The work is carried out on the Ryu controller, the source code of which is posted here: https://github.com/faucetsdn/ryu/tree/master/ryu In addition, the following software is used 1)!Ubuntu 20.04 LTS 2) Python 3.8.10 and 2.7.18 are installed. Using the command:
:~$ ryu-manager --observe-links multipath_app.py (don't pay attention to the file name, I changed it relative to the author's) trying to launch the app multipath_app.py This code is available at the link: https://github.com/wildan2711/multipath/blob/master/ryu_multipath.py After launching, I get an error in bash with the following errors:

  File "/home/temich/.local/lib/python3.8/site-packages/ryu/utils.py", line 99, in import_module
    return _import_module_file(modname)
  File "/home/temich/.local/lib/python3.8/site-packages/ryu/utils.py", line 88, in _import_module_file
    return load_source(modname, abspath)
  File "/home/temich/.local/lib/python3.8/site-packages/ryu/utils.py", line 42, in load_source
    return loader.load_module(name)
  File "<frozen importlib._bootstrap_external>", line 522, in _check_name_wrapper
  File "<frozen importlib._bootstrap_external>", line 1027, in load_module
  File "<frozen importlib._bootstrap_external>", line 852, in load_module
  File "<frozen importlib._bootstrap>", line 265, in _load_module_shim
  File "<frozen importlib._bootstrap>", line 702, in _load
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 844, in exec_module
  File "<frozen importlib._bootstrap_external>", line 981, in get_code
  File "<frozen importlib._bootstrap_external>", line 911, in source_to_code
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/temich/multipath_ap.py", line 65
    print "Available paths from ", src, " to ", dst, " : ", paths
          ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print("Available paths from ", src, " to ", dst, " : ", paths)?
...
ModuleNotFoundError: No module named 'multipath_ap'

After that, it became clear that python was swearing at the wrong syntax corresponding to python 2, but even so it is unclear why the error ModuleNotFoundError appears: No module named 'multipath_ap'. Realizing this, I tried to change the version of the Python interpreter that uses ryu-manager in the file of the same name with the following code:

#!/usr/bin/python3 (was changed to #!/usr/bin/python2)
# -*- coding: utf-8 -*-
import re
import sys
from ryu.cmd.manager import main
if __name__ == '__main__':
    sys.argv[0] = re.sub(r'(-script\.pyw|\.exe)?$', '', sys.argv[0])
    sys.exit(main())

But unfortunately, this did not lead to the expected launch of the application. Information about the error is provided below.

Traceback (most recent call last):
  File "/home/temich/.local/bin/ryu-manager", line 5, in <module>
    from ryu.cmd.manager import main
ImportError: No module named ryu.cmd.manager

This error does not occur when running standard applications. To summarize, the questions are as follows: 1) How do I still run this application? 2) Does Ru really work on Python3 or is it not important at all? 3) Is the solution to adapt the multipath_ap application code to Python3? 4) If so, how to do it most effectively? 5) If it is possible and the work is not so great, then it is interesting to find out the adaptation option.

Thanks

Code of multipath_app.py

from ryu.base import app_manager
from ryu.controller import mac_to_port
from ryu.controller import ofp_event
from ryu.controller.handler import CONFIG_DISPATCHER, MAIN_DISPATCHER
from ryu.controller.handler import set_ev_cls
from ryu.ofproto import ofproto_v1_3
from ryu.lib.mac import haddr_to_bin
from ryu.lib.packet import packet
from ryu.lib.packet import arp
from ryu.lib.packet import ethernet
from ryu.lib.packet import ipv4
from ryu.lib.packet import ipv6
from ryu.lib.packet import ether_types
from ryu.lib import mac, ip
from ryu.topology.api import get_switch, get_link
from ryu.app.wsgi import ControllerBase
from ryu.topology import event

from collections import defaultdict
from operator import itemgetter

import os
import random
import time

# Cisco Reference bandwidth = 1 Gbps
REFERENCE_BW = 10000000

DEFAULT_BW = 10000000

MAX_PATHS = 2

class ProjectController(app_manager.RyuApp):
    OFP_VERSIONS = [ofproto_v1_3.OFP_VERSION]

    def __init__(self, *args, **kwargs):
        super(ProjectController, self).__init__(*args, **kwargs)
        self.mac_to_port = {}
        self.topology_api_app = self
        self.datapath_list = {}
        self.arp_table = {}
        self.switches = []
        self.hosts = {}
        self.multipath_group_ids = {}
        self.group_ids = []
        self.adjacency = defaultdict(dict)
        self.bandwidths = defaultdict(lambda: defaultdict(lambda: DEFAULT_BW))

    def get_paths(self, src, dst):
        '''
        Get all paths from src to dst using DFS algorithm    
        '''
        if src == dst:
            # host target is on the same switch
            return [[src]]
        paths = []
        stack = [(src, [src])]
        while stack:
            (node, path) = stack.pop()
            for next in set(self.adjacency[node].keys()) - set(path):
                if next is dst:
                    paths.append(path + [next])
                else:
                    stack.append((next, path + [next]))
        print "Available paths from ", src, " to ", dst, " : ", paths
        return paths

