read data from serial port and publish over a topic that stores float32'

edit

def publish():

    rospy.init_node('modem',anonymous=True)
    acous_node=rospy.Publisher('acoustic_data',Float32,queue_size=10)
    r=rospy.Rate(10)
    while not rospy.is_shutdown():
            g.data=serial_node.read()
            acous_node.publish(g)
            localtime = time.asctime( time.localtime(time.time()) )
            print "Local current time :", localtime
            r.sleep()

this is what i tried but serial data is in str type,i want to convert to float32 type .how do i do? thanx