class BearerTokenGenerator(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"
        }
    """

    #: 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

    @staticmethod
    def get_allowed_scope(client, scope):
        if scope:
            scope = client.get_allowed_scope(scope)
        return scope

    def generate(self, grant_type, client, user=None, scope=None,
                 expires_in=None, include_refresh_token=True):
        """Generate a bearer token for OAuth 2.0 authorization token endpoint.

        :param client: the client that making the request.
        :param grant_type: current requested grant_type.
        :param user: current authorized user.
        :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
        """
        scope = self.get_allowed_scope(client, scope)
        access_token = self.access_token_generator(
            client=client, grant_type=grant_type, user=user, scope=scope)
        if expires_in is None:
            expires_in = self._get_expires_in(client, grant_type)

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

    def __call__(self, grant_type, client, user=None, scope=None,
                 expires_in=None, include_refresh_token=True):
        return self.generate(grant_type, client, user, scope, expires_in, include_refresh_token)