Re: EPICS7 and Python

[Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov>]

On 8/18/20 5:39 AM, Juan F. Esteban Müller via Tech-talk wrote:
> Hi again,
> 
>  
> 
> It seems that if I create a CA context before importing the p4p client, it all works fine. It is still a mystery for me, but it seems to be a good workaround for the moment.

Firstly, which OS are you working under (Linux?).

This suggests that the two python extensions are loading
two (maybe different) copies of libCom.so (or other libraries).

This is a likely enough situation with two unrelated python
modules loading some of the same libraries (eg. libCom.so).
Sometimes this just works, but when it doesn't the resulting
symptoms can be quite strange.

I'm not sure where your installation comes from, or if the
following recipe could be acceptable.

Install P4P and pyepics from pypi.org, using a bundled
version of EPICS (the epicscorelibs module seen below).

> python3 -m virtualenv -p python3 p4ptest
> . p4ptest/bin/activate
> pip install p4p pyepics

Then in python

> import time
> import epicscorelibs.path.pyepics
> from p4p.client.thread import Context
> import epics
> 
> pv = epics.PV('TST:Q1', connection_callback=lambda pvname,conn,pv: print('connected'))
> time.sleep(1)
> pv.connect(timeout=0)
> pv.get()

This works for me on Linux