a question regarding call-by-reference

[enjoying the view]

I am working on a school project, trying to build a simple RPC stub
generator. The idea is that the generator takes a normal python file
with functions defined in it and produces a client and server stub. The
client stub has the same functions defined, but they just connect to
the server and ask it to call the desired functions. The server stub is
a server listening for incoming requests and dispatching them to the
appropriate functions and sending back the results. Extremely simple
and I've gotten it to mostly work, but I have one problem:
call-by-reference parameters. An example:

A function like this:
def sum(a, b):
  return a + b

would yield a client stub like this (simplified):
def sum(a, b):
   send_message({'funcname': 'sum', 'args': (a, b)})
   result = receive_message()
   return result

Now, this works just fine. I get problems when I try to do something
like this:
def add_elm(list):
   list.append('elm')
   return

Imported normally this would work fine. The list given as a parameter
would be one element larger. But when the stubs are generated, the
function doesn't return anything and the list is appended in the server
and the client-side list is left untouched. At first I thought the
solution was easy, just return the changed parameters and place them in
the parameter variables in the client stub:
def add_elm(list):
   send_message({'funcname': 'add_elm', 'args': (list)})
   response = receive_message()
   (list) = response.get('args')
   return response.get('result')

The problem is, this doesn't work. The original list doesn't point to
the changed list. I have been trying to figure this out and it seems
that if I just assign to the list variable it just modifies the local
(to the function) name space, and that those changes aren't reflected
in the list in the original name space.

I believe there are some ways around this, but I haven't found one that
would not require any special handling in either the code calling the
client stub or the original functions. I want to maintain transparency.

etv