UDP Client/Server Socket in Python

Here is an alternative with asyncio.

import asyncio
import random

class EchoServerProtocol:
    def connection_made(self, transport):
        self.transport = transport

    def datagram_received(self, data, addr):
        message = data.decode()
        print('Received %r from %s' % (message, addr))
        rand = random.randint(0, 10)
        if rand >= 4:
            print('Send %r to %s' % (message, addr))
            self.transport.sendto(data, addr)
        else:
            print('Send %r to %s' % (message, addr))
            self.transport.sendto(data, addr)


loop = asyncio.get_event_loop()
print("Starting UDP server")

# One protocol instance will be created to serve all client requests
listen = loop.create_datagram_endpoint(
    EchoServerProtocol, local_addr=('127.0.0.1', 12000))
transport, protocol = loop.run_until_complete(listen)

try:
    loop.run_forever()
except KeyboardInterrupt:
    pass

transport.close()
loop.close()

I tested your code, and it works as expected on my machine. Your issue might not be your code. It could be a firewall or something else blocking all the packets on the loopback interface (127.0.0.1). Depending on your operating system, try testing with a packet monitor like Wireshark.

Also, here are a few suggestions on how to improve your code to be more Pythonic:

Server

import random
import socket

server_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
server_socket.bind(('', 12000))

while True:
    rand = random.randint(0, 10)
    message, address = server_socket.recvfrom(1024)
    message = message.upper()
    if rand >= 4:
        server_socket.sendto(message, address)

Client

import time
import socket

for pings in range(10):
    client_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    client_socket.settimeout(1.0)
    message = b'test'
    addr = ("127.0.0.1", 12000)

    start = time.time()
    client_socket.sendto(message, addr)
    try:
        data, server = client_socket.recvfrom(1024)
        end = time.time()
        elapsed = end - start
        print(f'{data} {pings} {elapsed}')
    except socket.timeout:
        print('REQUEST TIMED OUT')