序列中插入元素

def insert_l(l, v, old_index, type='append'):
    r = []
    # p<len(l)
    nl = len(l)
    for i in range(nl):
        if type == 'append':
            r.append(l[i])
            if i == old_index:
                r.append(v)
        elif type == 'before':
            if i == old_index:
                r.append(v)
            r.append(l[i])