Re: pyepics callback

[Matt Newville via Tech-talk <tech-talk@aps.anl.gov>]

Hi Alfio,

On Mon, Sep 9, 2019 at 10:08 AM Alfio Rizzo via Tech-talk <tech-talk@aps.anl.gov> 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