[Python-Dev] PEP 427: wheel

Daniel Holth dholth at gmail.com
Sat Feb 16 05:21:27 CET 2013

This is PEP 427: Wheel. A binary package format for Python. Because newegg
was taken.

Since the last submission, the signature specification has been made
clearly optional / informative and does not attempt to specify any specific
signing algorithm or how the signatures would fit into a security scheme
that would necessarily exist outside of a single archive. JWS signatures
are used in their current form by OpenID Connect and Mozilla Personas and
are a useful way to implement basically raw public or secret key
signatures. The embedded signature scheme in wheel should also not affect
the current effort to define end-to-end security for PyPI in any way; it
might be a useful complement for packages that are not hosted on PyPI at

This version also does not depend on the in-process Metadata PEP 426. It
has been cleaned up in several places regarding which PEPs it references to
describe its contents. The WHEEL metadata file should contain all the
information in the wheel filename itself, and the non-alphanumeric/unicode
filename escaping rules are made official.


For your consideration,

PEP: 427
Title: The Wheel Binary Package Format 0.1
Version: $Revision$
Last-Modified: $Date$
Author: Daniel Holth <dholth at fastmail.fm>
BDFL-Delegate: Nick Coghlan <ncoghlan at gmail.com>
Discussions-To: <distutils-sig at python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 20-Sep-2012
Post-History: 18-Oct-2012, 15-Feb-2013


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.  Although a specialized installer is recommended, 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.


This draft PEP describes version 0.1 of the "wheel" format. When the PEP
is accepted, the version will be changed to 1.0.  (The major version
is used to indicate potentially backwards-incompatible changes to the


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.


Installing a wheel 'distribution-1.0.py32.none.any.whl'

Wheel installation notionally consists of two phases:

- 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
  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 a dict of destination directories, such as
     The initially supported paths are taken from
  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
  e. Remove empty ``distribution-1.0.data`` directory.
  f. Compile any installed .py to .pyc. (Uninstallers should be smart
     enough to remove .pyc even if it is not mentioned in RECORD.)

Recommended installer features

Rewrite ``#!python``.
    In wheel, scripts are packaged in
    ``{distribution}-{version}.data/scripts/``.  If the first line of
    a file in ``scripts/`` starts with exactly ``b'#!python'``, rewrite to
    point to the correct interpreter.  Unix installers may need to add
    the +x bit to these files if the archive was created on Windows.

Generate script wrappers.
    In wheel, scripts packaged on Unix systems will certainly not have
    accompanying .exe wrappers.  Windows installers may want to add them
    during install.

File Format

File name convention

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

    Distribution name, e.g. 'django', 'pyramid'.

    Distribution 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 the empty string
    if unspecified, else sort the initial digits as a number, and the
    remainder lexicographically.

language implementation and version tag
    E.g. 'py27', 'py2', 'py3'.

abi tag
    E.g. 'cp33m', 'abi3', 'none'.

platform tag
    E.g. '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.

Escaping and Unicode

Each component of the filename is escaped by replacing runs of
non-alphanumeric characters with an underscore ``_``::

    re.sub("[^\w\d.]+", "_", distribution, re.UNICODE)

The filename is Unicode.  It will be some time before the tools are
updated to support non-ASCII filenames, but they are supported in this

File contents

The contents of a wheel file, where {distribution} is replaced with the
name of the package, e.g. ``beaglevote`` and {version} is replaced with
its version, e.g. ``1.0.0``, consist of:

#. ``/``, the root of the archive, contains all files to be installed in
   ``purelib`` or ``platlib`` as specified in ``WHEEL``.  ``purelib`` and
   ``platlib`` are usually both ``site-packages``.
#. ``{distribution}-{version}.dist-info/`` contains metadata.
#. ``{distribution}-{version}.data/`` contains one subdirectory
   for each non-empty install scheme key not already covered, where
   the subdirectory name is an index into a dictionary of install paths
   (e.g. ``data``, ``scripts``, ``include``, ``purelib``, ``platlib``).
#. Python scripts must appear in ``scripts`` and begin with exactly
   ``b'#!python'`` in order to enjoy script wrapper generation and
   ``#!python`` rewriting at install time.  They may have any or no
#. ``{distribution}-{version}.dist-info/METADATA`` is Metadata version 1.1
   or greater (PEP 314, PEP 345, PEP 426) format metadata.
#. ``{distribution}-{version}.dist-info/WHEEL`` is metadata about the
   itself in the same basic key: value format::

