Sending image over sockets (ONLY) in Python, image can not be open

Question

I want to upload the image from client to server using ONLY sockets in Python. I designed a very simple protocol flow of how I would like to do this:

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

Here's my client code:

#!/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()

And my server, which receives images from clients:

#!/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()

For the example image:

The client prints out messages, that suggest, that it send the image to server successfully:

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

The image is created at server's side, but I can not open it. And yes, I tried different images. With no success. I can't open them at server's side, even though they are available there.

Thanks to @BorrajaX, I managed to make it work! Thanks! :)

#!/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()

Without testing (and that's why I didn't make this an answer) it looks like in the server you're getting the first `4096` bytes into your `data` variable but those are never written into the file. In the `elif data:` you overwrite `data` with `data = sock.recv(40960000)`, so the first `4096` are never written into the `myfile` file. Maybe is that? — Savir, Feb 25 '17 at 16:32
@BorrajaX: Yes, that was it! I've added `myfile.write(data)` just after the `myfile = open(basename % imgcounter, 'wb')` line and now I can open the image! :) But, its just looks ugly now (the code). Do you have any idea how to improve it? To not to have two `myfile.write(data)`? — yak, Feb 25 '17 at 16:39
@BorrajaX: And how can I avoid errors when the client sends something different than the messages defined in the 'protocol' and the image? I will want to just disconnect it in such case. — yak, Feb 25 '17 at 16:41
Well... to improve... I guess you could always clean up a bit the data writing using `with open(basename % imgcounter, 'wb') as myfile`. As to avoiding errors... Since you don't know what `data` is (is binary) what I'd do is checking (after reading everything) that `data` is **actually** an image. Take a look to PIL (https://pypi.python.org/pypi/PIL) — Savir, Feb 26 '17 at 03:26

score 10 · Accepted Answer · answered Mar 01 '17 at 14:51

Okay your problem is, in the case of actual image received, you are reading the socket two times and ignoring the data in the first read and just writing the data received in the second read. So your image file is incomplete. See my fix below , you can optimize from this taking it as starting point. It works as such.

#!/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)
buffer_size = 4096

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:
                print ' Buffer size is %s' % buffer_size
                data = sock.recv(buffer_size)
                txt = str(data)

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

                    print 'got size'
                    print 'size is %s' % size

                    sock.send("GOT SIZE")
                    # Now set the buffer size for the image 
                    buffer_size = 40960000

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

                elif data:

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

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

                    sock.send("GOT IMAGE")
                    buffer_size = 4096
                    sock.shutdown()
            except:
                sock.close()
                connected_clients_sockets.remove(sock)
                continue
        imgcounter += 1
server_socket.close()

Maybe I misunderstand that but in the while loop, 'myfile' will be opened several times depending on the image size right? Lets say the image I want to receive/send can not be done with one chunk of buffersize 4096, so the remainings will be send afterwards. At that time 'myfile' will be closed and opened again and thus, overwrites the previous data? I tried your code but my image cant be opened afterwards. — V.Hunon, Aug 10 '20 at 11:53

Sending image over sockets (ONLY) in Python, image can not be open

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