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class Arm:
    def __init__(self):
        self.sub = rospy.Subscriber('/rx150/joint_states', JointState, self.callback)
        self.waistp = None
        self.shoulderp = None
        self.elbowp = None
        self.waistv = None
        self.shoulderv = None
        self.elbowv = None

def callback(self, data):
    self.waistp = math.degrees(data.position[5])+180
    self.shoulderp = abs(math.degrees(data.position[4])-90)
    self.elbowp = abs(math.degrees(data.position[0])-90)
    self.waistv = data.velocity[5]
    self.shoulderv = data.velocity[4]
    self.elbowv = data.velocity[0]

def return_waistp(self):
        if self.waistp is not None:
            return self.waistp
        else:
            rospy.loginfo("No waistp loaded yet!")

class Arm:
    def __init__(self):
        self.sub = rospy.Subscriber('/rx150/joint_states', JointState, self.callback)
        self.waistp = None
        self.shoulderp = None
        self.elbowp = None
        self.waistv = None
        self.shoulderv = None
        self.elbowv = None

def callback(self, data):
    self.waistp = math.degrees(data.position[5])+180
    self.shoulderp = abs(math.degrees(data.position[4])-90)
    self.elbowp = abs(math.degrees(data.position[0])-90)
    self.waistv = data.velocity[5]
    self.shoulderv = data.velocity[4]
    self.elbowv = data.velocity[0]

def return_waistp(self):
        if self.waistp is not None:
            return self.waistp
        else:
            rospy.loginfo("No waistp loaded yet!")

You have to have these variables in your class fields somewhere, with initial values. I suggest you something like code below.

class Arm:
    def __init__(self):
        self.sub = rospy.Subscriber('/rx150/joint_states', JointState, self.callback)
        self.waistp = None
        self.shoulderp = None
        self.elbowp = None
        self.waistv = None
        self.shoulderv = None
        self.elbowv = None

 def callback(self, data):
     self.waistp = math.degrees(data.position[5])+180
     self.shoulderp = abs(math.degrees(data.position[4])-90)
     self.elbowp = abs(math.degrees(data.position[0])-90)
     self.waistv = data.velocity[5]
     self.shoulderv = data.velocity[4]
     self.elbowv = data.velocity[0]

 def return_waistp(self):
         if self.waistp is not None:
             return self.waistp
         else:
             rospy.loginfo("No waistp loaded yet!")

You have to have these variables in your class fields somewhere, with initial values. values before the callback overwrites them with real values.. I suggest you something like code below.

class Arm:
    def __init__(self):
        self.sub = rospy.Subscriber('/rx150/joint_states', JointState, self.callback)
        self.waistp = None
        self.shoulderp = None
        self.elbowp = None
        self.waistv = None
        self.shoulderv = None
        self.elbowv = None

        self.is_everything_initialized = False

    def callback(self, data):
        if self.is_everything_initialized == False:
            self.is_everything_initialized = True
        self.waistp = math.degrees(data.position[5])+180
        self.shoulderp = abs(math.degrees(data.position[4])-90)
        self.elbowp = abs(math.degrees(data.position[0])-90)
        self.waistv = data.velocity[5]
        self.shoulderv = data.velocity[4]
        self.elbowv = data.velocity[0]

    def return_waistp(self):
            if self.waistp is not None:
self.is_everything_initialized:
                return self.waistp
            else:
                rospy.loginfo("No waistp loaded yet!")

You have to have these variables in your class fields somewhere, with initial values before the callback overwrites them with real values.. I suggest you something like code below.

class Arm:
    def __init__(self):
        self.sub = rospy.Subscriber('/rx150/joint_states', JointState, self.callback)
        self.waistp = None
        self.shoulderp = None
        self.elbowp = None
        self.waistv = None
        self.shoulderv = None
        self.elbowv = None

        self.is_everything_initialized = False

    def callback(self, data):
        if self.is_everything_initialized == False:
            self.is_everything_initialized = True
        self.waistp = math.degrees(data.position[5])+180
        self.shoulderp = abs(math.degrees(data.position[4])-90)
        self.elbowp = abs(math.degrees(data.position[0])-90)
        self.waistv = data.velocity[5]
        self.shoulderv = data.velocity[4]
        self.elbowv = data.velocity[0]

    def return_waistp(self):
            if self.is_everything_initialized:
                return self.waistp
            else:
                return -999

Try to use it like this now. In general, you have to have a longer initialization phase in your code, try to add some waits here and here, because it looks like you try to get these values before any callback is processed.