[Zope] Changing a COPY of metadata modifies ZClass instance?

[Milos Prudek]

I think I'm going slightly mad :-)

A Python Script that is supposed to modify a COPY of metadata in 
practice changes metadata itself, and ZClass instance :-(

for i in context.Catalog_Org(userid=uid):
   l=i.userid
   print "title: ",i.title
   l.remove(uid)

When this script runs from ZMI, it changes metadata in Catalog_Org (but 
index data in Catalog_Org is not changed[!]), and it changes data 
displayed in ../propertysheets of the given ZClass instances.

I believed that "l=i.userid" will put a copy of userid metadata into 
"l", and that anything I will do with "l" will not influence anything 
except "l".

Why is it behaving like that?

Zope 2.4.3, Linux

-- 
Milos Prudek