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To create a simple TCP server in Python, make a stream socket, bind it to an address and port, call listen(), then use accept() to get a separate socket for each client connection. The example below binds to IPv4 loopback, so it is reachable only from the same computer, and echoes bytes back to clients one at a time.
How do I create a simple TCP server in Python?
Python’s built-in socket module provides the low-level operations needed to accept TCP connections. This learning example uses 127.0.0.1 for a local test, repeats accept() for successive clients, and closes sockets with context managers.
import socket
HOST = "127.0.0.1" # IPv4 loopback: local connections only
PORT = 50007
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as server:
server.bind((HOST, PORT))
server.listen()
print(f"Listening on {HOST}:{PORT}")
while True:
conn, address = server.accept()
with conn:
print("Connected by", address)
while True:
data = conn.recv(1024)
if not data:
break
conn.sendall(data)
Save the file as server.py and run it with Python. The program waits for a client, prints its address, and sends back any bytes it receives. It continues accepting later connections after each client disconnects. This is an illustrative adaptation of the Python Software Foundation’s official socket example, not a production-ready service.
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socket.socket(socket.AF_INET, socket.SOCK_STREAM)creates an IPv4 stream socket;SOCK_STREAMis the socket type used for TCP.bind((HOST, PORT))assigns the listening socket a local address and port.listen()marks it as ready to accept incoming connections. The operating system manages a queue of connection requests that have not yet been accepted; omitting a backlog lets the system choose a default rather than guaranteeing a particular capacity.accept()waits for a connection and returns a new socket plus the peer address.recv(1024)reads bytes from that connection. If it returns an empty byte string, the peer has closed its sending side.sendall(data)sends the received bytes back, continuing until all of that byte string has been sent or an error occurs.
The Python Software Foundation describes the server sequence as socket(), bind(), listen(), and accept(), with accept() repeated to service more than one client. A client instead connects with connect(). See the socket reference.
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Why does accept() return another socket?
The original server socket remains in listening mode: it waits for new connections. Each call to accept() returns a distinct conn socket for communication with one connected client, along with that client’s address. Read and write client data through conn, not through server. Keeping these roles separate lets the server continue listening for connections after it finishes handling the current one.
How can I test a Python socket server locally?
Run the server in one terminal, then run this small client in another terminal on the same computer. It connects to the same loopback address and port, sends bytes, and prints the echoed response.
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import socket
HOST = "127.0.0.1"
PORT = 50007
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as client:
client.connect((HOST, PORT))
client.sendall(b"Hello, server")
print(client.recv(1024))
The expected output is b'Hello, server'. The server prints the client’s address when the connection is accepted. If the client reports that the connection was refused, check that the server is running, that both programs use the same port and address, and that no other process has taken that port.
What does the server’s bind address change?
127.0.0.1 is the IPv4 loopback address. Binding to it limits connections to the same machine, which is the safer choice for a beginner’s local exercise. The empty host string, "", means all available interfaces in the Python documentation’s example. That can make a service reachable through the machine’s network addresses by other devices, subject to firewall and network configuration. Use it only when you intend to allow that reachability.
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The example explicitly selects IPv4 with AF_INET. IPv6 and dual-stack behavior depends on the address family and platform. Python’s socket reference demonstrates resolving addresses with getaddrinfo() and warns that choosing only the first available address family may leave the server unable to accept traffic on another family. It also documents socket.create_server(), available since Python 3.8, with a platform-dependent dualstack_ipv6 option; neither guarantees identical behavior across systems.
How do I make a Python server handle more than one client?
The loop in the example accepts successive clients, but handles them serially: while it is reading from one client, it does not accept another. A client that stays connected without sending data can therefore keep later clients waiting. Concurrent client handling requires a different design, such as using threads or asynchronous I/O; the minimal example does not implement either.
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The listen backlog is not a substitute for concurrent handling. It concerns connection requests waiting to be accepted, and the operating system may choose the default when no backlog is specified. It is not a promise that the server can process a fixed number of clients simultaneously.
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TCP provides an ordered byte stream, not application-level message boundaries. recv(1024) means “read up to 1024 bytes”; it may return fewer bytes, and a single send by the client is not guaranteed to correspond to one receive on the server. The echo example sends back each chunk it gets, which is enough to demonstrate the socket lifecycle but does not define a general message protocol.
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For an application protocol, choose a framing rule, such as a fixed-size message, a delimiter, or a length prefix, and implement both sending and receiving around it. Also define how text is encoded and decoded rather than assuming that arbitrary received bytes are a complete text message. The Python Software Foundation’s Socket Programming HOWTO explains stream behavior and the need for applications to handle message boundaries.
When is this example appropriate?
- Use a raw socket example to learn TCP’s connection lifecycle or to build a deliberately custom byte-stream protocol.
- For an HTTP application, start with a higher-level server framework or library rather than treating this echo loop as a web server.
- Before exposing a service beyond loopback, design its protocol and error handling, decide how it will manage multiple clients, and account for the host’s network and firewall configuration.
The context managers close the listening socket when the outer block exits and each accepted connection when its inner block exits. A more robust program should also handle expected OSError conditions, such as a port that cannot be bound; this small teaching example intentionally keeps error handling minimal.
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