python-peps/pep-0610.rst

PEP: 610
Title: Recording the Direct URL Origin of installed distributions
Author: Stéphane Bidoul <stephane.bidoul@gmail.com>, Chris Jerdonek <chris.jerdonek@gmail.com>
Sponsor: Nick Coghlan <ncoghlan@gmail.com>
BDFL-Delegate: Pradyun Gedam <pradyunsg@gmail.com>
Discussions-To: https://discuss.python.org/t/recording-the-source-url-of-an-installed-distribution/1535
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 21-Apr-2019
Post-History:
Resolution: https://discuss.python.org/t/1535/56

Abstract
========

Following PEP 440, a distribution can be identified by a name and either a
version, or a direct URL reference (see `PEP440 Direct References`_).
After installation, the name and version are captured in the project metadata,
but currently there is no way to obtain details of the URL used when the
distribution was identified by a direct URL reference.

This proposal defines
additional metadata, to be added to the installed distribution by the
installation front end, which records the Direct URL Origin for use by
consumers which introspect the database of installed packages (see PEP 376).

Provisional acceptance
======================

In accordance with the PyPA's specification process, this PEP has been
`provisionally accepted <https://www.pypa.io/en/latest/specifications/#provisional-acceptance>`_
for initial implementation in ``pip`` and other PyPA tools.

During this time, the specification is still subject to revision based
on real world experience with those initial implementations.

Motivation
==========

The original motivation of this PEP was to permit tools with a "freeze"
operation allowing a Python environment to be recreated to work in a broader
range of situations.

Specifically, the PEP originated from the desire to address `pip issue #609`_:
i.e. improving the behavior of ``pip freeze`` in the presence of distributions
installed from direct URL references. It follows a
`thread on discuss.python.org`_ about the best course of action to implement
it.

Installation from direct URL references
---------------------------------------

Python installers such as pip are capable of downloading and installing
distributions from package indexes. They are also capable of downloading
and installing source code from requirements specifying arbitrary URLs of
source archives and Version Control Systems (VCS) repositories,
as standardized in `PEP440 Direct References`_.

In other words, two relevant installation modes exist.

1. the package to install is specified as a name and version specifier:

  In this case, the installer looks in a package index (or optionally
  using ``--find-links`` in the case of pip) to find the distribution to install.

2. The package to install is specified as a direct URL reference:

  In this case, the installer downloads whatever is specified by the URL
  (typically a wheel, a source archive or a VCS repository) and installs it.

  In this mode, installers typically download the source code in a
  temporary directory, invoke the PEP 517 build backend to produce a wheel
  if needed, install the wheel, and delete the temporary directory.

  After installation, no trace of the URL the user requested to download the
  package is left on the user system.

Freezing an environment
-----------------------

Pip also sports a command named ``pip freeze`` which examines the Database of
Installed Python Distributions to generate a list of requirements. The main
goal of this command is to help users generating a list of requirements that
will later allow the re-installation the same environment with the highest
possible fidelity.

As of pip version 19.3, the ``pip freeze`` command outputs a ``name==version``
line for each installed
distribution (except for editable installs). To achieve the goal of
reinstalling the same environment, this requires the (name, version)
tuple to refer to an immutable version of the
distribution. The immutability is guaranteed by package indexes
such as Warehouse. The package index to use is typically known from
environmental or command line parameters of the installer.

This freeze mechanism therefore works fine for installation mode 1 (i.e.
when the package to install was specified as a name plus version specifier).

For installation mode 2, i.e. when the package to install was specified as a
direct URL reference, the ``name==version`` tuple is obviously not sufficient
to reinstall the same distribution and users of the freeze command expect it
to output the URL that was originally requested.

The reasoning above is equally applicable to tools, other than ``pip freeze``,
that would attempt to generate a ``Pipfile.lock`` or any other similar format
from the Database of Installed Python Distributions. Unless specified
otherwise, "freeze" is used in this document as a generic term for such
an operation.

