'Constructor for AppAssertionCredentials
Args:
scope: string or iterable of strings, scope(s) of the credentials
being requested.
**kwargs: optional keyword args, including:
service_account_id: service account id of the application. If None
or unspecified, the default service account for
the app is used.'

@_helpers.positional(2)
def __init__(self, scope, **kwargs):

    self.scope = _helpers.scopes_to_string(scope)
    self._kwargs = kwargs
    self.service_account_id = kwargs.get('service_account_id', None)
    self._service_account_email = None
    super(AppAssertionCredentials, self).__init__(None)
