How to use the recurly.js.PRIVATE_KEY function in recurly

def setUp(self):
        super(TestJs, self).setUp()
        recurly.js.PRIVATE_KEY = '0cc86846024a4c95a5dfd3111a532d13'

import recurly

from django.conf import settings

recurly.API_KEY = settings.RECURLY_API_KEY

if hasattr(settings, 'RECURLY_JS_PRIVATE_KEY'):
    recurly.js.PRIVATE_KEY = settings.RECURLY_JS_PRIVATE_KEY

def generate_signature(self):
        # workaround for http://bugs.python.org/issue5285, map unicode to strings
        settings = get_settings()
        recurly.API_KEY = str(settings.recurly_api_key)
        recurly.js.PRIVATE_KEY = str(settings.recurly_private_key)
        plan_code = str(settings.recurly_plan_code)

        # Set a default currency for your API requests
        recurly.DEFAULT_CURRENCY = 'USD'

        return recurly.js.sign({'subscription': {'plan_code': plan_code}})