Quero fazer upload da imagem do cliente para o servidor usando SOMENTE soquetes em Python. Projetei um fluxo de protocolo muito simples de como gostaria de fazer isso:

CLIENT                                       SERVER
               SIZE 512 (send image size)
         ---------------------------------->
                      GOT SIZE   
         <---------------------------------- 
                 send image itself
         ---------------------------------->
                      GOT IMAGE
         <----------------------------------
                      BYE BYE 
         ---------------------------------->
                server closes socket

Aqui está o meu código de cliente:

#!/usr/bin/env python

import random
import socket, select
from time import gmtime, strftime
from random import randint

image = "tux.png"

HOST = '127.0.0.1'
PORT = 6666

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_address = (HOST, PORT)
sock.connect(server_address)

try:

    # open image
    myfile = open(image, 'rb')
    bytes = myfile.read()
    size = len(bytes)

    # send image size to server
    sock.sendall("SIZE %s" % size)
    answer = sock.recv(4096)

    print 'answer = %s' % answer

    # send image to server
    if answer == 'GOT SIZE':
        sock.sendall(bytes)

        # check what server send
        answer = sock.recv(4096)
        print 'answer = %s' % answer

        if answer == 'GOT IMAGE' :
            sock.sendall("BYE BYE ")
            print 'Image successfully send to server'

    myfile.close()

finally:
    sock.close()

E meu servidor, que recebe imagens de clientes:

#!/usr/bin/env python

import random
import socket, select
from time import gmtime, strftime
from random import randint

imgcounter = 1
basename = "image%s.png"

HOST = '127.0.0.1'
PORT = 6666

connected_clients_sockets = []

server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

server_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server_socket.bind((HOST, PORT))
server_socket.listen(10)

connected_clients_sockets.append(server_socket)

while True:

    read_sockets, write_sockets, error_sockets = select.select(connected_clients_sockets, [], [])

    for sock in read_sockets:

        if sock == server_socket:

            sockfd, client_address = server_socket.accept()
            connected_clients_sockets.append(sockfd)

        else:
            try:

                data = sock.recv(4096)
                txt = str(data)

                if txt.startswith('SIZE'):
                    tmp = txt.split()
                    size = int(tmp[1])

                    print 'got size'

                    sock.send("GOT SIZE")

                elif txt.startswith('BYE'):
                    sock.shutdown()

                elif data:

                    myfile = open(basename % imgcounter, 'wb')

                    data = sock.recv(40960000)
                    if not data:
                        myfile.close()
                        break
                    myfile.write(data)
                    myfile.close()

                    sock.send("GOT IMAGE")
                    sock.shutdown()
            except:
                sock.close()
                connected_clients_sockets.remove(sock)
                continue
        imgcounter += 1
server_socket.close()

Para a imagem de exemplo:

O cliente imprime mensagens, que sugerem, que envie a imagem ao servidor com êxito:

answer = GOT SIZE
answer = GOT IMAGE
Image successfully send to server

A imagem é criada no lado do servidor, mas não consigo abri-la. E sim, tentei imagens diferentes. Sem sucesso. Não consigo abri-los no servidor, mesmo que estejam disponíveis lá.

Graças a @BorrajaX, consegui fazê-lo funcionar! Obrigado! :)

#!/usr/bin/env python

import random
import socket, select
from time import gmtime, strftime
from random import randint

imgcounter = 1
basename = "image%s.png"

HOST = '127.0.0.1'
PORT = 6666

connected_clients_sockets = []

server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

server_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server_socket.bind((HOST, PORT))
server_socket.listen(10)

connected_clients_sockets.append(server_socket)

while True:

    read_sockets, write_sockets, error_sockets = select.select(connected_clients_sockets, [], [])

    for sock in read_sockets:

        if sock == server_socket:

            sockfd, client_address = server_socket.accept()
            connected_clients_sockets.append(sockfd)

        else:
            try:

                data = sock.recv(4096)
                txt = str(data)

                if data:

                    if data.startswith('SIZE'):
                        tmp = txt.split()
                        size = int(tmp[1])

                        print 'got size'

                        sock.sendall("GOT SIZE")

                    elif data.startswith('BYE'):
                        sock.shutdown()

                    else :

                        myfile = open(basename % imgcounter, 'wb')
                        myfile.write(data)

                        data = sock.recv(40960000)
                        if not data:
                            myfile.close()
                            break
                        myfile.write(data)
                        myfile.close()

                        sock.sendall("GOT IMAGE")
                        sock.shutdown()
            except:
                sock.close()
                connected_clients_sockets.remove(sock)
                continue
        imgcounter += 1
server_socket.close()