Creates a generator to perform SNMP GET query.

    When iterator gets advanced by :py:mod:`asyncio` main loop,
    SNMP GET request is sent (:RFC:`1905#section-4.2.1`).
    The iterator yields :py:class:`asyncio.Future` which gets done whenever
    response arrives or error occurs.

    Parameters
    ----------
    snmpDispatcher: :py:class:`~pysnmp.hlapi.v1arch.asyncore.SnmpDispatcher`
        Class instance representing asynio-based asynchronous event loop and
        associated state information.

    authData: :py:class:`~pysnmp.hlapi.v1arch.CommunityData`
        Class instance representing SNMPv1/v2c credentials.

    transportTarget: :py:class:`~pysnmp.hlapi.v1arch.asyncio.UdpTransportTarget` or
        :py:class:`~pysnmp.hlapi.v1arch.asyncio.Udp6TransportTarget` Class instance representing
        transport type along with SNMP peer address.

    \*varBinds: :class:`tuple` of OID-value pairs or :py:class:`~pysnmp.smi.rfc1902.ObjectType`
        One or more class instances representing MIB variables to place
        into SNMP request.

    Note
    ----
    The `SnmpDispatcher` object may be expensive to create, therefore it is
    advised to maintain it for the lifecycle of the application/thread for
    as long as possible.

    Other Parameters
    ----------------
    \*\*options :
        Request options:

        * `lookupMib` - load MIB and resolve response MIB variables at
          the cost of slightly reduced performance. Default is `False`,
          unless :py:class:`~pysnmp.smi.rfc1902.ObjectType` is present
          among `varBinds` in which case `lookupMib` gets automatically
          enabled.

    Yields
    ------
    errorIndication: str
        True value indicates SNMP engine error.
    errorStatus: str
        True value indicates SNMP PDU error.
    errorIndex: int
        Non-zero value refers to `varBinds[errorIndex-1]`
    varBinds: tuple
        A sequence of OID-value pairs in form of base SNMP types (if
        `lookupMib` is `False`) or :py:class:`~pysnmp.smi.rfc1902.ObjectType`
        class instances (if `lookupMib` is `True`) representing MIB variables
        returned in SNMP response.

    Raises
    ------
    PySnmpError
        Or its derivative indicating that an error occurred while
        performing SNMP operation.

    Examples
    --------
    >>> import asyncio
    >>> from pysnmp.hlapi.v1arch.asyncio import *
    >>>
    >>> @asyncio.coroutine
    ... def run():
    ...     errorIndication, errorStatus, errorIndex, varBinds = yield from getCmd(
    ...         SnmpDispatcher(),
    ...         CommunityData('public'),
    ...         UdpTransportTarget(('demo.snmplabs.com', 161)),
    ...         ObjectType(ObjectIdentity('SNMPv2-MIB', 'sysDescr', 0))
    ...     )
    ...     print(errorIndication, errorStatus, errorIndex, varBinds)
    >>>
    >>> asyncio.get_event_loop().run_until_complete(run())
    (None, 0, 0, [ObjectType(ObjectIdentity(ObjectName('1.3.6.1.2.1.1.1.0')), DisplayString('SunOS zeus.snmplabs.com 4.1.3_U1 1 sun4m'))])
    >>>

def getCmd(snmpDispatcher, authData, transportTarget,
           *varBinds, **options):
    """Creates a generator to perform SNMP GET query.

    When iterator gets advanced by :py:mod:`asyncio` main loop,
    SNMP GET request is sent (:RFC:`1905#section-4.2.1`).
    The iterator yields :py:class:`asyncio.Future` which gets done whenever
    response arrives or error occurs.

    Parameters
    ----------
    snmpDispatcher: :py:class:`~pysnmp.hlapi.v1arch.asyncore.SnmpDispatcher`
        Class instance representing asynio-based asynchronous event loop and
        associated state information.

    authData: :py:class:`~pysnmp.hlapi.v1arch.CommunityData`
        Class instance representing SNMPv1/v2c credentials.

    transportTarget: :py:class:`~pysnmp.hlapi.v1arch.asyncio.UdpTransportTarget` or
        :py:class:`~pysnmp.hlapi.v1arch.asyncio.Udp6TransportTarget` Class instance representing
        transport type along with SNMP peer address.

