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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def authenticate(self, username=None, password=None):
 """Test the authentication credentials Raises a ``ValueError`` if there is a problem authenticating with the human readable reason given by the institution. :param username: optional username (use self.username by default) :type username: string or None :param password: optional password (use self.password by default) :type password: string or None """


        u = self.username
        p = self.password
        if username and password:
            u = username
            p = password

        client = self.client()
        query = client.authenticated_query(username=u, password=p)
        res = client.post(query)
        ofx = BeautifulSoup(res, 'lxml')

        sonrs = ofx.find('sonrs')
        code = int(sonrs.find('code').contents[0].strip())

        try:
            status = sonrs.find('message').contents[0].strip()
        except Exception:
            status = ''

        if code == 0:
            return 1

        raise ValueError(status)