   constructors. To create credentials using a Google service account
    private key JSON file::

        credentials = service_account.Credentials.from_service_account_file(
            'service-account.json')

    Or if you already have the service account file loaded::

        service_account_info = json.load(open('service_account.json'))
        credentials = service_account.Credentials.from_service_account_info(
            service_account_info)

    Both helper methods pass on arguments to the constructor, so you can
    specify additional scopes and a subject if necessary::

        credentials = service_account.Credentials.from_service_account_file(
            'service-account.json',
            scopes=['email'],
            subject='user@example.com')

    The credentials are considered immutable. If you want to modify the scopes
    or the subject used for delegation, use :meth:`with_scopes` or
    :meth:`with_subject`::

        scoped_credentials = credentials.with_scopes(['email'])
        delegated_credentials = credentials.with_subject(subject)

    To add a quota project, use :meth:`with_quota_project`::

        credentials = credentials.with_quota_project('myproject-123')
    NFc
