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
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