zttp 0.0.29

A sans-IO HTTP parser for Python with a Zig core! ⚡

Documentation: https://zttp.marcelotryle.com

Source Code: https://github.com/Kludex/zttp

[!WARNING] zttp is experimental. The API and behaviour may change at any time, and it is not yet ready for production use.

zttp is a sans-IO HTTP parser whose engine is written in Zig. It speaks HTTP/1.1, HTTP/2, and HTTP/3, and it does no I/O of its own: you feed it bytes and pull out events, and you ask it for bytes to send. It never touches a socket, so it works with any I/O you like.

It's the same idea as h11, with a hand-written Zig engine underneath instead of pure Python.

The key features are:

• Sans-IO: a clean, event-based API. Feed HTTP/1.1 bytes with receive_event, then pull any additional Request / Data / EndOfMessage events with next_event. No callbacks, no sockets, no surprises.
• HTTP/1.1, HTTP/2, and HTTP/3: the same event API for all three, selected with one protocol= argument.
• Fast: faster than httptools (a C parser) on 13 of 14 benchmark workloads, and roughly 15x the pure-Python alternative.
• Safe: strict by default. It defends against request smuggling, rejects bare LF line endings, bounds every buffer, and ships in Zig's safety-checked build.
• Typed: a py.typed package with full type hints.
• No dependencies: the wheel ships the compiled engine and nothing else.

Requirements

zttp needs CPython 3.10+ and runs on Linux, macOS, and Windows.

Installation

$ pip install zttp

Wheels ship the Zig core already compiled, so there is nothing to build and nothing else to configure. See Installation for building from source.

Example

You play the server: bytes come in, events come out.

import zttp

conn = zttp.Connection(zttp.SERVER)
request = conn.receive_event(b"GET /path?q=1 HTTP/1.1\r\nHost: example.com\r\n\r\n")
request.end_stream           # True: this bodyless request is already complete
conn.next_event()   # NEED_DATA

# Build a response:
conn.send_response(200, [(b"Content-Length", b"5")])
conn.send_data(b"hello")
conn.end_message()
conn.data_to_send()  # b'HTTP/1.1 200 OK\r\nContent-Length: 5\r\n\r\nhello'

The read side yields Request / Response / Data / EndOfMessage, or the NEED_DATA sentinel when more bytes are required. A Request with end_stream=True needs no separate EndOfMessage. The write side serializes a head, body data, and the end of the message, framing the body (Content-Length or chunked) for you.

Pass receive_event whatever HTTP/1.1 bytes you have - a whole message, a fragment, or a single byte. It returns the first complete event, and next_event pulls any events that follow. zttp buffers partial input and resumes. There are no callbacks: you pull events when you are ready. That's what sans-IO means.

One API, three protocols

The protocol= argument selects the wire format; the event API stays the same:

import zttp

h1 = zttp.Connection(zttp.SERVER)                       # HTTP/1.1
h2 = zttp.Connection(zttp.SERVER, protocol=zttp.HTTP2)  # HTTP/2
h3 = zttp.Connection(zttp.SERVER, protocol=zttp.HTTP3)  # HTTP/3

On HTTP/2, one connection multiplexes many requests, so the Request / Response / Data / EndOfMessage events carry a stream_id and you send on a Stream handle. Outbound flow control is handled for you: send_data emits what the peer's window allows and parks the rest until credit arrives.

stream = h2.stream(request.stream_id)
stream.send_response(200, [(b"content-type", b"text/plain")])
stream.send_data(b"Hello, HTTP/2!")
stream.end_message()
h2.data_to_send()  # the HTTP/2 frames to put on the wire

On HTTP/3, the wire is UDP, so you feed whole datagrams with receive_datagram and pull the same events. The QUIC transport underneath (packet protection, loss recovery, congestion control, stream reassembly) is written from scratch in the Zig core. HTTP/3 uses the same stream-scoped send surface as HTTP/2.

h3.receive_datagram(datagram)
h3.next_event()  # the same Request / Data / EndOfMessage, tagged with stream_id

See HTTP/2 and HTTP/3 for the full story, including what each protocol changes and why. HTTP/3 support is experimental: the convenience server constructor creates an ephemeral TLS identity for local use, not a production server identity.

Performance

Against httptools, the C parser Uvicorn uses, on the same requests with each parser's default API:

zttp beats httptools on all 14 workloads while staying sans-IO and event-based. It is roughly 22x faster than the pure-Python alternative. A Uvicorn-shaped benchmark that materializes equivalent ASGI scopes measures a 1.16x to 1.52x lead. Run the benchmarks yourself with ./scripts/bench.

These are parser microbenchmarks, and single-digit-percent edges are close to run-to-run noise — see Performance for the full 14-workload table, the methodology, and the caveats.

Why it is fast

• SIMD-accelerated line and field scans with comptime-built character tables.
• Headers stay in a compact HeaderBlock until you access their Python values.
• The receive path emits one Data event per body span and can reuse Python-owned input for large standalone bodies.

Correctness & security

The core enforces the framing rules of RFC 9112 §6 against request smuggling: the Content-Length / Transfer-Encoding conflict, duplicate-Content-Length checks, and combining multiple Transfer-Encoding field-lines into one ordered list so chunked must be the sole, final coding. Line endings are strict CRLF by default (bare LF is rejected), chunk-size is strictly 1*HEXDIG, and obsolete line folding is rejected. Header blocks, trailers, and the receive buffer are all bounded by conservative built-in limits so a malicious peer cannot exhaust memory, and the outbound serializer rejects CR/LF/control bytes to prevent response splitting. The build defaults to Zig's safety-checked ReleaseSafe mode. Malformed input raises RemoteProtocolError; misusing the send API raises LocalProtocolError.

The HTTP/2 layer applies the same posture: the per-stream state machine enforces RFC 9113's stream lifecycle, with the exact stream-vs-connection error classification the RFC requires.

The parser has been through two adversarial security audits (a code review and a CVE-driven review against real HTTP-parser CVEs across Node, Go, Python, Rust, and C servers); zig build fuzz runs the adversarial-input net over the core. See THREAT_MODEL.md for what zttp defends against, the exact limits it enforces, and what the integrator is responsible for.

License

This project is licensed under the terms of the BSD-3-Clause license.