BUG: ScalerCH.select_channels() selects channel with no name text

[prjemian]

In BCDA-APS/use_bluesky#56, the name field of the first channel of the scaler was left blank in EPICS. The select_channels() code failed to handle this properly. Even when that name was set after the bluesky session started, select_channels() code still failed to handle this properly. If the channel had a name before bluesky connected to the scaler, then select_channels() code works properly.

Also, now is a good time to make the chan_names argument into a kwarg with default of None or []. Do that enhancement in a separate issue (#870).

[prjemian]

Further investigation with ophyd 1.5.1

test program

#!/usr/bin/env python

import ophyd
print(f"ophyd version: {ophyd.__version__}")

from ophyd.scaler import ScalerCH
from apstools.utils import device_read2table

scaler = ScalerCH("sky:scaler1", name="scaler")
scaler.wait_for_connection()

def procedure(text):
    scaler.channels.chan01.chname.put(text)
    scaler.select_channels(None)
    scaler.trigger()
    device_read2table(scaler)

procedure("")
procedure("clock")

output

(bluesky_2020_5) mintadmin@mint-vm:~$ ./scaler_869.py 
ophyd version: 1.5.1
=========== ===== ==========================
name        value timestamp                 
=========== ===== ==========================
            0.0   2020-07-20 23:40:59.445615
I0          0.0   2020-07-20 23:40:59.445615
scint       0.0   2020-07-20 23:40:59.445615
diode       0.0   2020-07-20 23:40:59.445615
I0Mon       0.0   2020-07-20 23:40:59.445615
ROI1        0.0   2020-07-20 23:40:59.445615
ROI2        0.0   2020-07-20 23:40:59.445615
scaler_time 0.0   2020-07-20 23:40:59.445615
=========== ===== ==========================

=========== ===== ==========================
name        value timestamp                 
=========== ===== ==========================
clock       0.0   2020-07-20 23:41:22.215437
I0          0.0   2020-07-20 23:41:22.215437
scint       0.0   2020-07-20 23:41:22.215437
diode       0.0   2020-07-20 23:41:22.215437
I0Mon       0.0   2020-07-20 23:41:22.215437
ROI1        0.0   2020-07-20 23:41:22.215437
ROI2        0.0   2020-07-20 23:41:22.215437
scaler_time 0.0   2020-07-20 23:41:22.215437
=========== ===== ==========================

Summary, when the first channel name is blank in EPICS, then select_channels() fails to remove that channel. When it is set, select_channels() works properly.

[prjemian]

Note if the second channel name is also blank, it is removed each time. Only the first channel name has this problem.

[prjemian]

Since scaler_time is always reported, then this line is not needed:

ophyd/ophyd/scaler.py

Line 145 in 09cc0c5

read_attrs = ['chan01'] # always include time

replace it with: read_attrs = []