[web2py] query using groupby and distinct count

[Fabiano Faver]

I`m trying to select distinct printers used in each day but i`m not 
managing to use the distinct argument

count = db.contadores.id_impressora.count()
for row in 
db((db.contadores.id>0)&(db.contadores.data<=today)).select(db.contadores.data,count,groupby=db.contadores.data):
        print row.contadores.data, row[count]

if I use something like
    
  db.contadores.id_impressora.count(distinct=db.contadores.id_impressora)

it gives:

Traceback (most recent call last):
  File "C:\web2py\gluon\restricted.py", line 204, in restricted
    exec ccode in environment
  File "C:/web2py/applications/OPM/controllers/default.py" 
<http://127.0.0.1:8000/admin/default/edit/OPM/controllers/default.py>, line 
581, in <module>
  File "C:\web2py\gluon\globals.py", line 172, in <lambda>
    self._caller = lambda f: f()
  File "C:/web2py/applications/OPM/controllers/default.py" 
<http://127.0.0.1:8000/admin/default/edit/OPM/controllers/default.py>, line 
516, in timeline_opm
    count = db.contadores.id_impressora.count(distinct=('id_impressora'))
TypeError: count() got an unexpected keyword argument 'distinct' 


How can I use distinct count here?