How to isolate a constant?

[MRAB]

On 23/10/2011 01:26, Gnarlodious wrote:
> 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?
>
'_someList' is part of the class itself.

This:

     someList = self._someList

just creates a new _reference to the list and this:

     someList += "X"

appends the items of the sequence "X" to the list.

Note that a string is also a sequence of characters, so:

 >>> x = []
 >>> x += "XY"
 >>> x
['X', 'Y']

Python will copy something only when you tell it to copy. A simple way
of copying a list is to slice it:

     someList = self._someList[:]