title: pysnmp - nextCmd - not retrieving next element
body: <p>I'm new to python. I'm trying to understand the usage of pysnmp.</p>
  
  <p>I tried the following:</p>
  
  <pre><code>import asyncio
  from pysnmp.hlapi.asyncio import *
  from pysnmp import debug
  
  @asyncio.coroutine
  def run():
      snmpEngine = SnmpEngine()
      while True:
          errorIndication, errorStatus, errorIndex, varBinds = yield from nextCmd(
              snmpEngine,
              CommunityData('public', mpModel=1),
              UdpTransportTarget(('giga-int-2', 161)),
              ContextData(),
              ObjectType(ObjectIdentity('1.3.6.1.2.1.31.1.1.1.1')),
              lexicographicMode=False
          )
  
          if errorIndication:
              print(errorIndication)
              break
          elif errorStatus:
              print('%s at %s' % (
                  errorStatus.prettyPrint(),
                  errorIndex and varBinds[int(errorIndex) - 1][0] or '?')
              )
          else:
              for varBind in varBinds:
                  for v in varBind:
                      print(' = '.join([x.prettyPrint() for x in v]))
  
      snmpEngine.transportDispatcher.closeDispatcher()
  
  asyncio.get_event_loop().run_until_complete(run())
  </code></pre>
  
  <p>As result I get always the same interface. What's wrong? Why is it not retrieving the next element?</p>
  
  <pre><code>SNMPv2-SMI::mib-2.31.1.1.1.1.1 = sc0
  SNMPv2-SMI::mib-2.31.1.1.1.1.1 = sc0
  SNMPv2-SMI::mib-2.31.1.1.1.1.1 = sc0
  </code></pre>