the terminal response global 'name ' is not defined

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It is generally better to wrap these two functions into a Python class, and you can then set your global variables to that class' class variables. Then the order in which your callback/savior functions are run doesn't matter. Sample untested code:

class Driver:
    def __init__(self):
        self.img_sub = rospy.Subscriber("/ball_location",Point, self.drive_callback)
        self.pub_vel = rospy.Publisher('cmd_vel_mux/input/teleop', Twist, queue_size=5)
        self.infra_sub = rospy.Subscriber("/scan", LaserScan, self.savoir)

        self.obstacle = False
        self.vel = None

    def drive_callback(self, data):
        ball_x  = data.x
        ball_y  = data.y
        width   = data.z

        # Create Twist() instance
        self.vel = Twist()

        if ball_x < 0 and ball_y < 0:
            self.vel.angular.z = 0
        else:
            # Determine center-x, normalized deviation from center
            mid_x   = int(width/2)
            delta_x = ball_x - mid_x
            norm_x  = delta_x/width

            if norm_x > 0.15:
                print ("delX: {:.3f}. Tournons a droite!".format(norm_x))
                self.vel.angular.z = -0.2 #tourner vers la droite avec une vitesse de 0.2 m/s
            elif norm_x < -0.15:
                print ("delX: {:.3f}. Tournons a gauche!".format(norm_x))
                self.vel.angular.z = 0.2 
            elif abs(norm_x<=0.15):
                print ("delX: {:.3f}. regardons devant!".format(norm_x))
                if self.obstacle==False:
                    self.vel.linear.x = 0.3 
                    print("en avant!")
                else:   
                    self.vel.linear.x=0

        # publish vel on the publisher
        self.pub_vel.publish(vel)

    def savoir(self, msg):
        if msg.ranges[320] >0.5:
            self.obstacle=False
        elif msg.ranges[320]<0.5:
            self.obstacle=True

if __name__ == '__main__':
    driver_obj = Driver()
    rospy.spin()