How to isolate a constant?

[Gnarlodious]

Say this:

class tester():
	_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? And how to prevent a classwide
constant from ever getting changed?

-- Gnarlie