably, without adding an __init__ method.

Answer 1

What’s happening here is that you actually have actually redefined x as an instance level attribute, and your class definition had them as both class level attributes.

If you do this, you can see your original x is still at 0.

>>> Object.x
0

As you don’t create a new list, it’s taking the class attribute. If you were to do this:

>>> a.y = []
>>> b.y = []
>>> a.y.append(1)
>>> b.y.append(2)
>>> print a.y, b.y
[1] [2]

That is what you are expecting. Really though you should be defining your class like this:

class Object(object):
    def __init__(self):
        self.y = []
        self.x = 0

(and don’t use Object as a classname!)