How to use the pyats.tcl.eval function in pyats

def get_current_packet_rate(self, first_sample=False):

        # Get the current packet rate
        # expect1.1> TGNGetCurrentPacketRate -array test_params
        result = tcl.eval('TGNGetCurrentPacketRate -array test_params')

        # Process result
        if result == '1':
            log.info("Successfully got current packet rate on device '{}'".format(self.device.name))

            # If this was the first sample of rates, save the timestamp
            if first_sample:
                fs_timestamp = tcl.eval('clock seconds')
                tcl.q.set('test_params(TGN,FirstRateSampleTimeStamp)', fs_timestamp)
        else:
            raise GenieTgnError("Unable to get current packet rate on device '{}'".format(self.device.name))

def send_arp_on_interface(self):

        # Send ARP on interface
        # expect1.1> TGNSendArpOnInterface
        result = tcl.eval('::psat-ng::TGNSendArpOnInterface')

        # Process result
        if result == '1':
            log.info("Successfully sent ARP on interface on TGN device '{}'".format(self.device.name))
        else:
            raise GenieTgnError("Unable to send ARP on interface on TGN device '{}'".format(self.device.name))

def start_routing(self):

        # Start routing
        # expect1.1> TGNRouting
        result = tcl.eval('TGNRouting')

        # Process result
        if result == '1':
            log.info("Successfully started routing on device '{}'".format(self.device.name))
        else:
            raise GenieTgnError("Failed to start routing on device '{}'".format(self.device.name))

def start_traffic(self):

        # Start traffic
        # expect1.1> TGNTraffic
        result = tcl.eval('TGNTraffic')

        # Process result
        if result == '1':
            log.info("Successfully started traffic on device '{}'".format(self.device.name))
        else:
            raise GenieTgnError("Failed to start traffic on device '{}'".format(self.device.name))

def learn_traffic_streams(self):

        # Learn traffic
        # expect1.1> LearnTGN
        result = tcl.eval('LearnTGN')

        # Process result
        if result == '1':
            log.info("Successfully learned traffic streams on device '{}'".format(self.device.name))
        else:
            raise GenieTgnError("Unable to learn traffic streams on device '{}'".format(self.device.name))

def poll_traffic_until_traffic_resumes(self, timeout=60, delay_check_traffic=10):

        # 'timeout' value in seconds [default 60 seconds]
        # 'delay' value in seconds [default 10 seconds]

        # Convert from seconds to milliseconds for PSAT TGN
        delay_check_traffic = delay_check_traffic * 1000

        # expect1.1> TGNPollUntilTrafficResumes -timeout $maxOutageTime -delayCheckTraffic $delayCheckTraffic
        result = tcl.eval('TGNPollUntilTrafficResumes -timeout {timeout}'
                          ' -delayCheckTraffic {delay}'.\
                          format(timeout=timeout, delay=delay_check_traffic))

        # Process result
        if result != '1':
            raise GenieTgnError("Traffic failure observed on device '{}'".\
                                format(self.device.name))

def get_reference_packet_rate(self):

        # Get the referecne packet rate
        # expect1.1> TGNGetReferencePacketRate -maxMinutes $test_params(TGNWaitForRefRateMinutes)
        result = tcl.eval('TGNGetReferencePacketRate -maxMinutes $test_params(TGNWaitForRefRateMinutes)')

        # Process result
        if result == '1':
            log.info("Successfully got reference packet rate on device '{}'".format(self.device.name))
        else:
            raise GenieTgnError("Unable to get reference packet rate on device '{}'".format(self.device.name))

else:
            raise Exception("env(ENA_TESTS) not set or its path does not exist")

        # Check for required env vars
        required_env_vars = ['AUTOTEST', 'ATS_EASY', 'IXIA_HOME', 'IXIA_VERSION',
                             'IXIA_HLTAPI_LIBRARY', 'XBU_SHARED', 'TCLLIBPATH',
                             'TCL_PKG_PREFER_LATEST']
        for env_var in required_env_vars:
            if env_var not in os.environ:
                raise Exception("env({}) has not been set".format(env_var))

        # Required TCL Packages
        tcl.q.package('require', 'cAAs')
        tcl.q.package('require', 'psat-ng')
        tcl.q.package('require', 'enaTgnUtils')
        tcl.eval('namespace import -force ::enaTgnUtils::*')

        # Source PSAT lib files
        try:
            tcl.q.source(psat_tgn)
            tcl.q.source(psat_lib)
        except:
            raise Exception("Unable to source PSAT files required for TGN")

        # Get arguments from TGN testbed YAML file
        try:
            address = self.connection_info['address']
            controller = self.connection_info['controller']
            handle = self.connection_info['handle']
            tgn_type = self.connection_info['tgn_type']
        except KeyError as e:
            raise Exception('Argument not provided in TGN YAML file') from e

def clear_stats(self):

        # Clear TGN statistics
        # expect1.1> TGNClearAllStats
        result = tcl.eval('TGNClearAllStats')

        # Process result
        if result == '1':
            log.info("Successfully cleared statistics on device '{}'".format(self.device.name))
        else:
            raise GenieTgnError("Unable to clear statistics device on '{}'".format(self.device.name))