We added the following method:
@staticmethod
@ndb.tasklet
def get_key_async(key):
"""
returns a future that upon calling get_result() will either
return the Future's result
or None if the key was None
"""
if not key: raise ndb.Return(None)
result = yield key.get_async()
raise ndb.Return(result)
to our code, a similar tasklet could be (which we've removed from out code):
@ndb.tasklet
def future_or_none(future):
if future:
result = yield future
raise ndb.Return(result)
raise ndb.Return(None)
Then in your case you'd do
yield (future_or_none(e.user.get_async() if e.user else None), ...)
On Wed, Jan 30, 2013 at 10:03 AM, Guido van Rossum <[email protected]> wrote:
> On Wed, Jan 30, 2013 at 9:00 AM, Moises Belchin <[email protected]>
> wrote:
>> Please take a look at this parallel tasklet code snippet #1.
>>
>>
>> @ndb.tasklet
>> def get_data_parallel(e):
>> usr, det = yield (e.user.get_async(),
>> MyKind.query(ancestor = e.key).fetch_async())
>> raise ndb.Return((e, usr, det))
>>
>>
>>
>> If e.user is None this raise an Exception.
>>
>>
>> I'm trying this snippet #2. However I still get Exception: "TypeError:
>> Expected Future, received <type 'NoneType'>: None"
>>
>>
>> @ndb.tasklet
>> def get_data_parallel(e):
>> usr, det = yield (e.user.get_async() if e.user else None,
>> MyKind.query(ancestor = e.key).fetch_async())
>> raise ndb.Return((e, usr, det))
>>
>>
>> How can I do something like snippet #2 ? How can I return future(None) or
>> future('')
>
> You can factor it out into two yields, one of which is optional. First
> create a future for the query that you always want to run:
>
> f = MyKind.query(ancestor = e.key).fetch_async() # No yield!
>
> Then conditionally yield the other async request:
>
> if e.user:
> usr = yield from e.user.get_async()
> else:
> usr = None
>
> Finally yield the future:
>
> det = yield f
>
> The trick is that the query will run when you yield the other operation.
>
> --
> --Guido van Rossum (python.org/~guido)
>
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--
Jim
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