You can enumerate() over u_deteminer, which will give you the indices and values. Then pass that to min with a key that looks for the smallest difference between the value and number. Then use that index to find the value in the other list:
u_deteminer = [0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0] u_H = [0.368, 0.303, 0.301, 0.301, 0.301, 0.301, 0.301, 0.301, 0.301, 0.301] n = 1.276819 i, val = min(enumerate(u_deteminer), key=lambda t: abs(t[1] - n)) # index and value from u_deteminer with the smallest difference print(i, val) # 5 1.2 # that index in u_H: print(u_H[i]) # 0.301
If you don't actually need the index, you can just zip the lists together and pass that to min. This will give you the values from both lists where the u_deteminer is closest to your target. Then just use the second:
n = 1.276819 D, H = min(zip(u_deteminer, u_H), key=lambda t: abs(t[0] - n) ) print(H) # 0.301