    def get_link_cost(self, s1, s2):
        '''
        Get the link cost between two switches 
        '''
        e1 = self.adjacency[s1][s2]
        e2 = self.adjacency[s2][s1]
        bl = min(self.bandwidths[s1][e1], self.bandwidths[s2][e2])
        ew = REFERENCE_BW/bl
        return ew

    def get_path_cost(self, path):
        '''
        Get the path cost
        '''
        cost = 0
        for i in range(len(path) - 1):
            cost += self.get_link_cost(path[i], path[i+1])
        return cost

    def get_optimal_paths(self, src, dst):
        '''
        Get the n-most optimal paths according to MAX_PATHS
        '''
        paths = self.get_paths(src, dst)
        paths_count = len(paths) if len(
            paths) < MAX_PATHS else MAX_PATHS
        return sorted(paths, key=lambda x: self.get_path_cost(x))[0:(paths_count)]

    def add_ports_to_paths(self, paths, first_port, last_port):
        '''
        Add the ports that connects the switches for all paths
        '''
        paths_p = []
        for path in paths:
            p = {}
            in_port = first_port
            for s1, s2 in zip(path[:-1], path[1:]):
                out_port = self.adjacency[s1][s2]
                p[s1] = (in_port, out_port)
                in_port = self.adjacency[s2][s1]
            p[path[-1]] = (in_port, last_port)
            paths_p.append(p)
        return paths_p

    def generate_openflow_gid(self):
        '''
        Returns a random OpenFlow group id
        '''
        n = random.randint(0, 2**32)
        while n in self.group_ids:
            n = random.randint(0, 2**32)
        return n


    def install_paths(self, src, first_port, dst, last_port, ip_src, ip_dst):
        computation_start = time.time()
        paths = self.get_optimal_paths(src, dst)
        pw = []
        for path in paths:
            pw.append(self.get_path_cost(path))
            print path, "cost = ", pw[len(pw) - 1]
        sum_of_pw = sum(pw) * 1.0
        paths_with_ports = self.add_ports_to_paths(paths, first_port, last_port)
        switches_in_paths = set().union(*paths)

        for node in switches_in_paths:

            dp = self.datapath_list[node]
            ofp = dp.ofproto
            ofp_parser = dp.ofproto_parser

            ports = defaultdict(list)
            actions = []
            i = 0

            for path in paths_with_ports:
                if node in path:
                    in_port = path[node][0]
                    out_port = path[node][1]
                    if (out_port, pw[i]) not in ports[in_port]:
                        ports[in_port].append((out_port, pw[i]))
                i += 1

            for in_port in ports:

                match_ip = ofp_parser.OFPMatch(
                    eth_type=0x0800, 
                    ipv4_src=ip_src, 
                    ipv4_dst=ip_dst
                )
                match_arp = ofp_parser.OFPMatch(
                    eth_type=0x0806, 
                    arp_spa=ip_src, 
                    arp_tpa=ip_dst
                )

                out_ports = ports[in_port]
                # print out_ports 

                if len(out_ports) > 1:
                    group_id = None
                    group_new = False

                    if (node, src, dst) not in self.multipath_group_ids:
                        group_new = True
                        self.multipath_group_ids[
                            node, src, dst] = self.generate_openflow_gid()
                    group_id = self.multipath_group_ids[node, src, dst]

                    buckets = []
                    # print "node at ",node," out ports : ",out_ports
                    for port, weight in out_ports:
                        bucket_weight = int(round((1 - weight/sum_of_pw) * 10))
                        bucket_action = [ofp_parser.OFPActionOutput(port)]
                        buckets.append(
                            ofp_parser.OFPBucket(
                                weight=bucket_weight,
                                watch_port=port,
                                watch_group=ofp.OFPG_ANY,
                                actions=bucket_action
                            )
                        )

                    if group_new:
                        req = ofp_parser.OFPGroupMod(
                            dp, ofp.OFPGC_ADD, ofp.OFPGT_SELECT, group_id,
                            buckets
                        )
                        dp.send_msg(req)
                    else:
                        req = ofp_parser.OFPGroupMod(
                            dp, ofp.OFPGC_MODIFY, ofp.OFPGT_SELECT,
                            group_id, buckets)
                        dp.send_msg(req)

                    actions = [ofp_parser.OFPActionGroup(group_id)]

                    self.add_flow(dp, 32768, match_ip, actions)
                    self.add_flow(dp, 1, match_arp, actions)

                elif len(out_ports) == 1:
                    actions = [ofp_parser.OFPActionOutput(out_ports[0][0])]

                    self.add_flow(dp, 32768, match_ip, actions)
                    self.add_flow(dp, 1, match_arp, actions)
        print "Path installation finished in ", time.time() - computation_start 
        return paths_with_ports[0][src][1]

    def add_flow(self, datapath, priority, match, actions, buffer_id=None):
        # print "Adding flow ", match, actions
        ofproto = datapath.ofproto
        parser = datapath.ofproto_parser

        inst = [parser.OFPInstructionActions(ofproto.OFPIT_APPLY_ACTIONS,
                                             actions)]
        if buffer_id:
            mod = parser.OFPFlowMod(datapath=datapath, buffer_id=buffer_id,
                                    priority=priority, match=match,
                                    instructions=inst)
        else:
            mod = parser.OFPFlowMod(datapath=datapath, priority=priority,
                                    match=match, instructions=inst)
        datapath.send_msg(mod)

