def backoff(test_func, reason=None):
    def decorator(func):
        @wraps(func)
        def inner(*args, **kwargs):
            if test_func(*args, **kwargs):
                return None

            result = None
            for wait_secs in [0, 1, 4, 9, 16, 32, 64, 128, 256, None]:
                result = func(*args, **kwargs)
                if test_func(*args, **kwargs):
                    break

                if wait_secs is not None:
                    log.info("Waiting %s seconds.", wait_secs)
                    time.sleep(wait_secs)
                else:
                    if reason:
                        raise Exception(reason)
                    else:
                        raise Exception('%s failed.' % func)

            return result
        return inner
    return decorator

def is_authenticated(xyz):
    try:
        authd = xyz.authenticate()  # returns True if successful, otherwise False.
    except Exception:
        return False    # Or, it might raise an Exception.
    return authd

@backoff(is_authenticated, reason="Cannot Authenticate.")
def do_xyz_things(xyz):
    xyz.get('/things')
    ...

if __name__ == '__main__':
    do_xyz_things(XyzApiClient(host))