calculate magnetic field strength from raw magnetometer

def mag_field(RAW_IMU, SENSOR_OFFSETS=None, ofs=None):
    '''calculate magnetic field strength from raw magnetometer'''
    mag_x = RAW_IMU.xmag
    mag_y = RAW_IMU.ymag
    mag_z = RAW_IMU.zmag
    if SENSOR_OFFSETS is not None and ofs is not None:
        mag_x += ofs[0] - SENSOR_OFFSETS.mag_ofs_x
        mag_y += ofs[1] - SENSOR_OFFSETS.mag_ofs_y
        mag_z += ofs[2] - SENSOR_OFFSETS.mag_ofs_z
    return sqrt(mag_x**2 + mag_y**2 + mag_z**2)