    @set_ev_cls(ofp_event.EventOFPSwitchFeatures, CONFIG_DISPATCHER)
    def _switch_features_handler(self, ev):
        print "switch_features_handler is called"
        datapath = ev.msg.datapath
        ofproto = datapath.ofproto
        parser = datapath.ofproto_parser

        match = parser.OFPMatch()
        actions = [parser.OFPActionOutput(ofproto.OFPP_CONTROLLER,
                                          ofproto.OFPCML_NO_BUFFER)]
        self.add_flow(datapath, 0, match, actions)

    @set_ev_cls(ofp_event.EventOFPPortDescStatsReply, MAIN_DISPATCHER)
    def port_desc_stats_reply_handler(self, ev):
        switch = ev.msg.datapath
        for p in ev.msg.body:
            self.bandwidths[switch.id][p.port_no] = p.curr_speed

    @set_ev_cls(ofp_event.EventOFPPacketIn, MAIN_DISPATCHER)
    def _packet_in_handler(self, ev):
        msg = ev.msg
        datapath = msg.datapath
        ofproto = datapath.ofproto
        parser = datapath.ofproto_parser
        in_port = msg.match['in_port']

        pkt = packet.Packet(msg.data)
        eth = pkt.get_protocol(ethernet.ethernet)
        arp_pkt = pkt.get_protocol(arp.arp)

        # avoid broadcast from LLDP
        if eth.ethertype == 35020:
            return

        if pkt.get_protocol(ipv6.ipv6):  # Drop the IPV6 Packets.
            match = parser.OFPMatch(eth_type=eth.ethertype)
            actions = []
            self.add_flow(datapath, 1, match, actions)
            return None

        dst = eth.dst
        src = eth.src
        dpid = datapath.id

        if src not in self.hosts:
            self.hosts[src] = (dpid, in_port)

        out_port = ofproto.OFPP_FLOOD

        if arp_pkt:
            # print dpid, pkt
            src_ip = arp_pkt.src_ip
            dst_ip = arp_pkt.dst_ip
            if arp_pkt.opcode == arp.ARP_REPLY:
                self.arp_table[src_ip] = src
                h1 = self.hosts[src]
                h2 = self.hosts[dst]
                out_port = self.install_paths(h1[0], h1[1], h2[0], h2[1], src_ip, dst_ip)
                self.install_paths(h2[0], h2[1], h1[0], h1[1], dst_ip, src_ip) # reverse
            elif arp_pkt.opcode == arp.ARP_REQUEST:
                if dst_ip in self.arp_table:
                    self.arp_table[src_ip] = src
                    dst_mac = self.arp_table[dst_ip]
                    h1 = self.hosts[src]
                    h2 = self.hosts[dst_mac]
                    out_port = self.install_paths(h1[0], h1[1], h2[0], h2[1], src_ip, dst_ip)
                    self.install_paths(h2[0], h2[1], h1[0], h1[1], dst_ip, src_ip) # reverse

        # print pkt

        actions = [parser.OFPActionOutput(out_port)]

        data = None
        if msg.buffer_id == ofproto.OFP_NO_BUFFER:
            data = msg.data

        out = parser.OFPPacketOut(
            datapath=datapath, buffer_id=msg.buffer_id, in_port=in_port,
            actions=actions, data=data)
        datapath.send_msg(out)

    @set_ev_cls(event.EventSwitchEnter)
    def switch_enter_handler(self, ev):
        switch = ev.switch.dp
        ofp_parser = switch.ofproto_parser

        if switch.id not in self.switches:
            self.switches.append(switch.id)
            self.datapath_list[switch.id] = switch

            # Request port/link descriptions, useful for obtaining bandwidth
            req = ofp_parser.OFPPortDescStatsRequest(switch)
            switch.send_msg(req)

    @set_ev_cls(event.EventSwitchLeave, MAIN_DISPATCHER)
    def switch_leave_handler(self, ev):
        print ev
        switch = ev.switch.dp.id
        if switch in self.switches:
            self.switches.remove(switch)
            del self.datapath_list[switch]
            del self.adjacency[switch]

    @set_ev_cls(event.EventLinkAdd, MAIN_DISPATCHER)
    def link_add_handler(self, ev):
        s1 = ev.link.src
        s2 = ev.link.dst
        self.adjacency[s1.dpid][s2.dpid] = s1.port_no
        self.adjacency[s2.dpid][s1.dpid] = s2.port_no

    @set_ev_cls(event.EventLinkDelete, MAIN_DISPATCHER)
    def link_delete_handler(self, ev):
        s1 = ev.link.src
        s2 = ev.link.dst
        # Exception handling if switch already deleted
        try:
            del self.adjacency[s1.dpid][s2.dpid]
            del self.adjacency[s2.dpid][s1.dpid]
        except KeyError:
            pass