Calculate Closing Velocity of 2 Objects - Given xyz Velocities

Hi there, I require an solution to track the speed of an incoming object (B) relative to object (A). Given only their x,y,z positions and velocities in 3D space. ie; (xyz=Current Position, v_=Current Velocity - No Acceleration) A: x=0, y=10000, z=0: v_x=0, v_y=-1000, v_z=0 B: x=0, y=0, z=0: v_x=0, v_y=500, v_z=0 result = 1500 (Easy to calculate in my head) But I need this math translated to a constant solution. It will be used in a Python program, so best the answer be coded in programer math :-). Thanks! Dan