Today I learned the basics of socket programming (in Python).
When the day started, I had very little knowledge about sockets. My intuitive understanding was that they were like tunnels that allowed different programs to communicate between themselves.
Now that the day is ending, I still don't know much about sockets, but I did spend the day reading about them and experimenting with them.
I have been documenting the process publicly:
Today, I'm spending the day learning in public about socket programming (in Python 🐍).— Rodrigo 🐍📝 (@mathsppblog) January 4, 2022
My starting point is this:
“I think sockets are like tunnels/roads that allow different programs to talk to each other directly.”
This 🧵 will evolve as I learn and experiment 👇
This helps me make sure I learn as much as possible. It also helps, because others chime in with interesting suggestions quite often.
So, “today I learned about sockets”.
I'll show you how you can create two sockets that communicate with each other.
In order to do this, let's open two Python terminals, and put them side by side.
I'll walk you through the things you have to do, just make sure to write each piece of code in the correct REPL:
To create the server, we
# (SERVER) >>> import socket >>> server = socket.socket(socket.AF_INET, socket.SOCK_STREAM) >>> server.bind(("localhost", 7342)) # 1 >>> server.listen() # 2 >>> client, address = server.accept() # 3
At this point, the code hangs because we are waiting for a client to connect.
To create a client, we do the exact same first step, but then we connect to the host and port:
# (CLIENT) >>> import socket >>> client = socket.socket(socket.AF_INET, socket.SOCK_STREAM) >>> client.connect(("localhost", 7342)) # 4
At this point, the server side is no longer hanging, because a client connected and the connection was automatically accepted.
Now, either side can send the first message. If this were a communication through an established protocol, we would know the socket that is expected to send the first message. For example, on the web, with the browsers, browsers are expected to send the first message (the request) to the server. Only then the server replies.
For our toy experiment, let's have the client send the first (and only) message with some data, which the server will give back in reverse.
Sending the data means calling the method
# (CLIENT) >>> client.send(b"Hello, world!") # 5 13
Now that we sent some data, the server can receive it and handle it. In our case, we receive the data, and then return it reversed.
After we reverse the data and send it back, we close the socket. In our toy example, we assume that the client can only use the socket once, which means that the server has no reason to keep the socket open after returning the reversed data.
Here is the code that implements the steps described:
# (SERVER) >>> data = client.recv(1024) >>> print(data.decode("utf8")) Hello, world! >>> response = bytes(reversed(data)) >>> client.send(response) 13 >>> client.close()
While this is happening, the client side is waiting to receive a message back, which we can receive and print:
# (CLIENT) >>> back = client.recv(1024) >>> print(back.decode("utf8")) !dlrow ,olleH
After this, we can see that the connection on the server side was indeed closed.
This is signalled by the fact that calling the method
.recv once more returns an empty bytes sequence.
This is how sockets signal that the other end was closed;
if the server side wasn't closed but the server hadn't sent any (more) data yet,
the method call would just hang and wait for data to be received.
So, here is our verification that the server side was indeed closed, after which we close our end of the connection too:
# (CLIENT) >>> client.recv(1024) b'' >>> client.close()
That's it for now! Stay tuned and I'll see you around!
I hope you learned something new! If you did, consider following the footsteps of the readers who bought me a slice of pizza 🍕. Your small contribution helps me produce this content for free and without spamming you with annoying ads.