full text of wheel PEP
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Daniel Holth
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pep-0427.txt
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pep-0427.txt
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PEP: 426
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PEP: 427
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Title: The Wheel Binary Package Format 1.0
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Version: $Revision$
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Last-Modified: $Date$
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@ -13,18 +13,291 @@ Created: 20 Sep 2012
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Abstract
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========
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This PEP describes a binary format for Python called wheel.
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This PEP describes a built-package format for Python called wheel.
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A wheel is a ZIP-format archive with a specially formatted file name and
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the .whl extension. It contains a single distribution nearly as it would
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be installed according to PEP 376 with a particular installation scheme.
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A wheel file may be installed by simply unpacking into site-packages
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with the standard ‘unzip’ tool, while preserving enough information
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to spread its contents out onto their final paths at any later time.
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Rationale
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=========
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Python needs a package format that is easier to install than sdist.
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Python’s sdist packages are defined by and require the distutils and
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setuptools build systems, running arbitrary code to build-and-install,
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and re-compile, code just so it can be installed into a new virtualenv.
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This system of conflating build-install is slow, hard to maintain, and
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hinders innovation in both build systems and installers.
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Wheel attempts to remedy these problems by providing a simpler interface
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between the build system and the installer. The wheel binary package
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format frees installers from having to know about the build system,
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saves time by amortizing compile time over many installations, and
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removes the need to install a build system in the target environment.
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Details
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=======
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Installing a wheel ‘distribution-1.0.py32.none.any.whl’
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-------------------------------------------------------
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#. Unpack.
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a. Parse distribution-1.0.dist-info/WHEEL.
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b. Check that installer is compatible with Wheel-Version. Warn if minor
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version is greater, abort if major version is greater.
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c. If Root-Is-Purelib == ‘true’, unpack archive into purelib
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(site-packages).
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d. Else unpack archive into platlib (site-packages).
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#. Spread.
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a. Unpacked archive includes distribution-1.0.dist-info/ and (if there
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is data) distribution-1.0.data/
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b. Move each subtree of distribution-1.0.data/ onto its destination
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path. Each subdirectory of distribution-1.0.data/ is a key into
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sysconfig.get\_paths(), such as
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distribution-1.0.data/(config\|purelib\|platlib\|scripts).
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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>`_
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c. If applicable, update scripts starting with “#!python” to point to
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the correct interpreter.
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d. Update distribution-1.0.dist.info/RECORD with the installed paths.
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e. Remove empty distribution-1.0.data directory.
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File Format
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-----------
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File name convention
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~~~~~~~~~~~~~~~~~~~~
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The wheel filename is {distribution}-{version}(-{build tag})?-{python
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tag}-{abi tag}-{platform tag}.whl
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distribution
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Distribution name, e.g. ‘django’, ‘pyramid’
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version
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PEP-386 compliant version, e.g. 1.0
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build tag
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Optional build number. Must start with a digit. A tie breaker if two
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wheels have the same version. Sort as None if unspecified, else sort
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the initial digits as a number, and the remainder lexicographically.
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language implementation and version tag
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‘py27’ ‘py2’ ‘py3’
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abi tag
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‘cp33m’, ‘abi3’, ‘none’
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platform tag
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‘linux\_x86\_64’, ‘any’
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For example, distribution-1.0-1-py27-none-any.whl is the first build
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of a package called 'distribution', and is compatible with Python 2.7
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(any Python 2.7 implementation), with no abi (pure Python), on any CPU
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architecture.
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The last three components of the filename before the extension are
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called “compatibility tags.” The compatibility tags express
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the package’s basic interpreter requirements and are detailed
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in `PEP 425 <http://hg.python.org/peps/file/tip/pep-0425.txt>`_.
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File contents
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~~~~~~~~~~~~~
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#. Wheel files contain a folder {distribution}-{version}.dist-info/ with
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the PEP 426 metadata (Metadata version 1.3 or greater) and an
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additional file WHEEL with metadata about the archive itself.
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#. The root of a .whl is installed into one of purelib or platlib.
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#. Wheel files contain metadata about the wheel format itself in
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{distribution}-{version}.dist-info/WHEEL::
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Wheel-Version: 1.0
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Generator: bdist_wheel 0.7
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Root-Is-Purelib: true
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#. Wheel-Version is the version number of the Wheel specification.
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Generator is the name and optionally the version of the software that
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produced the archive. Root-Is-Purelib is true if the top level
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directory of the archive should be installed into purelib; otherwise
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the root should be installed into platlib.
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#. A wheel installer should warn if Wheel-Version is greater than the
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version it supports, and fail if Wheel-Version has a greater major
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version than the version it supports.
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#. If a .whl contains scripts, both purelib and platlib, or any other
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files that are not installed on sys.path, they are found in
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{distribution}-{version}.data/{key}, where {key} is an index into
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sysconfig.get\_paths().
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#. Wheel, being an installation format that is intended to work across
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multiple versions of Python, does not generally include .pyc files.
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#. Wheel does not contain setup.py or setup.cfg.
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The .dist-info directory
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^^^^^^^^^^^^^^^^^^^^^^^^
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#. Wheel .dist-info directories include at a minimum METADATA, WHEEL,
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and RECORD.
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#. METADATA is the PEP 426 metadata (Metadata version 1.3 or greater)
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#. WHEEL is the wheel metadata, specific to a build of the package.
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#. RECORD is a list of (almost) all the files in the wheel and their
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secure hashes. Unlike PEP 376, every file except RECORD, which cannot
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contain a hash of itself, must include its hash. The hash algorithm
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must be sha256 or better; specifically, md5 and sha1 are not
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permitted, as signed wheel files rely on the strong hashes in RECORD to
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validate the integrity of the archive.
