how I convert string to float?

edit

I'm using arduino to send values from 0.00 to 5.00 and I'm trying to read them but I get this error

raise ROSSerializationException(str(e)) rospy.exceptions.ROSSerializationException: <class 'struct.error'="">: 'required argument is not a float' when writing 'x: "0.92"

this is the code, how can I convert the value to float?

def __init__(self): 
rospy.init_node('my_tf_broadcaster')
self.ser = serial.Serial('/dev/ttyACM0', 115200)
    self.get_odometry()

def get_odometry(self):
self.pub_tf = rospy.Publisher("/tf", tf.msg.tfMessage)
    change = 0.0
    while not rospy.is_shutdown():
            # Run this loop at about 10Hz
            rospy.sleep(0.1)

    message = self.ser.readline()       
    t = geometry_msgs.msg.TransformStamped()
    t.header.stamp = rospy.Time.now()
        t.header.frame_id = "odom"
        t.child_frame_id = "base_link"
        t.transform.translation.x = message.rstrip()    #trying to read this value
        t.transform.translation.y = 1.0
        t.transform.translation.z = 0.0
        t.transform.rotation.x = 0.0
        t.transform.rotation.y = 0.0
        t.transform.rotation.z = 0.0
        t.transform.rotation.w = 1.0
    tfm = tf.msg.tfMessage([t])
        self.pub_tf.publish(tfm)