import grass.script as grass

b1=grass.list_grouped('rast', pattern='rec_2*_b01')["sinnubes"]

name1 = [i.split('_')[1] for i in b1]
dates=[i.split('_')[0] for i in name1]

grass.run_command('r.mask', rast='puntosmonitoreo', maskcats=1)

for date in dates:
    expr1 = "f_{date} = (rec_{date}_b01 / rec_{date}_b02)".format(date=date)
    grass.mapcalc(expr1, overwrite=True)