Socket Programming in Python (Guide)

Background

Sockets have a long history. Their use originated with ARPANET in 1971 and later became an API in the Berkeley Software Distribution (BSD) operating system released in 1983 called Berkeley sockets.

When the Internet took off in the 1990s with the World Wide Web, so did network programming. Web servers and browsers weren’t the only applications taking advantage of newly connected networks and using sockets. Client-server applications of all types and sizes came into widespread use.

Today, although the underlying protocols used by the socket API have evolved over the years, and new ones have developed, the low-level API has remained the same.

The most common type of socket applications are client-server applications, where one side acts as the server and waits for connections from clients. This is the type of application that you’ll be creating in this tutorial. More specifically, you’ll focus on the socket API for Internet sockets, sometimes called Berkeley or BSD sockets. There are also Unix domain sockets, which can only be used to communicate between processes on the same host.

Socket API Overview

Python’s socket module provides an interface to the Berkeley sockets API. This is the module that you’ll use in this tutorial.

The primary socket API functions and methods in this module are:

• socket()
• .bind()
• .listen()
• .accept()
• .connect()
• .connect_ex()
• .send()
• .recv()
• .close()

Python provides a convenient and consistent API that maps directly to system calls, their C counterparts. In the next section, you’ll learn how these are used together.

As part of its standard library, Python also has classes that make using these low-level socket functions easier. Although it’s not covered in this tutorial, you can check out the socketserver module, a framework for network servers. There are also many modules available that implement higher-level Internet protocols like HTTP and SMTP. For an overview, see Internet Protocols and Support.

TCP Sockets

You’re going to create a socket object using socket.socket(), specifying the socket type as socket.SOCK_STREAM. When you do that, the default protocol that’s used is the Transmission Control Protocol (TCP). This is a good default and probably what you want.

Why should you use TCP? The Transmission Control Protocol (TCP):

• Is reliable: Packets dropped in the network are detected and retransmitted by the sender.
• Has in-order data delivery: Data is read by your application in the order it was written by the sender.

In contrast, User Datagram Protocol (UDP) sockets created with socket.SOCK_DGRAM aren’t reliable, and data read by the receiver can be out-of-order from the sender’s writes.

Why is this important? Networks are a best-effort delivery system. There’s no guarantee that your data will reach its destination or that you’ll receive what’s been sent to you.

Network devices, such as routers and switches, have finite bandwidth available and come with their own inherent system limitations. They have CPUs, memory, buses, and interface packet buffers, just like your clients and servers. TCP relieves you from having to worry about packet loss, out-of-order data arrival, and other pitfalls that invariably happen when you’re communicating across a network.

To better understand this, check out the sequence of socket API calls and data flow for TCP:

The left-hand column represents the server. On the right-hand side is the client.

Starting in the top left-hand column, note the API calls that the server makes to set up a “listening” socket:

• socket()
• .bind()
• .listen()
• .accept()

A listening socket does just what its name suggests. It listens for connections from clients. When a client connects, the server calls .accept() to accept, or complete, the connection.

The client calls .connect() to establish a connection to the server and initiate the three-way handshake. The handshake step is important because it ensures that each side of the connection is reachable in the network, in other words that the client can reach the server and vice-versa. It may be that only one host, client, or server can reach the other.

In the middle is the round-trip section, where data is exchanged between the client and server using calls to .send() and .recv().

At the bottom, the client and server close their respective sockets.

Echo Client and Server

Now that you’ve gotten an overview of the socket API and how the client and server communicate, you’re ready to create your first client and server. You’ll begin with a simple implementation. The server will simply echo whatever it receives back to the client.

# echo-server.py

import socket

HOST = "127.0.0.1"  # Standard loopback interface address (localhost)
PORT = 65432  # Port to listen on (non-privileged ports are > 1023)

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()
    conn, addr = s.accept()
    with conn:
        print(f"Connected by {addr}")
        while True:
            data = conn.recv(1024)
            if not data:
                break
            conn.sendall(data)

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    pass  # Use the socket object without calling s.close().

# echo-server.py

# ...

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    # ...

# echo-server.py

# ...

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()
    conn, addr = s.accept()
    # ...