On Sat, Oct 22, 2011 at 5:26 PM, Gnarlodious <gnarlodi...@gmail.com> wrote: > Say this: > > class tester():
Style note: either have it explicitly subclass `object`, or don't include the parens at all. Empty parens for the superclasses is just weird. > _someList = [0, 1] > def __call__(self): > someList = self._someList > someList += "X" > return someList > > test = tester() > > But guess what, every call adds to the variable that I am trying to > copy each time: > test() >> [0, 1, 'X'] > test() >> [0, 1, 'X', 'X'] > > > Can someone explain this behavior? The line `someList = self._someList` does NOT copy the list. It make `someList` point to the same existing list object. Hence, modifications to that object from either variable will affect the other. Similarly, `someList += "X"` modifies someList *in-place*; it does not produce a new list object. The upshot is that you're just modifying and returning references to *the same list* repeatedly, never producing a new list object. > And how to prevent a classwide > constant from ever getting changed? Python doesn't have any language-enforced notion of constants. So, short of writing/using a library to try and enforce such a notion, you're out of luck. You could use an immutable datatype (e.g. a tuple) instead of a mutable one (e.g. a list) as some level of safeguard though. Cheers, Chris -- http://rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
