i'd like to try save data from subscriber to a variabel for my condition statement, but when i launch my program, program only execute the subscriber only but not the condition

this is my program

class sukreb():
def __init__(self):
    rospy.init_node('sukreb')
    rospy.Subscriber('mode',std_msgs.msg.String, self.callback)
    # rospy.Subscriber()
    rospy.spin()

def callback(self,data):
    mode = data
    return mode
    if(mode == "a"):
        lowobj = lowMagenta
        upobj  = upMagenta

    if(mode == "b"):
        lowobj = lowCyan
        upobj = upCyan