The solution is to use self.db.query instead of self.db.get!
self.lock_tables("read", ['nets_permissions as n', 'devices_permissions as d'])
usrs = self.db.query("SELECT n.user_id FROM nets_permissions as n \
left join devices_permissions as d \
on n.user_id = d.user_id \
where d.user_id is null \
and n.network_id=%s and n.perm<>3", netid)
self.unlock_tables()
for usr in usrs:
self.lock_tables("write", ['devices_permissions'])
self.db.execute("INSERT devices_permissions SET \
user_id=%s, network_id=%s, device_id=%s, perm=%s",\
usr['user_id'], netid, sensid, perm)
self.unlock_tables();