習(xí)題練習(xí)：

def f1(s1,s2,op=4):
    if op == 1:
        temp = s1.intersection(s2)
    elif op == 2:
        temp = s1.difference(s2)
    else:
        temp = s1.union(s2)
    return temp
set1= set('ab')
set2 = set('cd')

print f1(set1,set2)
print f1(set1,set2,1)
print f1(set1,set2,2)

結(jié)果輸出：

set(['a', 'c', 'b', 'd'])
set([])
set(['a', 'b'])