Re: How to set timeout for reading a pyepics device attribute [SEC=UNOFFICIAL]

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

Hi Terry, 

On Mon, Dec 2, 2019 at 11:00 PM CORNALL, Terry via Tech-talk <tech-talk@aps.anl.gov> wrote:

Hi All.

If I use pyepics devices, such as a motor, via:

m=epics.Motor('SR10BM01SPAL03:SPIN’, timeout=5)

And then try to read some attribute, like:

jvel=m.JVEL

it sometimes happens that the attempt to read the attribute times out in 1 second. (rare, but it does happen)

UserWarning: ca.get('SR10BM01SPAL03:SPIN.JVEL') timed out after 1.00 seconds.

 

The timeout=5 specified in creating the device instance above in my example appears to be a ‘connection timeout’ used when first making the device instance rather than the timeout used when getting an attribute value.

 

I’ve pondered the code and doco but can’t see how to set  the timeout that applies to reading an attribute.

If I use an epics.Motor.get() like

Jvel=m.get(‘JVEL’, timeout=5)

I get an error about unexpected keyword argument ‘timeout’, and looking at the code in epics/motor.py supports that I can’t do it that way.

 

According to code in epics/motor.py, Motor.get() uses PV.get() but doesn’t specify any timeout  so I get stuck with a hard-coded 1 second.

 

I guess I can get the epics.Motor’s PV as a string and then attach the attribute string and then use PV.get(pvstring, timeout=5) but it seems a bit hacky if there is a better way that I’ve missed.

 

Any thoughts?

Terry

You are right that the `timeout` to the underlying `PV.get()` is not exposed through the Motor `get()` method.  It probably should be, and that would be easy to fix. 

You could first lookup the PV for the specific motor attribute and use its `get()` method, perhaps like

   m = epics.Motor('SR10BM01SPAL03:SPIN’,  

   jog_speed = m.PV('JVEL').get(timeout=10.0)

That would require knowing how the Motor Devices'  PVs are indexed which is by "field suffix" ('VAL', 'JVAL', 'HOPR', and so forth), so that whereas you could get 

that jog speed using

  

  jog_speed = m.get('jog_speed') 

you could not do

  

  jog_speed = m.PV('jog_speed').get()  # Nope, have to use 'JVEL'

Note also that there is sort of a double use for the `timeout` argument in `PV.get()`.  First, it is used as a connection timeout if the PV is not already connected.   Then it is used as the timeout to get the actual data.  That means that if the PV really is not connected, `PV.get(timeout=5.0)` really will return after 5+delta seconds (delta ~1ms).  If the PV is very slow to connect and fetch the data, it could take longer than 5 seconds.  I don't think I've ever seen that situation, but it is possible. 

I'll put "expose get() timeout to Devices and Motors" on the to-do list.

--Matt