Step-by-step guide
You need to use the GRIB Filter icon, which is implemented as the read() function in script. The following Python example shows how to read a GRIB file and extract temperature fields on pressure levels out of it:
import metview as mv f = mv.read(source='my_grib_file', param='t', levtype='pl')
Alternatively the GRIB file can be read into a fieldset object and you can apply the filter on it:
import metview as mv
g = mv.read('my_grib_file')
f = mv.read(data=g, param='t', levtype='pl')