# value to search
search = 11.5
# list where search
a = [25.75443, 26.7803, 25.79099, 24.17642, 24.3526, 22.79056, 20.84866, 19.49222, 18.38086, 18.0358, 16.57819, 15.71255, 14.79059, 13.64154, 13.09409, 12.18347, 11.33447, 10.32184, 9.544922, 8.813385, 8.181152, 6.983734, 6.048035, 5.505096, 4.65799]
# get index of the closest value
index = min(range(len(a)), key=lambda i: abs(a[i]-search))
print "The closest value / index = %s - value = %s"%(index,a[index])
# out: The closest value / index = 16 - value = 11.33447