Is it safe to use images.get_serving_url_async() inside a ndb tasklet?

Question

NDB tasklets and yield are a great way to do async/parallel code. However, it is not quite clear from the documentation if this mecanism can be safely used with non-ndb async functions such as images.get_serving_url_async().

The NDB Asynchronous Operation documentation page has a very tiny section about using the NDB context's own version of urlfetch_async(), where it is stated (emphasis mine):

The URL Fetch service has its own asynchronous request API. It's fine, but not always easy to use with NDB tasklets.

It is not quite clear to me why it is "not always easy to use with NDB tasklets", and this makes me wonder if the same statement applies to images.get_serving_url_async().

So my question is: will I get into trouble if I do this?

@ndb.tasklet
def foo():
    url = yield images.get_serving_url_async(image_key)

Answer 1

It is fine. The yield will wait for the RPC and allow other tasklets to run while waiting. The comment about urlfetch was just about its clumsy API.