Right, of course it was the mutable use of an internal data structure.

  class Foo:
	bar = [1]

  foo1 = Foo()
  foo2 = Foo()
  foo2.bar.append(2)
  print foo1.bar
  print foo2.bar

prints [1, 2] [1, 2]

And that's the reason why I don't like that the standard way I define common instance members is an init function. Are there any decorators I could use to do that outside of any class function?