Given a list L in Python, and two elements x and y that are guaranteed to be in L, I wish to know whether x appears before y in L.

If there is only one pair, I can simply loop over L until I find either x or y. This takes time linear in the length of L. But, I have to answer many such queries for the same L (for different x and y). Is there an efficient way, maybe a data structure or iterator in Python, that I can use to answer many queries on the same L quickly?