Re: Issue of PV callback of PyEpics

[Tong Zhang via Tech-talk <tech-talk@aps.anl.gov>]

mypv.get(as_string=True) returns the string value as expected. Please 
see the detailed constructed fully functional demo below:

Script works as expected with mbbi record:

  7 import epics
  8 import time
  9
 10
 11 pv_name = 'INVALID_PV_NAME'
 12
 13 def pv_with_callback(pv_name):
 14     pv = epics.PV(pv_name)
 15     time.sleep(2)
 16     def on_value_changed(char_value, **kws):
 17         print("Char Value: ", char_value)
 18     pv.add_callback(on_value_changed)
 19
 20
 21 pv_with_callback(pv_name)
 22
 23 time.sleep(2)
 24 pv_name = 'status'
 25
 26 pv_with_callback(pv_name)

If commented out line 15th, and run it again, the behavior is changed, 
only value is returned in the callback rather than expected char_value.

This typically is the case when switching from one PV from another PV, 
while not all of them are reachable in the controls network.

Thanks,
Tong

On 2/18/19 5:49 PM, Matthew Newville via Tech-talk wrote:
> On Mon, Feb 18, 2019, 4:30 PM Zhang, Tong via Tech-talk 
> <tech-talk@aps.anl.gov <mailto:tech-talk@aps.anl.gov> wrote:
>
>     Hello,
>
>     When I work with PyEpics.PV object, I've found sometimes the value
>     of 'char_value' in the added callback just return what 'value' meant
>     to be. I tried with a MBBI type record, it should be expected as a
>     string in the callback, but an integer was got. Could somebody let
>     me know what is the possible reason?
>
>
> An mbbi record definitely should return the appropriate string in 
> 'char_value' in a callback.  That does need to be able to lookup the 
> CTRL values, especially including the enumeration strings.
>
> Can you do
>      mypv = epics.PV(pvname)
>      print(mypv.get(as_string=True))
>      print(mypv.get_info())
>
> and see the enumeration strings?
>
> Matt