Re: How to ensure with_ctrlvars monitor in PyEpics if PVs are initially unavailable

[Matthew Newville via Tech-talk <tech-talk at aps.anl.gov>]

Try this:

import time, epics

def onchange(pvname, value, **kws):

    print(f'Change {pvname=}, {value=}, units={kws.get(‘units', '?')}', list(kws.keys()))

mypv = epics.PV(‘calcExample', form='ctrl')

mypv.add_callback(on change)

for _ in range(10):

    time.sleep(1)

# or to get the units once:

mypv = epics.PV(‘calcExample') # the 'time' form is the default

mypv.wait_for_connection()

mypv_units = mypv.get_ctrlvars().get('units', '?’)

--Matthew Newville (newville at cars.uchicago.edu)

From: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of Wang, Lin via Tech-talk <tech-talk at aps.anl.gov>
Date: Thursday, June 25, 2026 at 6:57 AM
To: tech-talk at aps.anl.gov <tech-talk at aps.anl.gov>
Subject: How to ensure with_ctrlvars monitor in PyEpics if PVs are initially unavailable

Dear all,

If the PV is ready, the following PyEpics code works normally and metadata is always available in callback,

import time, epics

def cb(**kw):

    print("units =", kw.get("units"))

pv = epics.PV("calcExample")

pv.wait_for_connection(timeout=5)

pv.add_callback(cb, with_ctrlvars=True)

time.sleep(10)

However, it seems that if the PV is not ready at first, the with_ctrlvars parameter will not take effect. In this case, even if the PV becomes accessible later, callback can never access metadata in subsequent updates.

So the questions: if PV metadata is intended to be available in callback no matter what status the PV is when calling add_callback(), is there any workaround? Or am I missing something?

Thanks,

Lin