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<== Date ==> <== Thread ==>

Subject: RE: Zero length length requests, EPICS upgrades, and cothread
From: <michael.abbott@diamond.ac.uk>
To: <newville@cars.uchicago.edu>, <anj@aps.anl.gov>
Cc: tech-talk@aps.anl.gov
Date: Wed, 18 Dec 2013 07:20:53 +0000
From: tech-talk-bounces@aps.anl.gov [mailto:tech-talk-
> I'm not a user of cothread.catools either, but I think it might be
> useful to have cothreads.catools and pyepics have similar behaviors
> whenever possible.    I've been confused about what the correct
> behavior for waveforms should be too.

Hi Matt!

> >> QUESTION: What should be the correct *default* result length when
> >> using cothread to perform caget() or camonitor() on a waveform?
> >>
> >> A. The "natural" length, encoded as zero: this returns NORD elements
> >> on a .12 and above server, but NELM elements on older version of
> >> EPICS.  (This is the current default.)
> >
> > I think you should stick with this meaning.
> >
> >> B. The "full" length, which for a waveform is NELM elements.
> >>
> >>
> >> I am proposing to make the following *incompatible* change: the
> >> default result length (encoded by the 'count' parameter to 'caget'
> >> and 'camonitor') will change from 0 ("natural" length) to "full"
> >> length, and if the "natural" length is wanted it will be necessary to
> >> explicitly specify 'count=0' as an argument.
> >>
> >> Please shout if you think this is a bad idea...
> >
> > Shout!
> 
> Oh, Micheal's proposal is (mostly) exactly what pyepics ca.get() does.
>  It takes a count argument with a default value of None.  That is
> interpreted as "use ca_element_count() to figure out count".   One can
> pass in count=0 to get the natural length (NORD elements), but the
> default is the full array.

That's what I was in the middle of rewriting cothread to do when I sent this mail, but on reflection and thinking about Andrew's response, I've decided to keep cothread's current behaviour which is to default count to 0 with the behaviour we're discussion.

I also very much like Andrew's suggestion of making, in effect, this decision (a grotesque caricature of the real code):

	if ca_element_count(channel) > 1:
	    return numpy.array(data)
	else:
	    return scalar(data)

> Should the ca.get() be changed to have a default count of 0 (meaning,
> return NORD elements)?  I think that would mean the user has to
> explicitly do a ca_element_count() to get the full array, which does
> seem reasonable.
> 
> That is, if we know that elements NORD through NELM-1 are not valid
> data then it seems better to not return them, unless the user
> explicitly asks for it.  Then again, I think it would be preferable to
> have uniform behavior.  Looking forward to hearing what the best
> approach should be...

It's a tricky one.  Looking like I'm probably sticking with count=0, which is going to disappoint my colleague with the array access problem!

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Replies:
Re: Zero length length requests, EPICS upgrades, and cothread Matt Newville
References:
Zero length length requests, EPICS upgrades, and cothread michael.abbott
Re: Zero length length requests, EPICS upgrades, and cothread Andrew Johnson
Re: Zero length length requests, EPICS upgrades, and cothread Matt Newville

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