The importance of installing from (VCS) URLs for application integrators
------------------------------------------------------------------------

For an application integrator, it is important to be able to reliably install
and freeze unreleased version of python distributions.
For instance when a developer needs to deploy an unreleased patched version
of a dependency, it is common to install the dependency directly from a VCS
branch that has the patch, while waiting for the maintainer to release an
updated version.

In such cases, it is important for "freeze" to pin the exact VCS
reference (commit-hash if available) that was installed, in order to create
reproducible builds with the highest possible fidelity.

Additional origin metadata available for VCS URLs
-------------------------------------------------

For VCS URLs, there is additional origin information available only at
install time useful for introspection and certain workflows. For example,
when installing a revision from a VCS URL, a tool can determine if the
revision corresponds to a branch, tag or (in the case of Git) a ref. This
information can be used when introspecting the Database of Installed Distributions
to communicate to users more information about what version was installed
(e.g. whether a branch or tag was installed and, if so, the name of the
branch or tag). This also permits one to know whether a PEP 440 direct
reference URL can be constructed using the tag form, as only tags have the
semantics of immutability.

In cases where the revision is mutable (e.g. branches and Git refs), knowing
this information enables workflows where users can e.g. update to the latest
version of a branch they are tracking, or update to the latest version of a
pull request they are reviewing locally. In contrast, when the revision is a
tag, tools can know in advance (e.g. without network calls) that no update is
needed.

As with the URL itself, if this information isn't recorded at install time
when the VCS repository is available, it would otherwise be lost.

Note about "editable" installs
------------------------------

The editable installation mode of pip roughly lets a user insert a
local directory in sys.path for development purpose. This mode is somewhat
abused to work around the fact that a non-editable install from a VCS URL
loses track of the origin after installation.
Indeed, editable installs implicitly record the VCS origin in the checkout
directory, so the information can be recovered when running "freeze".

The use of this workaround, although useful, is fragile, creates confusion
about the purpose of the editable mode, and works only when the distribution
can be installed with setuptools (i.e. it is not usable with other PEP 517
build backends).

When this PEP is implemented, it will not be necessary anymore to use
editable installs for the purpose of making pip freeze work correctly with
VCS references.

Rationale
=========

This PEP specifies a new ``direct_url.json`` metadata file in the
``.dist-info`` directory of an installed distribution.

The fields specified are sufficient to reproduce the source archive and `VCS
URLs supported by pip`_. They are also sufficient to reproduce `PEP440 Direct
References`_, as well as `Pipfile and Pipfile.lock`_ entries. Finally, they
are sufficient to record the branch, tag, and/or Git ref origin of the
installed version that is already available for editable installs by virtue
of a VCS checkout being present.

Since at least three different ways already exist to encode this type of
information, this PEP uses a dictionary format, so as not to make any
assumption on how a direct
URL reference must ultimately be encoded in a requirement or lockfile. See also
the `Alternatives`_ section below for more discussion about this choice.

Information has been taken from Ruby's bundler manual to verify it has similar
capabilities and inform the selection and naming of fields in this
specifications.

The JSON format allows for the addition of additional fields in the future.

Specification
=============

This PEP specifies a ``direct_url.json`` file in the ``.dist-info`` directory
of an installed distribution, to record the Direct URL Origin of the distribution.

The canonical source for the name and semantics of this metadata file is
the `Recording the Direct URL Origin of installed distributions`_ document.

This file MUST be created by installers when installing a distribution
from a requirement specifying a direct URL reference (including a VCS URL).

This file MUST NOT be created when installing a distribution from an other
type of requirement (i.e. name plus version specifier).

This JSON file MUST be a dictionary, compliant with `RFC 8259`_ and UTF-8
encoded.

If present, it MUST contain at least two fields. The first one is ``url``, with
type ``string``. Depending on what ``url`` refers to, the second field MUST be
one of ``vcs_info`` (if ``url`` is a VCS reference), ``archive_info`` (if
``url`` is a source archives or a wheel), or ``dir_info`` (if ``url``  is a
local directory). These info fields have a (possibly empty) subdictionary as
value, with the possible keys defined below.

