Experimental Physics and
Industrial Control System

Geoff Savage <[email protected]> · Wed, 20 Apr 2011 10:43:10 -0500

Hi Luca,

I don't know anything about the internals of ca.py. But I have read

some python tracebacks over the years.

This is telling you that the __on_connect() function does not understand

the argument named "count" that is being passed to it.

-------------------------------------------------------------------------------
TypeError: __on_connect() got an unexpected keyword argument 'count'
Traceback (most recent call last):
File "_ctypes/callbacks.c", line 295, in 'calling callback function'

File "/usr/local/lib/python2.6/dist-packages/epics/ca.py", line 420, in

_onGetEvent

 args.usr(value=value, **kwds)
-------------------------------------------------------------------------------

My guess is that you added a function that is called when the CA

connection state changes and for this function you have included a count

argument. In the python interfaces to CA you almost never need to

specify a count yourself. The python code can determine the count with

a query to the record.

A connection callback should simply notify a user or send an alarm that

the record can no longer be reached. If you receive a disconnect

callback you don't need to do anything except the default CA action.

When the record is available again CA will automatically reconnect and

you will receive a connect callback.

The default CA action depends on your threading model. We use single

threaded applications with python GUIs with Tkinter. In this case we

use the after method of Tkinter widgets to call ca_poll() at 10Hz. This

allows CA to call the connection and monitor callbacks.

CA is actually simpler to use than you think. The connection handling

has been coded at a level below the python interface. At this point I

should recognize the efforts of Jeff Hill and others to make the CA

interface convenient to use.

Geoff

On 4/20/2011 2:21 AM, Luca Luisa wrote:

Hi,
I tried to types of code:

1. in the first one I don't disconnect the PV when the Server
disconnects and I just wait an automatic reconnection.
2. in the second I disconnect the PV, and re-create it again when the
server connects again.

In both cases I get the same error.

Luca

On 04/19/2011 12:19 AM, Jeff Hill wrote:

Hi Matt, Luca,

Sorry about the delay responding - I was away on business at the end
of last
week.

EPICS server, I get a lot of unknown errors in the terminal
from where I launch my application, those are:

----------------------------------------------------------------------
TypeError: __on_connect() got an unexpected keyword argument 'count'
Traceback (most recent call last):
File "_ctypes/callbacks.c", line 295, in 'calling callback function'
File "/usr/local/lib/python2.6/dist-packages/epics/ca.py",
line 420, in _onGetEvent
args.usr(value=value, **kwds)
-----------------------------------------------------------------------

Despite some similarities, I don’t believe that this particular message
originates from the ca client library.

Note also that if you do not unsubscribe when the circuit becomes
unresponsive then the ca client library will auto-magically
re-subscribe on
your behalf should the channel need to be reconnected on a fresh TCP
circuit.

Jeff
______________________________________________________
Jeffrey O. Hill Email [email protected]
LANL MS H820 Voice 505 665 1831
Los Alamos NM 87545 USA FAX 505 665 5107

Message content: TSPA

With sufficient thrust, pigs fly just fine. However, this is
not necessarily a good idea. It is hard to be sure where they
are going to land, and it could be dangerous sitting under them
as they fly overhead. -- RFC 1925

-----Original Message-----
From: [email protected] [mailto:tech-talk-
[email protected]] On Behalf Of Matt Newville
Sent: Thursday, April 14, 2011 10:07 AM
To: [email protected]; EPICS Tech Talk
Subject: Re: ca.py - Throws error - workaround

Hi Luca,

I hope it's OK with you that I'm sending the message to Tech-Talk,
where there are many people with much better understanding of Channel
Access than I have.

On Thu, Apr 14, 2011 at 1:58 AM, Luca Luisa<[email protected]> wrote:

Hi,
I'm working with PyEpics version 3.1.1.
My QT Python application communicates with many EPICS servers where

hundred

of PVs are delivered.

Our application has to deal with any EPICS server
disconnection/re-connection that can happen during the application run.

Each PV used by this QT Python application has a callback function

added

with the module "add_callback()".

When I noticed an EPICS server disconnection, I clear the callbacks

with the

method "clear_callbacks()".

When any EPICS server re-connect again, I try to add again another

callback

function to the same PV instance previously created, if fails (the
add_callback returns None) I re-create the PV instance again and try to

add

the callback function.

But in the cycle above described used to handle the dynamic connection

to

EPICS server, I get a lot of unknown errors in the terminal from where

I

launch my application, those are:

-----------------------------------------------------------------------

--------

TypeError: __on_connect() got an unexpected keyword argument 'count'
Traceback (most recent call last):
File "_ctypes/callbacks.c", line 295, in 'calling callback function'
File "/usr/local/lib/python2.6/dist-packages/epics/ca.py", line 420,

in

_onGetEvent
args.usr(value=value, **kwds)
-----------------------------------------------------------------------

--------

Our application is very complex so I cannot send you an example.

In order to remove this error that full-fills the terminal window, I

had to

change the "ca.py" source with a try: except: statement.
The diff is:

419,423c419,420
< try:
< if hasattr(args.usr, '__call__'):
< args.usr(value=value, **kwds)
< except:
< """"""
---

if hasattr(args.usr, '__call__'):
args.usr(value=value, **kwds)

My solution is a pure workaround to make my application working and I

got

success but I would like to know if this is a known error or my way to
handle connection/disconnection of a EPICS server is wrong.

Many thanks

Ing. Luca Luisa

I'm reluctant to make that change, as it's the only place ca.py can
tell the programmer that there was an error in running the
user-supplied callback and this change silences that message.

This message:

TypeError: __on_connect() got an unexpected keyword argument 'count'
Traceback (most recent call last):
File "_ctypes/callbacks.c", line 295, in 'calling callback function'
File "/usr/local/lib/python2.6/dist-packages/epics/ca.py", line 420,

in

_onGetEvent
args.usr(value=value, **kwds)

says that PV.__on_connect() -- the internally defined connection
callback for a PV -- is being run as a change-in-value callback. I'm
not sure how that's even possible.

When I noticed an EPICS server disconnection, I clear the callbacks

with the

method "clear_callbacks()".

I'm not sure this is necessary. Have you tried not doing this?
Obviously, you won't get a callback from a disconnected PV, and will
get a callback upon reconnection, but I would think that's not worse
than explicitly clearing and re-setting callbacks. I can verify that
it does works for me in simple test scripts to not clear "change
value" callbacks and restart the server IOC.

When any EPICS server re-connect again, I try to add again another

callback

function to the same PV instance previously created, if fails (the
add_callback returns None) I re-create the PV instance again and try to

add

the callback function.

For PV.add_callback() to return None, the PV has to be unconnected --
or at least that's the intention. So it's hard to understand what's
happening if you wait for reconnection to add a callback. I do think
it shouldn't be necessary to re-create the PV.

I'm not sure whether this is related to using QT. A minimal example
would be really helpful.

--Matt

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System