创建一个函数，接收已读取的两个字符串，返回两个字符串集合的并集，交集，

差集和对称差集。函数名为 set_operations。

def set_operations(str1, str2):
    set1 = set(str1.split())
    set2 = set(str2.split())

    union = set1.union(set2)
    intersection = set1.intersection(set2)
    difference = set1.difference(set2)
    symmetric_difference = set1.symmetric_difference(set2)

    return {
        'union': union,
        'intersection': intersection,
        'difference': difference,
        'symmetric_difference': symmetric_difference
    }

函数使用示例：

str1 = 'this is a test string'
str2 = 'this is another test string'

result = set_operations(str1, str2)

print(result['union'])  # {'this', 'a', 'is', 'test', 'string', 'another'}
print(result['intersection'])  # {'this', 'is', 'test', 'string'}
print(result['difference'])  # {'a'}
print(result['symmetric_difference'])  # {'another', 'a'}