``url`` MUST be stripped of any sensitive authentication information,
for security reasons.

The user:password section of the URL MAY however
be composed of environment variables, matching the following regular
expression::

    \$\{[A-Za-z0-9-_]+\}(:\$\{[A-Za-z0-9-_]+\})?

Additionally, the user:password section of the URL MAY be a
well-known, non security sensitive string. A typical example is ``git``
in the case of an URL such as ``ssh://git@gitlab.com``.

When ``url`` refers to a VCS repository, the ``vcs_info`` key MUST be present
as a dictionary with the following keys:

- A ``vcs`` key (type ``string``) MUST be present, containing the name of the VCS
  (i.e. one of ``git``, ``hg``, ``bzr``, ``svn``). Other VCS's SHOULD be registered by
  writing a PEP to amend this specification.
  The ``url`` value MUST be compatible with the corresponding VCS,
  so an installer can hand it off without transformation to a
  checkout/download command of the VCS.
- A ``requested_revision`` key (type ``string``) MAY be present naming a
  branch/tag/ref/commit/revision/etc (in a format compatible with the VCS)
  to install.
- A ``commit_id`` key (type ``string``) MUST be present, containing the
  exact commit/revision number that was installed.
  If the VCS supports commit-hash
  based revision identifiers, such commit-hash MUST be used as
  ``commit_id`` in order to reference the immutable
  version of the source code that was installed.
- If the installer could discover additional information about
  the requested revision, it MAY add a ``resolved_revision`` and/or
  ``resolved_revision_type`` field. If no revision was provided in
  the requested URL, ``resolved_revision`` MAY contain the default branch
  that was installed, and ``resolved_revision_type`` will be ``branch``.
  If the installer determines that ``requested_revision`` was a tag, it MAY
  add ``resolved_revision_type`` with value ``tag``.

When ``url`` refers to a source archive or a wheel, the ``archive_info`` key
MUST be present as a dictionary with the following key:

- A ``hash`` key (type ``string``) SHOULD be present, with value
  ``<hash-algorithm>=<expected-hash>``.
  It is RECOMMENDED that only hashes which are unconditionally provided by
  the latest version of the standard library's ``hashlib`` module be used for
  source archive hashes. At time of writing, that list consists of 'md5',
  'sha1', 'sha224', 'sha256', 'sha384', and 'sha512'.

When ``url`` refers to a local directory, the ``dir_info`` key MUST be
present as a dictionary with the following key:

- ``editable`` (type: ``boolean``): ``true`` if the distribution was installed
  in editable mode, ``false`` otherwise. If absent, default to ``false``.

When ``url`` refers to a local directory, it MUST have the ``file`` scheme
and be compliant with `RFC 8089`_. In particular, the path component must
be absolute. Symbolic links SHOULD be preserved when making relative
paths absolute.

.. note::

  When the requested URL has the file:// scheme and points to a local directory that happens to contain a
  VCS checkout, installers MUST NOT attempt to infer any VCS information and
  therefore MUST NOT output any VCS related information (such as ``vcs_info``)
  in ``direct_url.json``.

A top-level ``subdirectory`` field MAY be present containing a directory path,
relative to the root of the VCS repository, source archive or local directory,
to specify where ``pyproject.toml`` or ``setup.py`` is located.

.. note::

   As a general rule, installers should as much as possible preserve the
   information that was provided in the requested URL when generating
   ``direct_url.json``. For example, user:password environment variables
   should be preserved and ``requested_revision`` should reflect the revision that was
   provided in the requested URL as faithfully as possible. This information is
   however *enriched* with more precise data, such as ``commit_id``.

Registered VCS
--------------

This section lists the registered VCS's; expanded, VCS-specific information
on how to use the ``vcs``, ``requested_revision``, and other fields of
``vcs_info``; and in
some cases additional VCS-specific fields.
Tools MAY support other VCS's although it is RECOMMENDED to register
them by writing a PEP to amend this specification. The ``vcs`` field SHOULD be the command name
(lowercased). Additional fields that would be necessary to
support such VCS SHOULD be prefixed with the VCS command name.

