EPICS Re: pyepics callback

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Hi Alfio,

On Mon, Sep 9, 2019 at 10:08 AM Alfio Rizzo via Tech-talk <[email protected]> wrote:

Hi

I made a simple python script to get the updated value of my PV

with a simple callback function :

import epics

import time

def onChanges(**kw):

    print ('PV Changed! ', kw["pvname"], kw["value"], kw["units"], time.ctime())

mypv = epics.PV("TCU6:RF")

mypv.add_callback(onChanges)

while True:

    pass



The issue is that the unit is shown as None even if it defined in the .db

PV Changed! TCU6:RF -39.69 None Mon Sep 9 15:03:58 2019

PV Changed! TCU6:RF -39.73 None Mon Sep 9 15:03:59 2019

PV Changed! TCU6:RF -39.71 None Mon Sep 9 15:04:00 2019

PV Changed! TCU6:RF -39.7 None Mon Sep 9 15:04:01 2019

PV Changed! TCU6:RF -39.67 None Mon Sep 9 15:04:02 2019

But if I add this line on the script before to add the callback

……

mypv.units

mypv.add_callback(onChanges)

……

Then when I restart the script it shows the unit correctly

PV Changed! TCU6:RF -39.79 dBm Mon Sep 9 15:04:31 2019

PV Changed! TCU6:RF -39.71 dBm Mon Sep 9 15:04:32 2019

PV Changed! TCU6:RF -39.69 dBm Mon Sep 9 15:04:33 2019

Is this a bug/feature or I am doing something wrong ?

Hm, I think this is mostly a timing issue, but we could try to make this better. Basically, in the first variation, the PV never asked for its CTRL fields, so it does not know what they are. When you ask for the units (or any other CTRL fields), it goes and fetches all of them (once) and includes those in the monitor callback.

So doing

mypv = epics.PV(pvname)

mypv.get_ctrlvars()

mypyv.add_callback(onChanges)

is probably the best way to get the units into your callback.

If you expect that the units might actually change, you should create the PV with `form='ctrl'` as Eric suggested.

Hope that helps,

--Matt

Subject:	Re: pyepics callback
From:	Matt Newville via Tech-talk <[email protected]>
To:	Alfio Rizzo <[email protected]>
Cc:	"[email protected]" <[email protected]>
Date:	Mon, 9 Sep 2019 10:50:21 -0500

Experimental Physics and Industrial Control System