Performs SNMP GET query.

    Based on passed parameters, prepares SNMP GET packet
    (:RFC:`1905#section-4.2.1`) and schedules its transmission by
    I/O framework at a later point of time.

    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 :

        * `lookupMib` (bool) - load MIB and resolve response MIB variables at
          the cost of slightly reduced performance. Default is `True`.
        * `cbFun` (callable) - user-supplied callable that is invoked
           to pass SNMP response data or error to user at a later point
           of time. Default is `None`
        * `cbCtx` (object) - user-supplied object passing additional
           parameters to/from `cbFun`. Default is `None`.

    Notes
    -----
    User-supplied `cbFun` callable must have the following call
    signature:

    * snmpEngine (:py:class:`~pysnmp.hlapi.SnmpEngine`):
      Class instance representing SNMP engine.
    * sendRequestHandle (int): Unique request identifier. Can be used
      for matching multiple ongoing requests with received responses.
    * 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 in exactly
      the same order as `varBinds` in request.
    * `cbCtx` : Original user-supplied object.

    Returns
    -------
    sendRequestHandle : int
        Unique request identifier. Can be used for matching received
        responses with ongoing requests.

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

    Examples
    --------
    >>> from pysnmp.hlapi.asyncore import *
    >>> def cbFun(snmpEngine, sendRequestHandle, errorIndication, errorStatus, errorIndex, varBinds, cbCtx):
    ...     print(errorIndication, errorStatus, errorIndex, varBinds)
    >>>
    >>> snmpEngine = SnmpEngine()
    >>> getCmd(snmpEngine,
    ...        CommunityData('public'),
    ...        UdpTransportTarget(('demo.snmplabs.com', 161)),
    ...        ContextData(),
    ...        ObjectType(ObjectIdentity('SNMPv2-MIB', 'sysDescr', 0)),
    ...        cbFun=cbFun)
    >>> snmpEngine.transportDispatcher.runDispatcher()
    (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):
    """Performs SNMP GET query.

    Based on passed parameters, prepares SNMP GET packet
    (:RFC:`1905#section-4.2.1`) and schedules its transmission by
    I/O framework at a later point of time.

    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 :

        * `lookupMib` (bool) - load MIB and resolve response MIB variables at
          the cost of slightly reduced performance. Default is `True`.
        * `cbFun` (callable) - user-supplied callable that is invoked
           to pass SNMP response data or error to user at a later point
           of time. Default is `None`
        * `cbCtx` (object) - user-supplied object passing additional
           parameters to/from `cbFun`. Default is `None`.

    Notes
    -----
    User-supplied `cbFun` callable must have the following call
    signature:

    * snmpEngine (:py:class:`~pysnmp.hlapi.SnmpEngine`):
      Class instance representing SNMP engine.
    * sendRequestHandle (int): Unique request identifier. Can be used
      for matching multiple ongoing requests with received responses.
    * 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 in exactly
      the same order as `varBinds` in request.
    * `cbCtx` : Original user-supplied object.

    Returns
    -------
    sendRequestHandle : int
        Unique request identifier. Can be used for matching received
        responses with ongoing requests.

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

    Examples
    --------
    >>> from pysnmp.hlapi.asyncore import *
    >>> def cbFun(snmpEngine, sendRequestHandle, errorIndication, errorStatus, errorIndex, varBinds, cbCtx):
    ...     print(errorIndication, errorStatus, errorIndex, varBinds)
    >>>
    >>> snmpEngine = SnmpEngine()
    >>> getCmd(snmpEngine,
    ...        CommunityData('public'),
    ...        UdpTransportTarget(('demo.snmplabs.com', 161)),
    ...        ContextData(),
    ...        ObjectType(ObjectIdentity('SNMPv2-MIB', 'sysDescr', 0)),
    ...        cbFun=cbFun)
    >>> snmpEngine.transportDispatcher.runDispatcher()
    (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(snmpEngine, sendRequestHandle,
                errorIndication, errorStatus, errorIndex,
                varBinds, cbCtx):

        lookupMib, cbFun, cbCtx = cbCtx

        if cbFun:
            varBinds = VB_PROCESSOR.unmakeVarBinds(
                snmpEngine.cache, varBinds, lookupMib)

            return cbFun(snmpEngine, sendRequestHandle, errorIndication,
                         errorStatus, errorIndex, varBinds, cbCtx)

    addrName, paramsName = LCD.configure(
        snmpEngine, authData, transportTarget, contextData.contextName)

    varBinds = VB_PROCESSOR.makeVarBinds(snmpEngine.cache, varBinds)

    return cmdgen.GetCommandGenerator().sendVarBinds(
        snmpEngine, addrName, contextData.contextEngineId,
        contextData.contextName, varBinds, __cbFun,
        (options.get('lookupMib', True),
         options.get('cbFun'), options.get('cbCtx')))