def generate_url(self, service_scope=None, state_value='', additional_fields=None):

        ''' a method to generate an authorization url to oauth2 service for client

        :param service_scope: [optional] list with scope of permissions for agent
        :param state_value: [optional] string with unique identifier for callback
        :param additional_fields: [optional] dictionary with key value strings for service
        :return: string with authorization url
        '''

        title = '%s.generate_url' % self.__class__.__name__

    # validate inputs
        input_fields = {
            'state_value': state_value,
            'service_scope': service_scope,
            'additional_fields': additional_fields
        }
        for key, value in input_fields.items():
            if value:
                object_title = '%s(%s=%s)' % (title, key, str(value))
                self.fields.validate(value, '.%s' % key, object_title)

    # validate additional fields
        if additional_fields:
            for key, value in additional_fields.items():
                if not isinstance(value, str):
                    raise TypeError("%s(additional_fields={'%s':'...'}) must be a string datatype." % (title, key))

    # construct query parameters
        query_params = {
            'response_type': 'code',
            'client_id': self.client_id,
            'redirect_uri': self.redirect_uri
        }
        if state_value:
            query_params['state'] = state_value
        if service_scope:
            for item in service_scope:
                if 'scope' not in query_params.keys():
                    query_params['scope'] = ''
                if query_params['scope']:
                    query_params['scope'] += ' '
                query_params['scope'] += item
        if additional_fields:
            query_params.update(**additional_fields)

    # encode query parameters
        from urllib.parse import urlencode
        url_string = '%s?%s' % (self.auth_endpoint, urlencode(query_params))

        return url_string