@classmethod
def get(cls, user=None, subject=None, token=None):
    """Fetches a user token.

        :param user:
            User unique ID.
        :param subject:
            The subject of the key. Examples:

            - 'auth'
            - 'signup'
        :param token:
            The existing token needing verified.
        :returns:
            A :class:`UserToken` or None if the token does not exist.
        """
    if user and subject and token:
        return cls.get_key(user, subject, token).get()
    assert subject and token, 'subject and token must be provided to UserToken.get().'
    return cls.query(cls.subject == subject, cls.token == token).get()
