'Creates the Argus object.
fdesc: a file-like object or a filename
pcap_filter: a PCAP filter to use with racluster'

def __init__(self, fdesc, pcap_filter=None):

    cmd = ['racluster', '-u', '-n', '-c', ',', '-m']
    cmd.extend(self.aggregation)
    cmd.append('-s')
    cmd.extend(self.fields)
    cmd.extend(['-r', (fdesc if isinstance(fdesc, basestring) else '-')])
    if (pcap_filter is not None):
        cmd.extend(['-', pcap_filter])
    super(Argus, self).__init__(cmd, ({} if isinstance(fdesc, basestring) else {'stdin': fdesc}))
    self.fdesc.readline()
