h11

h11 is an open-source, pure-Python implementation of the HTTP/1.1 protocol state machine. It provides a lightweight, event-driven interface for encoding and decoding HTTP/1.1 messages in accordance with RFC 7230 through RFC 7235. The library is designed to be a building block rather than a full networking stack, focusing on correctness and robustness.

Design and API

h11 exposes a Connection object that can operate in either client or server mode. The library translates between a stream of bytes and a sequence of high-level events, such as requests, responses, data chunks, and message termination. Users feed incoming bytes into the connection and obtain bytes to send back over a socket, while the connection manages the HTTP/1.1 state machine, including header parsing, body streaming, and message boundaries. It handles protocol details such as keep-alive behavior and the interplay between Content-Length and chunked transfer encoding. The API is designed to be simple and dependency-free beyond the Python standard library.

Usage and role

As a protocol engine, h11 does not perform socket I/O or concurrency control. Applications integrate h11 with their I/O loop or framework, using the library to interpret and construct HTTP messages while managing buffering and network transport themselves. This separation makes h11 a common choice as a low-level component in higher-level HTTP clients and servers.

Limitations and scope

h11 implements HTTP/1.1 only and does not provide HTTP/2 support or TLS functionality. It is intended for use as part of larger systems that implement the networking, persistence, and higher-level application logic around HTTP communications.