    \*varBinds: :class:`tuple` of OID-value pairs or :py:class:`~pysnmp.smi.rfc1902.ObjectType`
        One or more class instances representing MIB variables to place
        into SNMP request.

    Note
    ----
    The `SnmpDispatcher` object may be expensive to create, therefore it is
    advised to maintain it for the lifecycle of the application/thread for
    as long as possible.

    Other Parameters
    ----------------
    \*\*options :
        Request options:

        * `lookupMib` - load MIB and resolve response MIB variables at
          the cost of slightly reduced performance. Default is `False`,
          unless :py:class:`~pysnmp.smi.rfc1902.ObjectType` is present
          among `varBinds` in which case `lookupMib` gets automatically
          enabled.

    Yields
    ------
    errorIndication: str
        True value indicates SNMP engine error.
    errorStatus: str
        True value indicates SNMP PDU error.
    errorIndex: int
        Non-zero value refers to `varBinds[errorIndex-1]`
    varBinds: tuple
        A sequence of OID-value pairs in form of base SNMP types (if
        `lookupMib` is `False`) or :py:class:`~pysnmp.smi.rfc1902.ObjectType`
        class instances (if `lookupMib` is `True`) representing MIB variables
        returned in SNMP response.

    Raises
    ------
    PySnmpError
        Or its derivative indicating that an error occurred while
        performing SNMP operation.

    Examples
    --------
    >>> import asyncio
    >>> from pysnmp.hlapi.v1arch.asyncio import *
    >>>
    >>> @asyncio.coroutine
    ... def run():
    ...     errorIndication, errorStatus, errorIndex, varBinds = yield from getCmd(
    ...         SnmpDispatcher(),
    ...         CommunityData('public'),
    ...         UdpTransportTarget(('demo.snmplabs.com', 161)),
    ...         ObjectType(ObjectIdentity('SNMPv2-MIB', 'sysDescr', 0))
    ...     )
    ...     print(errorIndication, errorStatus, errorIndex, varBinds)
    >>>
    >>> asyncio.get_event_loop().run_until_complete(run())
    (None, 0, 0, [ObjectType(ObjectIdentity(ObjectName('1.3.6.1.2.1.1.1.0')), DisplayString('SunOS zeus.snmplabs.com 4.1.3_U1 1 sun4m'))])
    >>>
    """

    def _cbFun(snmpDispatcher, stateHandle, errorIndication, rspPdu, _cbCtx):
        if future.cancelled():
            return

        errorStatus = pMod.apiPDU.getErrorStatus(rspPdu)
        errorIndex = pMod.apiPDU.getErrorIndex(rspPdu)

        varBinds = pMod.apiPDU.getVarBinds(rspPdu)

        if lookupMib:
            try:
                varBinds = VB_PROCESSOR.unmakeVarBinds(
                    snmpDispatcher.cache, varBinds, lookupMib)

            except Exception as e:
                future.set_exception(e)
                return

        future.set_result(
            (errorIndication, errorStatus, errorIndex, varBinds)
        )

    lookupMib = options.get('lookupMib')

    if not lookupMib and any(isinstance(x, ObjectType) for x in varBinds):
        lookupMib = True

    if lookupMib:
        varBinds = VB_PROCESSOR.makeVarBinds(snmpDispatcher.cache, varBinds)

    pMod = api.PROTOCOL_MODULES[authData.mpModel]

    reqPdu = pMod.GetRequestPDU()
    pMod.apiPDU.setDefaults(reqPdu)
    pMod.apiPDU.setVarBinds(reqPdu, varBinds)

    future = asyncio.Future()

    snmpDispatcher.sendPdu(authData, transportTarget, reqPdu, cbFun=_cbFun)

    return future