python mapscript filteritem not working

[Marcos Sánchez Provencio]

Good morning (in Spain). This is my first post to the list.

I have set up a django app that uses mapscript to serve maps (not using
http://code.djangoproject.com/wiki/GeoDjango whatsoever).

I have a shp file with several layers of detail (wind predictions,
aggregated in 1x, 4x, 16x, etc.). I wanted to use filteritem and filter
to show only the level of desired detail.

If I put 
 FILTERITEM escala
 FILTER "16"
in a mapfile, it works as expected.

but

            self.capa.filteritem='escala'
            self.capa.setFilter('16')
does not.

Any hint why?