怎样转换a中元素的前两个元素

Question

a=[(1,5,2),(1,2,3)]
def sw(x,y,z):

b=x
x=y
y=b
return((x,y,z))

for i in a:

for j in i:
    for k in i:
        for l in i:
           i = sw(j,k,l)

print(a)