s1和s2有重合，为什么if语句if s1.isdisjoint(s2)==a:输出是2呢

s1 = set([1, 2, 3, 4, 5])

s2 = set([1, 2, 3, 4, 5, 6, 7, 8, 9])  

a="False"

if  s1.isdisjoint(s2)==a:

    print(1)

else:

    print(2)