Creates a generator to perform one or more SNMP GET queries.

    On each iteration, new SNMP GET request is send (:RFC:`1905#section-4.2.1`).
    The iterator blocks waiting for response to arrive or error to occur.

    Parameters
    ----------
    snmpEngine : :py:class:`~pysnmp.hlapi.SnmpEngine`
        Class instance representing SNMP engine.

    authData : :py:class:`~pysnmp.hlapi.CommunityData` or :py:class:`~pysnmp.hlapi.UsmUserData`
        Class instance representing SNMP credentials.

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

    contextData : :py:class:`~pysnmp.hlapi.ContextData`
        Class instance representing SNMP ContextEngineId and ContextName values.

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

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

            * `lookupMib` - load MIB and resolve response MIB variables at
              the cost of slightly reduced performance. Default is `True`.

    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 :py:class:`~pysnmp.smi.rfc1902.ObjectType` class
        instances representing MIB variables returned in SNMP response.

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

    Notes
    -----
    The `getCmd` generator will be exhausted immediately unless
    a new sequence of `varBinds` are send back into running generator
    (supported since Python 2.6).

    Examples
    --------
    >>> from pysnmp.hlapi import *
    >>> g = getCmd(SnmpEngine(),
    ...            CommunityData('public'),
    ...            UdpTransportTarget(('demo.snmplabs.com', 161)),
    ...            ContextData(),
    ...            ObjectType(ObjectIdentity('SNMPv2-MIB', 'sysDescr', 0)))
    >>> next(g)
    (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(snmpEngine, authData, transportTarget, contextData,
           *varBinds, **options):
    """Creates a generator to perform one or more SNMP GET queries.

    On each iteration, new SNMP GET request is send (:RFC:`1905#section-4.2.1`).
    The iterator blocks waiting for response to arrive or error to occur.

    Parameters
    ----------
    snmpEngine : :py:class:`~pysnmp.hlapi.SnmpEngine`
        Class instance representing SNMP engine.

    authData : :py:class:`~pysnmp.hlapi.CommunityData` or :py:class:`~pysnmp.hlapi.UsmUserData`
        Class instance representing SNMP credentials.

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

    contextData : :py:class:`~pysnmp.hlapi.ContextData`
        Class instance representing SNMP ContextEngineId and ContextName values.

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

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

            * `lookupMib` - load MIB and resolve response MIB variables at
              the cost of slightly reduced performance. Default is `True`.

    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 :py:class:`~pysnmp.smi.rfc1902.ObjectType` class
        instances representing MIB variables returned in SNMP response.

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

    Notes
    -----
    The `getCmd` generator will be exhausted immediately unless
    a new sequence of `varBinds` are send back into running generator
    (supported since Python 2.6).

    Examples
    --------
    >>> from pysnmp.hlapi import *
    >>> g = getCmd(SnmpEngine(),
    ...            CommunityData('public'),
    ...            UdpTransportTarget(('demo.snmplabs.com', 161)),
    ...            ContextData(),
    ...            ObjectType(ObjectIdentity('SNMPv2-MIB', 'sysDescr', 0)))
    >>> next(g)
    (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'))])
    >>>

    """

    # noinspection PyShadowingNames
    def cbFun(snmpEngine, sendRequestHandle,
              errorIndication, errorStatus, errorIndex,
              varBinds, cbCtx):
        cbCtx['errorIndication'] = errorIndication
        cbCtx['errorStatus'] = errorStatus
        cbCtx['errorIndex'] = errorIndex
        cbCtx['varBinds'] = varBinds

    cbCtx = {}

    while True:
        if varBinds:
            cmdgen.getCmd(snmpEngine, authData, transportTarget,
                          contextData, *varBinds,
                          cbFun=cbFun, cbCtx=cbCtx,
                          lookupMib=options.get('lookupMib', True))

            snmpEngine.transportDispatcher.runDispatcher()

            errorIndication = cbCtx['errorIndication']
            errorStatus = cbCtx['errorStatus']
            errorIndex = cbCtx['errorIndex']
            varBinds = cbCtx['varBinds']

        else:
            errorIndication = errorStatus = errorIndex = None
            varBinds = []

        varBinds = (yield errorIndication, errorStatus, errorIndex, varBinds)

        if not varBinds:
            break