当条件不满足时，如何引发Exception并返回none值

我是python新手，只需要学习python。我已经编写了一个代码来查找两个字符串中的公共字符，并且得到了所需的输出。如果出现以下情况，我想修改代码，对于以下情况，它应该返回

None

1) For two string, if there is no match
2) any of string1 or string2 is nil/empty
3) any of string1 or string2 is hash/array/set/Fixnum [i.e anything other than string]

我应该为上述情况提出一个例外。我已经通过论坛和链接，但不能正确地理解它。有人能帮我解决上述情况下如何引发异常吗

1) For two string, if there is no match
2) any of string1 or string2 is nil/empty
3) any of string1 or string2 is hash/array/set/Fixnum [i.e anything other than string]

这是密码

1) For two string, if there is no match
2) any of string1 or string2 is nil/empty
3) any of string1 or string2 is hash/array/set/Fixnum [i.e anything other than string]

class CharactersInString:
    def __init__(self, value1, value2):
        self.value1 = value1
        self.value2 = value2


    def find_chars_order_n(self):
        new_string = [ ]
        new_value1 = list(self.value1)
        new_value2 = list(self.value2)
        print( "new_value1: ", new_value1)
        print( "new_value2: ", new_value2)
        for i in new_value1:
            if i in new_value2 and i not in new_string:
                new_string.append(i)
        final_list = list(new_string)
        return ''.join(final_list)

if __name__ == "__main__":
    obj = CharactersInString("ho", "killmse")
    print(obj.find_chars_order_n())

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有人能帮我吗有人能帮我吗。。我无法为try-and-except块找出正确的synatx