def triangles():    
    ls = [1]
    i = 2
    while 1:
        yield ls
        # 两边的1不参与列表推导式，中间的每个值等于上一列的下标和下标减一的和
        ls = [1] + [(ls[x-1] + ls[x]) for x in range(1, i-1)] + [1]
        i += 1