Float64 to float in python

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I'm publishing Float64 type msg via rosserial python node to ros and I want to subscribe those messages. But the python code that subscribing Float64 msgs gives error and now I Float64 cannot convert to float.. Is there any way to do this? I tried with strings and convert it to float. But it failed.

import roslib; roslib.load_manifest('UWU_Bot')
import rospy
from geometry_msgs.msg  import Twist
from std_msgs.msg       import Float64

def callback(volt):
    Distance = 16.2537*volt**4 - 129.893 * volt**3 + 382.268 * volt**2 - 512.611 * volt + 306.439

def nav():
    rospy.init_node('UWU_Bot')
    pub = rospy.Publisher('cmd_vel', Twist)
    rospy.Subscriber("/IR_volts", Float64 , callback)
    rospy.spin()

if __name__ == '__main__':
    try:
        nav()
    except rospy.ROSInterruptException: pass

This is the python code. And I used rosserial IR_ranger example to send the Float64 data.