python-peps/pep-0427.txt

PEP: 427
Title: The Wheel Binary Package Format 1.0
Version: $Revision$
Last-Modified: $Date$
Author: Daniel Holth <dholth@fastmail.fm>
Discussions-To: Distutils SIG
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 20 Sep 2012


Abstract
========

This PEP describes a built-package format for Python called wheel.

A wheel is a ZIP-format archive with a specially formatted file name and
the .whl extension. It contains a single distribution nearly as it would
be installed according to PEP 376 with a particular installation scheme.
A wheel file may be installed by simply unpacking into site-packages
with the standard ‘unzip’ tool, while preserving enough information
to spread its contents out onto their final paths at any later time.

Rationale
=========

Python needs a package format that is easier to install than sdist.
Python’s sdist packages are defined by and require the distutils and
setuptools build systems, running arbitrary code to build-and-install,
and re-compile, code just so it can be installed into a new virtualenv.
This system of conflating build-install is slow, hard to maintain, and
hinders innovation in both build systems and installers.

Wheel attempts to remedy these problems by providing a simpler interface
between the build system and the installer. The wheel binary package
format frees installers from having to know about the build system,
saves time by amortizing compile time over many installations, and
removes the need to install a build system in the target environment.

Details
=======

Installing a wheel ‘distribution-1.0.py32.none.any.whl’
-------------------------------------------------------

#. Unpack.

    a. Parse distribution-1.0.dist-info/WHEEL.
    b. Check that installer is compatible with Wheel-Version. Warn if minor
       version is greater, abort if major version is greater.
    c. If Root-Is-Purelib == ‘true’, unpack archive into purelib
       (site-packages).
    d. Else unpack archive into platlib (site-packages).

#. Spread.

    a. Unpacked archive includes distribution-1.0.dist-info/ and (if there
       is data) distribution-1.0.data/
    b. Move each subtree of distribution-1.0.data/ onto its destination
       path. Each subdirectory of distribution-1.0.data/ is a key into
       sysconfig.get\_paths(), such as
       distribution-1.0.data/(config\|purelib\|platlib\|scripts).
       See` <http://docs.python.org/dev/packaging/setupcfg.html#resources>`_`http://docs.python.org/dev/packaging/setupcfg.html#resources <http://docs.python.org/dev/packaging/setupcfg.html#resources>`_
    c. If applicable, update scripts starting with “#!python” to point to
       the correct interpreter.
    d. Update distribution-1.0.dist.info/RECORD with the installed paths.
    e. Remove empty distribution-1.0.data directory.

File Format
-----------

File name convention
~~~~~~~~~~~~~~~~~~~~

The wheel filename is {distribution}-{version}(-{build tag})?-{python
tag}-{abi tag}-{platform tag}.whl

distribution
    Distribution name, e.g. ‘django’, ‘pyramid’

version
    PEP-386 compliant version, e.g. 1.0

build tag
    Optional build number. Must start with a digit. A tie breaker if two
    wheels have the same version. Sort as None if unspecified, else sort
    the initial digits as a number, and the remainder lexicographically.

language implementation and version tag
    ‘py27’ ‘py2’ ‘py3’

abi tag
    ‘cp33m’, ‘abi3’, ‘none’

platform tag
    ‘linux\_x86\_64’, ‘any’

For example, distribution-1.0-1-py27-none-any.whl is the first build
of a package called 'distribution', and is compatible with Python 2.7
(any Python 2.7 implementation), with no abi (pure Python), on any CPU
architecture.

The last three components of the filename before the extension are
called “compatibility tags.” The compatibility tags express
the package’s basic interpreter requirements and are detailed
in `PEP 425 <http://hg.python.org/peps/file/tip/pep-0425.txt>`_.


File contents
~~~~~~~~~~~~~

#. Wheel files contain a folder {distribution}-{version}.dist-info/ with
   the PEP 426 metadata (Metadata version 1.3 or greater) and an
   additional file WHEEL with metadata about the archive itself.
#. The root of a .whl is installed into one of purelib or platlib.
#. Wheel files contain metadata about the wheel format itself in
   {distribution}-{version}.dist-info/WHEEL::

    Wheel-Version: 1.0
    Generator: bdist_wheel 0.7
    Root-Is-Purelib: true

#. Wheel-Version is the version number of the Wheel specification.
   Generator is the name and optionally the version of the software that
   produced the archive. Root-Is-Purelib is true if the top level
   directory of the archive should be installed into purelib; otherwise
   the root should be installed into platlib.
#. A wheel installer should warn if Wheel-Version is greater than the
   version it supports, and fail if Wheel-Version has a greater major
   version than the version it supports.
#. If a .whl contains scripts, both purelib and platlib, or any other
   files that are not installed on sys.path, they are found in
   {distribution}-{version}.data/{key}, where {key} is an index into
   sysconfig.get\_paths().
#. Wheel, being an installation format that is intended to work across
   multiple versions of Python, does not generally include .pyc files.
#. Wheel does not contain setup.py or setup.cfg.


