Send and receive objects through sockets in Python

Question

I have searched a lot on the Internet, but I haven't been able to find the solution to send an object over the socket and receive it as is. I know it needs pickling which I have already done. And that converts it to bytes and is received on the other hand. But how can I convert those bytes to that type of object?

process_time_data = (current_process_start_time, current_process_end_time)
prepared_process_data = self.prepare_data_to_send(process_time_data)
data_string = io.StringIO(prepared_process_data)
data_string =  pack('>I', len(data_string)) + data_string
self.send_to_server(data_string)

This is the code which is converting the object to StringIO on the client and sending to the server. And on the server side I am getting bytes. Now I am searching for bytes to be converted to StringIO again so that I can get the object value.

In the code, Object is wrapped in StringIO and is being sent over the socket. Is there a better approach?

The server-side code is as follows.

server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
#server.setblocking(0)
server.bind(('127.0.0.1', 50000))
server.listen(5)
inputs = [server]
outputs = []
message_queues = {}

while inputs:
    readable, writeable, exceptional = select.select(inputs, outputs, inputs)
    for s in readable:
        if s is server:
            connection, client_address = s.accept()
            print(client_address)
            connection.setblocking(0)
            inputs.append(connection)
            message_queues[connection] = queue.Queue()
            print('server started...')
        else:
            print('Getting data step 1')
            raw_msglen = s.recv(4)
            msglen = unpack('>I', raw_msglen)[0]
            final_data = b''
            while len(final_data) < msglen:
                data = s.recv(msglen - len(final_data))
                if data:
                    #print(data)
                    final_data += data
                    message_queues[s].put(data)
                    if s not in outputs:
                        outputs.append(s)
                    else:
                        if s in outputs:
                            outputs.remove(s)
                else:
                    break
            inputs.remove(connection)
            #s.close()
            del message_queues[s]

            process_data = ProcessData()
            process_screen = ProcessScreen()

            if final_data is not None:
                try:
                    deserialized_data = final_data.decode("utf-8")
                    print(deserialized_data)
                except (EOFError):
                    break
            else:
                print('final data is empty.')

            print(process_data.project_id)
            print(process_data.start_time)
            print(process_data.end_time)
            print(process_data.process_id)

The two helper functions are as follows:

def receive_all(server, message_length, message_queues, inputs, outputs):
    # Helper function to recv message_length bytes or return None if EOF is hit
    data = b''
    while len(data) < message_length:
        packet = server.recv(message_length - len(data))
        if not packet:
            return None
        data += packet
        message_queues[server].put(data)
        if server not in outputs:
            outputs.append(server)
        else:
            if server in outputs:
                outputs.remove(server)
    inputs.remove(server)
    del message_queues[server]
    return data


def receive_message(server, message_queues, inputs, outputs):
    # Read message length and unpack it into an integer
    raw_msglen = receive_all(server, 4, message_queues, inputs, outputs)
    if not raw_msglen:
        return None
    message_length = unpack('>I', raw_msglen)[0]

    return receive_all(server, message_length, message_queues, inputs, outputs)

And two of the model classes are as follows:

class ProcessData:
    process_id = 0
    project_id = 0
    task_id = 0
    start_time = 0
    end_time = 0
    user_id = 0
    weekend_id = 0

# Model class to send image data to the server
class ProcessScreen:
    process_id = 0
    image_data = bytearray()

Answer 1

You're looking for pickle and the loads and dumps operations. Sockets are basically byte streams. Let us consider the case you have.

class ProcessData:
    process_id = 0
    project_id = 0
    task_id = 0
    start_time = 0
    end_time = 0
    user_id = 0
    weekend_id = 0

An instance of this class needs to be pickled into a data string by doing data_string = pickle.dumps(ProcessData()) and unpickled by doing data_variable = pickle.loads(data) where data is what is received.

So let us consider a case where the client creates an object of ProcessData and sends it to server. Here's what the client would look like. Here's a minimal example.

Client

import socket, pickle

class ProcessData:
    process_id = 0
    project_id = 0
    task_id = 0
    start_time = 0
    end_time = 0
    user_id = 0
    weekend_id = 0


HOST = 'localhost'
PORT = 50007
# Create a socket connection.
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))

# Create an instance of ProcessData() to send to server.
variable = ProcessData()
# Pickle the object and send it to the server
data_string = pickle.dumps(variable)
s.send(data_string)

s.close()
print 'Data Sent to Server'

Now your server which receives this data looks as follows.

Server

import socket, pickle

class ProcessData:
    process_id = 0
    project_id = 0
    task_id = 0
    start_time = 0
    end_time = 0
    user_id = 0
    weekend_id = 0


HOST = 'localhost'
PORT = 50007
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)
conn, addr = s.accept()
print 'Connected by', addr

data = conn.recv(4096)
data_variable = pickle.loads(data)
conn.close()
print data_variable
# Access the information by doing data_variable.process_id or data_variable.task_id etc..,
print 'Data received from client'

Running the server first creates a bind on the port and then running the client makes the data transfer via the socket. You could also look at this answer.