itertools.product（）方法出错

Question

from itertools import product
a=[[1],[1,2],[1,2,3]]
r=len(a)
c=r-1
for k, l in product(range(c),range(c-k,0,-1)):
    a[k]=a[k]+[0]
print(a)

我收到错误“NameError：名称'k'未定义”

预期产出：

[[1,0,0],[1,2,0],[1,2,3]]