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#. INSTALLER and REQUESTED are not included in the archive.
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#. RECORD.jws is used for digital signatures. It is not mentioned in
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RECORD.
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#. RECORD.p7s is reserved as a courtesy to anyone who would prefer to
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use s/mime signatures to secure their wheel files. It is not
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mentioned in RECORD.
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#. During extraction, wheel installers verify all the hashes in RECORD
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against the file contents. Apart from RECORD and its signatures,
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installation will fail if any file in the archive is not both
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mentioned and correctly hashed in RECORD.
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The .data directory
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^^^^^^^^^^^^^^^^^^^
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Any file that is not normally installed inside site-packages goes into
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the .data directory, named as the .dist-info directory but with the
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.data/ extension::
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distribution-1.0.dist-info/
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distribution-1.0.data/
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The .data directory contains subdirectories named after the keys of
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sysconfig.get\_paths(), and those subdirectories contain the scripts,
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headers, documentation and so forth from the distribution. During
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installation the contents of these subdirectories are moved onto their
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destination paths.
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Signed wheel files
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------------------
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Wheel files include an extended RECORD that enables digital signatures.
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PEP 376’s RECORD is altered to include
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digestname=urlsafe\_b64encode\_nopad(digest) (urlsafe base64 encoding
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with no trailing = characters) as the second column instead of an
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md5sum. All possible entries are hashed, including any generated files
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such as .pyc files, but not RECORD. For example::
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file.py,sha256=AVTFPZpEKzuHr7OvQZmhaU3LvwKz06AJw8mT\_pNh2yI,3144
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distribution-1.0.dist-info/RECORD,,
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RECORD.jws is not mentioned in RECORD at all. Every other file in the
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archive must have a correct hash in RECORD, or the installation will
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fail.
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The signature is one or more JSON Web Signature JSON Serialization
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(JWS-JS) signatures stored in a file RECORD.jws adjacent to RECORD.
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A signature-aware installer can be instructed to check for a particular
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Ed25519 public key by using an extended “extras” syntax.::
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# request a normal optional feature "extra", and indicate
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# the package should be signed by a particular
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# urlsafe-b64encode-nopad encoded ed25519 public key:
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package[extra, ed25519=ouBJlTJJ4SJXoy8Bi1KRlewWLU6JW7HUXTgvU1YRuiA]
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An application could distribute a requires.txt file with many such lines
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for all its dependencies and their public keys. By installing from this
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file an application’s users would know whether the applicaton’s
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dependencies came from the correct publishers.
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Applications that wish to “fail open” for backwards compatibility with
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non-signature-aware installers should specify that their package
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provides the extra ed25519=(key) with no associated dependencies.
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Key distribution is outside the scope of this spec. Public wheel signing
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keys could be signed with the packager’s GPG key or stored at
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an https://-protected URL.
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JSON Web Signatures Extensions
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The Ed25519 algorithm is used as an extension to the JSON Web Signatures
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specification. Wheel uses alg="Ed25519" in the header. The key attribute
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holds the signature’s public JSON Web Key. For JSON Web Key / JSON
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Private Key the verifying (public) key is called vk and the signing
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(private) key is called sk.
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Example header::
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{
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"alg": "Ed25519",
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"typ": "JWT",
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"key": {
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"alg": "Ed25519",
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"vk": "tmAYCrSfj8gtJ10v3VkvW7jOndKmQIYE12hgnFu3cvk"
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}
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}
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A future version of wheel may omit typ.
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Example payload::
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{ "hash": "sha256=ADD-r2urObZHcxBW3Cr-vDCu5RJwT4CaRTHiFmbcIYY" }
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A future version of wheel may include timestamps in the payload or in
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the signature.
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See
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- http://self-issued.info/docs/draft-ietf-jose-json-web-signature.html
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- http://self-issued.info/docs/draft-jones-json-web-signature-json-serialization-01.html
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- http://self-issued.info/docs/draft-ietf-jose-json-web-key-05.html
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- http://self-issued.info/docs/draft-jones-jose-json-private-key-00.html
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Comparison to .egg
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------------------
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#. Wheel is an installation format; egg is importable. Wheel archives do
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not need to include .pyc and are less tied to a specific Python
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version or implementation. Wheel can install (pure Python) packages
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built with previous versions of Python so you don’t always have to
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wait for the packager to catch up.
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#. Wheel uses .dist-info directories; egg uses .egg-info. Wheel is
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compatible with the new world of Python packaging and the new
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concepts it brings.
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#. Wheel has a richer file naming convention for today’s
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multi-implementation world. A single wheel archive can indicate its
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compatibility with a number of Python language versions and
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implementations, ABIs, and system architectures. Historically the ABI
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has been specific to a CPython release, wheel is ready for the stable
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ABI.
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#. Wheel is lossless. The first wheel implementation bdist\_wheel always
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generates egg-info, and then converts it to a .whl. It is also
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possible to convert existing eggs and bdist\_wininst distributions.
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#. Wheel is versioned. Every wheel file contains the version of the
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wheel specification and the implementation that packaged it.
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Hopefully the next migration can simply be to Wheel 2.0.
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#. Wheel is a reference to the other Python.
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FAQ
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===
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Wheel defines a .data directory. Should I put all my data there?
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No. The word 'data' is used only because it is short, and is only
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meant to indicate files that are not installed into site-packages.
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This specification does not have an opinion on how you should organize
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your code.
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Copyright
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=========
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This document has been placed in the public domain.
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This document has been placed into the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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End:
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