hpack: HTTP/2 Header Compression for Python

hpack provides a simple Python interface to the HPACK compression algorithm, used to compress HTTP headers in HTTP/2. Used by some of the most popular HTTP/2 implementations in Python, HPACK offers a great Python interface as well as optional upgrade to optimised C-based compression routines from nghttp2.

Using hpack is easy:

from hpack import Encoder, Decoder

e = Encoder()
encoded_bytes = e.encode(headers)

d = Decoder()
decoded_headers = d.decode(encoded_bytes)

hpack will transparently use nghttp2 on CPython if it’s available, gaining even better compression efficiency and speed, but it also makes available a pure-Python implementation that conforms strictly to RFC 7541.

Contents

Installing hpack

hpack is trivial to install from the Python Package Index. Simply run:

$ pip install hpack

Alternatively, feel free to download one of the release tarballs from our GitHub page, extract it to your favourite directory, and then run

$ python setup.py install

hpack has no external dependencies.

Using nghttp2

If you want to use nghttp2 with hpack, all you need to do is install it along with its Python bindings. Consult nghttp2’s documentation for instructions on how to install it.

hpack API

This document provides the HPACK API.

class hpack.Encoder

An HPACK encoder object. This object takes HTTP headers and emits encoded HTTP/2 header blocks.

encode(headers, huffman=True)

Takes a set of headers and encodes them into a HPACK-encoded header block.

Parameters:
  • headers –

    The headers to encode. Must be either an iterable of tuples, an iterable of HeaderTuple, or a dict.

    If an iterable of tuples, the tuples may be either two-tuples or three-tuples. If they are two-tuples, the tuples must be of the format (name, value). If they are three-tuples, they must be of the format (name, value, sensitive), where sensitive is a boolean value indicating whether the header should be added to header tables anywhere. If not present, sensitive defaults to False.

    If an iterable of HeaderTuple, the tuples must always be two-tuples. Instead of using sensitive as a third tuple entry, use NeverIndexedHeaderTuple to request that the field never be indexed.

    Warning

    HTTP/2 requires that all special headers (headers whose names begin with : characters) appear at the start of the header block. While this method will ensure that happens for dict subclasses, callers using any other iterable of tuples must ensure they place their special headers at the start of the iterable.

    For efficiency reasons users should prefer to use iterables of two-tuples: fixing the ordering of dictionary headers is an expensive operation that should be avoided if possible.

  • huffman – (optional) Whether to Huffman-encode any header sent as a literal value. Except for use when debugging, it is recommended that this be left enabled.
Returns:

A bytestring containing the HPACK-encoded header block.
header_table_size

Controls the size of the HPACK header table.

class hpack.Decoder

An HPACK decoder object.

decode(data, raw=False)

Takes an HPACK-encoded header block and decodes it into a header set.

Parameters:
  • data – A bytestring representing a complete HPACK-encoded header block.
  • raw – (optional) Whether to return the headers as tuples of raw byte strings or to decode them as UTF-8 before returning them. The default value is False, which returns tuples of Unicode strings
Returns:

A list of two-tuples of (name, value) representing the HPACK-encoded headers, in the order they were decoded.
Raises:

HPACKDecodingError – If an error is encountered while decoding the header block.
header_table_size

Controls the size of the HPACK header table.

class hpack.HeaderTuple

A data structure that stores a single header field.

HTTP headers can be thought of as tuples of (field name, field value). A single header block is a sequence of such tuples.

In HTTP/2, however, certain bits of additional information are required for compressing these headers: in particular, whether the header field can be safely added to the HPACK compression context.

This class stores a header that can be added to the compression context. In all other ways it behaves exactly like a tuple.

class hpack.NeverIndexedHeaderTuple

A data structure that stores a single header field that cannot be added to a HTTP/2 header compression context.

class hpack.HPACKError

The base class for all hpack exceptions.

class hpack.HPACKDecodingError

An error has been encountered while performing HPACK decoding.

class hpack.InvalidTableIndex

An invalid table index was received.