snmpd pass to run python - python-2.7

Question

Im trying to query a modbus device trough snmp using snmpd to pass a python script to retrieve data.
#! /bin/bash
if [ "$1" = "-g" ]
then
echo .1.3.6.1.4.1.52612.10.3.1
echo string
python /usr/local/bin/readvolt.py
fi
exit 0
And this is the readvolt.py looks like :
#!/usr/bin/python
import minimalmodbus
eqp = minimalmodbus.Instrument('/dev/ttyUSB0',1) # port name, slave address (in decimal)
# skip some other lines for serial port initialization
volt = eqp.read_float(0,4,2) # getting data from serial modbus
print volt
and this line from my snmpd.conf :
pass .1.3.6.1.4.1.52612.10.3.1 /bin/sh /usr/local/bin/volt.sh
my question : I got traceback from python, couldnot find minimalmodbus module, but when i tried to run the readvolt.py from directly from host, it is working as expected (it can print out the result (volt) )
pi#raspberrypi:/usr/local/bin $ readvolt.py
220.25
I also tried using simple python script (test.py) just to make sure if snmpd pass can run python script on respond of snmpget from snmp manager
#!/usr/bin/python
import sys
print "test"
It run OK :
suryo#r50e:~$ snmpwalk -v2c -c public 192.168.1.5 .1.3.6.1.4.1.52612.10.3.1
iso.3.6.1.4.1.52612.10.3.1 = STRING: "test"
suryo#r50e:~$
what is the problem here ? seems that python could not import external module when it is run by snmpd pass.
I'm thinking if this is an access control issue, Debian-snmp doesnt have right to access serial port..

Answer 1

Problem is solved, by finding out the username of the snmpd daemon. I put whoami in the script and got 'Debian-snmp', then becoming straight forward, checking group membership by running :
pi#rraspberrypi:~$ groups Debian-snmp
Debian-snmp : Debian-snmp
Put Debian-snmp in the dialout membership to grant full access to serial ports:
pi#raspberrypi:~ $ sudo usermod -a -G dialout Debian-snmp
Restart the snmpd to logon with new membership, and voilla..It can read the slave modbus device from snmp command /snmpget