       Wheel-Version: 0.1
       Generator: bdist_wheel 0.7
       Root-Is-Purelib: true
       Tag: py2-none-any
       Tag: py3-none-any
       Build: 1

#. ``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. ``Tag`` is the
   wheel's expanded compatibility tags; in the example the filename would
   contain ``py2.py3-none-any``. ``Build`` is the build number and is
   omitted if there is no build number.
#. A wheel installer should warn if Wheel-Version is greater than the
   version it supports, and must fail if Wheel-Version has a greater
   major version than the version it supports.
#. 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.

This version of the wheel specification is based on the distutils install
schemes and does not define how to install files to other locations.
The layout offers a superset of the functionality provided by the existing
wininst and egg binary formats.

The .dist-info directory

#. Wheel .dist-info directories include at a minimum METADATA, WHEEL,
   and RECORD.
#. METADATA is the package metadata, the same format as PKG-INFO as
   found at the root of sdists.
#. 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.
#. PEP 376's INSTALLER and REQUESTED are not included in the archive.
#. RECORD.jws is used for digital signatures.  It is not mentioned in
#. RECORD.p7s is allowed 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::



The .data directory contains subdirectories with 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 a secure hash
``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 which cannot contain its
own hash. For example::


The signature file(s) RECORD.jws and RECORD.p7s are not mentioned in
RECORD at all since they can only be added after RECORD is generated.
Every other file in the archive must have a correct hash in RECORD
or the installation will fail.

If JSON web signatures are used, one or more JSON Web Signature JSON
Serialization (JWS-JS) signatures is stored in a file RECORD.jws adjacent
to RECORD.  JWS is used to sign RECORD by including the SHA-256 hash of
RECORD as the signature's JSON payload::

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

If RECORD.p7s is used, it must contain a detached S/MIME format signature

A wheel installer is not required to understand digital signatures but
MUST verify the hashes in RECORD against the extracted file contents.
When the installer checks file hashes against RECORD, a separate signature
checker only needs to establish that RECORD matches the signature.


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


Wheel defines a .data directory.  Should I put all my data there?
    This specification does not have an opinion on how you should organize
    your code.  The .data directory is just a place for any files that are
    not normally installed inside ``site-packages`` or on the PYTHONPATH.
    In other words, you may continue to use ``pkgutil.get_data(package,
    resource)`` even though *those* files will usually not be distributed
    in *wheel's* ``.data`` directory.

Why does wheel include attached signatures?
    Attached signatures are more convenient than detached signatures
    because they travel with the archive.  Since only the individual files
    are signed, the archive can be recompressed without invalidating
    the signature or individual files can be verified without having
    to download the whole archive.

Why does wheel allow JWS signatures?
    The JOSE specifications of which JWS is a part are designed to be easy
    to implement, a feature that is also one of wheel's primary design
    goals.  JWS yields a useful, concise pure-Python implementation.

Why does wheel also allow S/MIME signatures?
    S/MIME signatures are allowed for users who need or want to use
    existing public key infrastructure with wheel.

    Signed packages are only a basic building block in a secure package
    update system.  Wheel only provides the building block.


Example urlsafe-base64-nopad implementation::

    # urlsafe-base64-nopad for Python 3
    import base64

    def urlsafe_b64encode_nopad(data):
        return base64.urlsafe_b64encode(data).rstrip(b'=')

    def urlsafe_b64decode_nopad(data):
        pad = b'=' * (4 - (len(data) & 3))
        return base64.urlsafe_b64decode(data + pad)


This document has been placed into the public domain.

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