Re: ca.py - Throws error - workaround

[Matt Newville <newville@cars.uchicago.edu>]

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 <luca.luisa@i-tech.si> 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