查找列表中第一个出现的特定数字的位置 [python]

Question

所以我想找到一个在列表中出现不止一次的数字。我想要第一个的位置。

示例：说我想要 3

     s = [1,2,3,4,5,3,9,8]  => s[2] appears first

Answer 1

def first_dup( seq ):
    # keep track of the positions
    seen = {}
    for pos,item in enumerate(seq):
        if item in seen:
            # saw it before, so its a duplicate
            return seen[item]
        else:
            # first time we see it, store the pos
            seen[item] = pos