using return upper

Hello,
I'm trying to modify my admin.py from my app "ordis":

from ordis.models import Ordi, Maintenance, OperatingSystem
from django.contrib import admin

#class MaintenanceAdmin(admin.ModelAdmin):
#list_display = (???) here I would like to see my Computer id, and
the OS installed on it

def renvoi_os(Ordi):
#return ("%d" % (Ordi.id)).upper()
return ("%d %d" % (Ordi.id, Ordi.operatingsystemused)).upper()
class MaintenanceAdmin(admin.ModelAdmin):
list_display = (renvoi_os,)
#class OrdiAdmin(admin.ModelAdmin):

admin.site.register(Ordi)
admin.site.register(Maintenance,MaintenanceAdmin)
admin.site.register(OperatingSystem)

That admin.py is associated with this models.py:

from django.db import models

from django.db.models import (Model, BooleanField,
CharField, DateTimeField, TextField, URLField,
EmailField, ManyToManyField, ForeignKey, IntegerField,
FileField, ImageField)

# Create your models here.

class OperatingSystem (models.Model):
operatingsystem = CharField (max_length=30, blank=True, null=True)
class Ordi(models.Model):
architecture = CharField (max_length=30, blank=True, null=True)
operatingsystemused = ManyToManyField(OperatingSystem, null=True,
blank=True)
class Maintenance(models.Model):
ordi=ForeignKey(Ordi, blank=True, null=True)
action = TextField(null=True, blank=True)

When I just call Ordi.id in admin.py (commented line), I get the Ordi
id associated with "Maintenance".
But the line with Ordi.id + Ordi.operatingsystemused gives me back a
(None)

What am I doing wrong??

THanks for your help

Florian

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