How to read msg from /odom at time series n and (n-1)

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Hi there, I hope to compare the message from the consecutive time series n and (n-1) from /odom in python. I am confused about how to store the msg at time n-1 and time n at the same time, especially for the time sequence t=1.

def listener():
    rospy.init_node('listener', anonymous=True)
    rospy.Subscriber("/odom", Odometry, simple_callback)
    rospy.spin()
def simple_callback():

if __name__== '__main__':
    listener()

The problem which confused me is that: I was thinking to use some variables to store the msg at sequence n-1 and n like : current_odom=[ ] and last_odom=[ ]. However, I don't how to store the msg at time t=1, because listener() and simple_callback are in loop, I don't know when to pass the data in current_odom=[ ] and last_odom=[ ]. Could you please give me any hints?

Thank you.