[Zope] Python script: problem with session

[p.t.]

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.