# GET Command Generator
from pysnmp.carrier.asynsock.dispatch import AsynsockDispatcher
from pysnmp.carrier.asynsock.dgram import udp
from pyasn1.codec.ber import encoder, decoder
from pysnmp.proto import api
from time import time

# Protocol version to use
pMod = api.protoModules[api.protoVersion1]

# Build PDU
reqPDU =  pMod.GetRequestPDU()
pMod.apiPDU.setDefaults(reqPDU)
pMod.apiPDU.setVarBinds(
    reqPDU, (((1,3,6,1,2,1,1,1,0), pMod.Null('')),
             ((1,3,6,1,2,1,1,3,0), pMod.Null('')))
    )

# Build message
reqMsg = pMod.Message()
pMod.apiMessage.setDefaults(reqMsg)
pMod.apiMessage.setCommunity(reqMsg, 'public')
pMod.apiMessage.setPDU(reqMsg, reqPDU)

def cbTimerFun(timeNow, startedAt=time()):
    if timeNow - startedAt > 3:
        raise Exception("Request timed out")
    
def cbRecvFun(transportDispatcher, transportDomain, transportAddress,
              wholeMsg, reqPDU=reqPDU):
    while wholeMsg:
        rspMsg, wholeMsg = decoder.decode(wholeMsg, asn1Spec=pMod.Message())
        rspPDU = pMod.apiMessage.getPDU(rspMsg)
        # Match response to request
        if pMod.apiPDU.getRequestID(reqPDU)==pMod.apiPDU.getRequestID(rspPDU):
            # Check for SNMP errors reported
            errorStatus = pMod.apiPDU.getErrorStatus(rspPDU)
            if errorStatus:
                print(errorStatus.prettyPrint())
            else:
                for oid, val in pMod.apiPDU.getVarBinds(rspPDU):
                    print('%s = %s' % (oid.prettyPrint(), val.prettyPrint()))
            transportDispatcher.jobFinished(1)
    return wholeMsg

transportDispatcher = AsynsockDispatcher()
transportDispatcher.registerTransport(
    udp.domainName, udp.UdpSocketTransport().openClientMode()
    )
transportDispatcher.registerRecvCbFun(cbRecvFun)
transportDispatcher.registerTimerCbFun(cbTimerFun)
transportDispatcher.sendMessage(
    encoder.encode(reqMsg), udp.domainName, ('localhost', 161)
    )
transportDispatcher.jobStarted(1)
transportDispatcher.runDispatcher()
transportDispatcher.closeDispatcher()
