A simple hole-punching system with sender, receiver and introductor. Launch introductor first, then sender, then receiver. Presumably, may work with full-cone NAT, but not the other type.

hole-punch-introductor.py

#!/usr/bin/python3
# -*- coding: utf-8 -*-
from socket import AF_INET, SOCK_DGRAM
import socket

PORT = 9091

sock = socket.socket(AF_INET, SOCK_DGRAM)

# assign socket to host (empty for any) and port (as tuple, not as two params!)
sock.bind(('', PORT))
print("binded to port", PORT)

adreses = []

while True:

	data, addr = sock.recvfrom(1024)
	print(addr, "Data:",data)
	if data.decode("utf-8") == "ping":
		adreses += [addr]
		sock.sendto(b"pong", addr)

	if len(adreses) >= 2:
		# Once there are two addresses, send address of one peer to another and vice versa
		adr1 = adreses.pop(0)
		adr2 = adreses.pop(0)

		send_to1 = adr2 
		send_to2 = adr1 

		print("Sending data about receiver to sender")
		sock.sendto(" ".join(map(str,send_to1)).encode("utf-8"),adr1)

		break

receiver.py

#!/usr/bin/python3
# -*- coding: utf-8 -*-
import socket
from socket import AF_INET, SOCK_DGRAM
from time import sleep

HOST = 'localhost'
NAT_PORT = 9091
PORT = 44568
NAT_ADDR = (HOST,NAT_PORT)

sock = socket.socket(AF_INET, SOCK_DGRAM)
sock.bind(('', PORT))

sock.sendto(b"ping", NAT_ADDR)

# collect the data about other peer
data = sock.recv(1024)

data_text = data.decode("utf-8")
print("Reply is:", data)

########################
#Establish P2P
########################

data = sock.recv(1024)
print("Reply is:", data)

sender.py

#!/usr/bin/python3
# -*- coding: utf-8 -*-
from socket import AF_INET, SOCK_DGRAM
import socket
from time import sleep

HOST = 'localhost'
NAT_PORT = 9091
PORT = 44567
NAT_ADDR = (HOST,NAT_PORT)

sock = socket.socket(AF_INET, SOCK_DGRAM)
sock.bind(('', PORT))

sock.sendto(b"ping", NAT_ADDR)

data = sock.recv(1024)
print("Reply is:" + data.decode("utf-8"))

# collect the data about other peer
data = sock.recv(1024)
print("Reply is:" + data.decode("utf-8"))

data = data.decode("utf-8").split(" ")

PEER_ADDR = data[0]
PEER_PORT = int(data[1])
PEER_FULL_ADDR = (PEER_ADDR, PEER_PORT)

#####################
#Establish P2P
#####################
sleep(1)

sock.sendto(b"Hello, receiver!", PEER_FULL_ADDR)