<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def edit(self, id):
 """ Edit a prefix. """


        # find prefix
        c.prefix = Prefix.get(int(id))

        # we got a HTTP POST - edit object
        if request.method == 'POST':
            c.prefix.prefix = request.params['prefix_prefix']
            c.prefix.description = request.params['prefix_description']

            if request.params['prefix_node'].strip() == '':
                c.prefix.node = None
            else:
                c.prefix.node = request.params['prefix_node']

            if request.params['prefix_country'].strip() == '':
                c.prefix.country = None
            else:
                c.prefix.country = request.params['prefix_country']

            if request.params['prefix_comment'].strip() == '':
                c.prefix.comment = None
            else:
                c.prefix.comment = request.params['prefix_comment']

            if request.params['prefix_order_id'].strip() == '':
                c.prefix.order_id = None
            else:
                c.prefix.order_id = request.params['prefix_order_id']

            if request.params['prefix_customer_id'].strip() == '':
                c.prefix.customer_id = None
            else:
                c.prefix.customer_id = request.params['prefix_customer_id']

            if request.params['prefix_vrf'].strip() == '':
                c.prefix.vrf = None
            else:
                # TODO: handle non-existent VRF...
                c.prefix.vrf = VRF.list({ 'rt': request.params['prefix_vrf'] })[0]

            if request.params.get('prefix_monitor') is not None:
                c.prefix.monitor = True
            else:
                c.prefix.monitor = False

            c.prefix.alarm_priority = request.params['prefix_alarm_priority']
            c.prefix.save()
            redirect(url(controller='prefix', action='list'))


        return render('/prefix_edit.html')