[Zope] Python script: problem with session

I did already post this with a different thread: probably the formulation was to complex. Or may be there is some very basic knowledge I'm missing. I'll try again. If I assign to variable in a Python script (say context.REQUEST['foo']) a value stored in SESSION (say context.SESSION['foo']), and then I change the retrieved value (the one in REQUEST, not the original one), also the variable in SESSION is changed. In this way I have lost my original information stored in SESSION! I've tryied to avoid this, but I can't manage to find how. Any suggestion? TIA, p.t.