The .dist-info directory
^^^^^^^^^^^^^^^^^^^^^^^^

#. Wheel .dist-info directories include at a minimum METADATA, WHEEL,
   and RECORD.
#. METADATA is the PEP 426 metadata (Metadata version 1.3 or greater)
#. WHEEL is the wheel metadata, specific to a build of the package.
#. RECORD is a list of (almost) all the files in the wheel and their
   secure hashes. Unlike PEP 376, every file except RECORD, which cannot
   contain a hash of itself, must include its hash. The hash algorithm
   must be sha256 or better; specifically, md5 and sha1 are not
   permitted, as signed wheel files rely on the strong hashes in RECORD to
   validate the integrity of the archive.
#. INSTALLER and REQUESTED are not included in the archive.
#. RECORD.jws is used for digital signatures. It is not mentioned in
   RECORD.
#. RECORD.p7s is reserved as a courtesy to anyone who would prefer to
   use s/mime signatures to secure their wheel files. It is not
   mentioned in RECORD.
#. During extraction, wheel installers verify all the hashes in RECORD
   against the file contents. Apart from RECORD and its signatures,
   installation will fail if any file in the archive is not both
   mentioned and correctly hashed in RECORD.


The .data directory
^^^^^^^^^^^^^^^^^^^

Any file that is not normally installed inside site-packages goes into
the .data directory, named as the .dist-info directory but with the
.data/ extension::

    distribution-1.0.dist-info/

    distribution-1.0.data/

The .data directory contains subdirectories named after the keys of
sysconfig.get\_paths(), and those subdirectories contain the scripts,
headers, documentation and so forth from the distribution. During
installation the contents of these subdirectories are moved onto their
destination paths.


Signed wheel files
------------------

Wheel files include an extended RECORD that enables digital signatures.
PEP 376’s RECORD is altered to include
digestname=urlsafe\_b64encode\_nopad(digest) (urlsafe base64 encoding
with no trailing = characters) as the second column instead of an
md5sum. All possible entries are hashed, including any generated files
such as .pyc files, but not RECORD. For example::

    file.py,sha256=AVTFPZpEKzuHr7OvQZmhaU3LvwKz06AJw8mT\_pNh2yI,3144
    distribution-1.0.dist-info/RECORD,,

RECORD.jws is not mentioned in RECORD at all. Every other file in the
archive must have a correct hash in RECORD, or the installation will
fail.

The signature is one or more JSON Web Signature JSON Serialization
(JWS-JS) signatures stored in a file RECORD.jws adjacent to RECORD.

A signature-aware installer can be instructed to check for a particular
Ed25519 public key by using an extended “extras” syntax.::

    # request a normal optional feature "extra", and indicate
    # the package should be signed by a particular 
    # urlsafe-b64encode-nopad encoded ed25519 public key:
    package[extra, ed25519=ouBJlTJJ4SJXoy8Bi1KRlewWLU6JW7HUXTgvU1YRuiA]

An application could distribute a requires.txt file with many such lines
for all its dependencies and their public keys. By installing from this
file an application’s users would know whether the applicaton’s
dependencies came from the correct publishers.

Applications that wish to “fail open” for backwards compatibility with
non-signature-aware installers should specify that their package
provides the extra ed25519=(key) with no associated dependencies.

Key distribution is outside the scope of this spec. Public wheel signing
keys could be signed with the packager’s GPG key or stored at
an https://-protected URL.


JSON Web Signatures Extensions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Ed25519 algorithm is used as an extension to the JSON Web Signatures
specification. Wheel uses alg="Ed25519" in the header. The key attribute
holds the signature’s public JSON Web Key. For JSON Web Key / JSON
Private Key the verifying (public) key is called vk and the signing
(private) key is called sk.

Example header::

    {
     "alg": "Ed25519",
     "typ": "JWT",
     "key": {
     "alg": "Ed25519",
     "vk": "tmAYCrSfj8gtJ10v3VkvW7jOndKmQIYE12hgnFu3cvk"
     }
    }

A future version of wheel may omit typ.

Example payload::

    { "hash": "sha256=ADD-r2urObZHcxBW3Cr-vDCu5RJwT4CaRTHiFmbcIYY" }

A future version of wheel may include timestamps in the payload or in
the signature.

See

- http://self-issued.info/docs/draft-ietf-jose-json-web-signature.html
- http://self-issued.info/docs/draft-jones-json-web-signature-json-serialization-01.html 
- http://self-issued.info/docs/draft-ietf-jose-json-web-key-05.html
- http://self-issued.info/docs/draft-jones-jose-json-private-key-00.html 


Comparison to .egg
------------------

#. Wheel is an installation format; egg is importable. Wheel archives do
   not need to include .pyc and are less tied to a specific Python
   version or implementation. Wheel can install (pure Python) packages
   built with previous versions of Python so you don’t always have to
   wait for the packager to catch up.
#. Wheel uses .dist-info directories; egg uses .egg-info. Wheel is
   compatible with the new world of Python packaging and the new
   concepts it brings.
#. Wheel has a richer file naming convention for today’s
   multi-implementation world. A single wheel archive can indicate its
   compatibility with a number of Python language versions and
   implementations, ABIs, and system architectures. Historically the ABI
   has been specific to a CPython release, wheel is ready for the stable
   ABI.
#. Wheel is lossless. The first wheel implementation bdist\_wheel always
   generates egg-info, and then converts it to a .whl. It is also
   possible to convert existing eggs and bdist\_wininst distributions.
#. Wheel is versioned. Every wheel file contains the version of the
   wheel specification and the implementation that packaged it.
   Hopefully the next migration can simply be to Wheel 2.0.
#. Wheel is a reference to the other Python.


FAQ
===

Wheel defines a .data directory. Should I put all my data there?
    No. The word 'data' is used only because it is short, and is only
    meant to indicate files that are not installed into site-packages.
    This specification does not have an opinion on how you should organize
    your code.


Copyright
=========

This document has been placed into the public domain.