class BearerToken(object):
    """Bearer Token generator which can create the payload for token response
    by OAuth 2 server. A typical token response would be:

    .. code-block:: http

        HTTP/1.1 200 OK
        Content-Type: application/json;charset=UTF-8
        Cache-Control: no-store
        Pragma: no-cache

        {
            "access_token":"mF_9.B5f-4.1JqM",
            "token_type":"Bearer",
            "expires_in":3600,
            "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA"
        }

    :param access_token_generator: a function to generate access_token.
    :param refresh_token_generator: a function to generate refresh_token,
        if not provided, refresh_token will not be added into token response.
    :param expires_generator: The expires_generator can be an int value or a
        function. If it is int, all token expires_in will be this value. If it
        is function, it can  generate expires_in depending on client and
        grant_type::

            def expires_generator(client, grant_type):
                if is_official_client(client):
                    return 3600 * 1000
                if grant_type == 'implicit':
                    return 3600
                return 3600 * 10
    :return: Callable

    When BearerToken is initialized, it will be callable::

        token_generator = BearerToken(access_token_generator)
        token = token_generator(client, grant_type, expires_in=None,
                    scope=None, include_refresh_token=True)

    The callable function that BearerToken created accepts these parameters:

    :param client: the client that making the request.
    :param grant_type: current requested grant_type.
    :param expires_in: if provided, use this value as expires_in.
    :param scope: current requested scope.
    :param include_refresh_token: should refresh_token be included.
    :return: Token dict
    """

    #: default expires_in value
    DEFAULT_EXPIRES_IN = 3600
    #: default expires_in value differentiate by grant_type
    GRANT_TYPES_EXPIRES_IN = {
        'authorization_code': 864000,
        'implicit': 3600,
        'password': 864000,
        'client_credentials': 864000
    }

    def __init__(self, access_token_generator,
                 refresh_token_generator=None,
                 expires_generator=None):
        self.access_token_generator = access_token_generator
        self.refresh_token_generator = refresh_token_generator
        self.expires_generator = expires_generator

    def _get_expires_in(self, client, grant_type):
        if self.expires_generator is None:
            expires_in = self.GRANT_TYPES_EXPIRES_IN.get(
                grant_type, self.DEFAULT_EXPIRES_IN)
        elif callable(self.expires_generator):
            expires_in = self.expires_generator(client, grant_type)
        elif isinstance(self.expires_generator, int):
            expires_in = self.expires_generator
        else:
            expires_in = self.DEFAULT_EXPIRES_IN
        return expires_in

    def __call__(self, client, grant_type, user=None, scope=None,
                 expires_in=None, include_refresh_token=True):
        access_token = self.access_token_generator(client, grant_type, user, scope)
        if expires_in is None:
            expires_in = self._get_expires_in(client, grant_type)

        token = {
            'token_type': 'Bearer',
            'access_token': access_token,
            'expires_in': expires_in
        }
        if include_refresh_token and self.refresh_token_generator:
            token['refresh_token'] = self.refresh_token_generator(
                client, grant_type, user, scope)
        if scope:
            token['scope'] = scope
        return token