To create a simple TCP server in Python, open 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 intended for testing on the same computer—not for accepting connections from other devices.
How do I create a simple TCP server in Python?
Python’s built-in socket module provides the low-level pieces. This small echo server accepts connections one at a time and sends back the bytes each client sends:
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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)The server follows the sequence socket(), bind(), listen(), and accept(). The Python Software Foundation’s socket reference describes this as the server-side sequence, with accept() repeated as needed for further clients.
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This is an educational echo server, not a general-purpose service. It handles one connection at a time: while it is receiving from a connected client, it does not accept another. It also assumes a simple byte-stream exchange rather than defining a complete application protocol.
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What do bind(), listen(), and accept() do?
- Create:
socket.socket(socket.AF_INET, socket.SOCK_STREAM)creates an IPv4 stream socket.SOCK_STREAMis the socket type used for TCP in the documented examples. - Bind:
bind((HOST, PORT))assigns the local address and port on which the server will wait. - Listen:
listen()marks the socket as ready to accept incoming connection requests. The optional backlog concerns requests queued before they are accepted; it is not a guarantee of how many clients the server can serve at once. If omitted, the operating system may choose a reasonable default. - Accept:
accept()waits for a connection and returns a pair: a new socket for that client and the client’s address.
Why does accept() return another socket?
The original socket remains the listening socket: it waits for new connections. The socket returned by accept() is the connection socket, and it is the one used for that client’s recv() and sendall() calls. Keeping those roles separate lets the server continue listening while a connection is handled—although this example does so serially.
How can I test a Python socket server locally?
Save the server as server.py, run it with Python, and leave it running. In another terminal, run this client:
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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"hellon")
print(client.recv(1024))The client should print the bytes it sent, and the server should print the connecting peer’s address. This simple exchange works because the server echoes received bytes. Both programs use the same loopback address and port; a mismatch prevents the client from reaching the server.
Which address should the server bind to?
| Host value | Reachability and use |
|---|---|
127.0.0.1 |
IPv4 loopback. A client on the same machine can connect; other devices cannot use this address to reach the server. |
"" (empty string) |
In the documented example, binds to all available interfaces. Other devices may be able to reach the service through an address on the machine, subject to firewall and network configuration. |
For a local exercise, loopback limits exposure. Binding to all interfaces is a deliberate reachability change, not just a different spelling of localhost. The Python Socket Programming HOWTO explains that localhost binding is visible only on the same machine, while an empty host makes the socket reachable through the machine’s addresses.
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What does recv(1024) actually read?
recv(1024) requests up to 1,024 bytes; it does not promise to return exactly that many bytes or one complete application message. TCP delivers a stream of bytes and does not preserve the boundaries between separate sends. The loop echoes each chunk it receives until recv() returns an empty byte string, which indicates that the peer has closed its sending side.
For an actual protocol, define how the receiver recognizes message boundaries. Common approaches include fixed-size messages, a delimiter, or a length prefix. Also define text encoding explicitly if the protocol carries text; the example uses bytes and makes no encoding assumption. Use sendall() when the intention is to send all bytes in a given byte string.
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How do I make a Python server handle more than one client?
The repeated accept() loop in the example serves successive clients, but serially. It waits for a client to finish before accepting another. To handle overlapping connections, a server needs a concurrency strategy, such as worker threads, processes, or asynchronous I/O; each accepted connection still has its own connection socket. The right choice depends on the workload and protocol, and the minimal example does not implement one.
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The example explicitly creates an IPv4 socket with AF_INET. IPv6 requires appropriate address-family handling, and whether a socket accepts both IPv4 and IPv6 traffic depends on platform configuration. The Python reference demonstrates using getaddrinfo() to work with available address families and cautions that selecting only the first result can leave the other family unsupported.
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Python also provides socket.create_server(), available since Python 3.8, as a convenience for creating a server socket. Its optional dualstack_ipv6 behavior is platform-dependent, so do not assume an IPv6 bind has identical dual-stack behavior on every system. For HTTP applications, a higher-level HTTP server or framework is generally a better fit than implementing the protocol directly with raw TCP sockets; the socket API is useful for learning TCP mechanics or building a custom protocol.
How are the sockets closed?
The outer with block closes the listening socket when execution leaves it, and the inner with conn closes each accepted connection socket when that client’s loop ends. Context managers make cleanup explicit even when an error interrupts normal handling. In a more robust program, handle expected OSError cases—for example, when the requested port cannot be used—and decide whether to retry, report the failure, or exit.
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