'Constructor for AppAssertionCredentials
Args:
email: an email that specifies the service account to use.
Only necessary if using custom service accounts
(see https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances#createdefaultserviceaccount).'

def __init__(self, email=None, *args, **kwargs):

    if ('scopes' in kwargs):
        warnings.warn(_SCOPES_WARNING)
        kwargs['scopes'] = None
    super(AppAssertionCredentials, self).__init__(None, *args, **kwargs)
    self.service_account_email = email
    self.scopes = None
    self.invalid = True