Git
+++

Home page

   https://git-scm.com/

vcs command

   git

``vcs`` field

   git

``requested_revision`` field

   A tag name, branch name, Git ref, commit hash, shortened commit hash,
   or other commit-ish.

``commit_id`` field

   A commit hash (40 hexadecimal characters sha1).

.. note::

   Installers can use the ``git show-ref`` and ``git symbolic-ref`` commands
   to determine if the ``requested_revision`` corresponds to a Git ref.
   In turn, a ref beginning with ``refs/tags/`` corresponds to a tag, and
   a ref beginning with ``refs/remotes/origin/`` after cloning corresponds
   to a branch.

Mercurial
+++++++++

Home page

   https://www.mercurial-scm.org/

vcs command

   hg

``vcs`` field

   hg

``requested_revision`` field

   A tag name, branch name, changeset ID, shortened changeset ID.

``commit_id`` field

   A changeset ID (40 hexadecimal characters).

Bazaar
++++++

Home page

   https://bazaar.canonical.com/

vcs command

   bzr

``vcs`` field

   bzr

``requested_revision`` field

   A tag name, branch name, revision id.

``commit_id`` field

   A revision id.

Subversion
++++++++++

Home page

   https://subversion.apache.org/

vcs command

   svn

``vcs`` field

   svn

``requested_revision`` field

   ``requested_revision`` must be compatible with ``svn checkout`` ``--revision`` option.
   In Subversion, branch or tag is part of ``url``.

``commit_id`` field

   Since Subversion does not support globally unique identifiers,
   this field is the Subversion revision number in the corresponding
   repository.

Examples
========

Example direct_url.json
-----------------------

Source archive:

.. code::

    {
        "url": "https://github.com/pypa/pip/archive/1.3.1.zip",
        "archive_info": {
            "hash": "sha256=2dc6b5a470a1bde68946f263f1af1515a2574a150a30d6ce02c6ff742fcc0db8"
        }
    }

Git URL with tag and commit-hash:

.. code::

    {
        "url": "https://github.com/pypa/pip.git",
        "vcs_info": {
            "vcs": "git",
            "requested_revision": "1.3.1",
            "resolved_revision_type": "tag",
            "commit_id": "7921be1537eac1e97bc40179a57f0349c2aee67d"
        }
    }

Local directory:

.. code::

   {
       "url": "file:///home/user/project",
       "dir_info": {}
   }

Local directory installed in editable mode:

.. code::

   {
       "url": "file:///home/user/project",
       "dir_info": {
           "editable": true
       }
   }


Example pip commands and their effect on direct_url.json
--------------------------------------------------------

Commands that generate a ``direct_url.json``:

* pip install https://example.com/app-1.0.tgz
* pip install https://example.com/app-1.0.whl
* pip install "git+https://example.com/repo/app.git#egg=app&subdirectory=setup"
* pip install ./app
* pip install file:///home/user/app
* pip install --editable "git+https://example.com/repo/app.git#egg=app&subdirectory=setup"
  (in which case, ``url`` will be the local directory where the git repository has been
  cloned to, and ``dir_info`` will be present with ``"editable": true`` and no
  ``vcs_info`` will be set)
* pip install -e ./app

Commands that *do not* generate a ``direct_url.json``

* pip install app
* pip install app --no-index --find-links https://example.com/

Use cases
=========

"Freezing" an environment

  Tools, such as ``pip freeze``, which generate requirements from the Database
  of Installed Python Distributions SHOULD exploit ``direct_url.json``
  if it is present, and give it priority over the Version metadata in order
  to generate a higher fidelity output. In the presence of a ``vcs`` direct URL reference,
  the ``commit_id`` field SHOULD be used in priority in order to provide
  the highest possible fidelity to the originally installed version. If
  supported by their requirement format, tools are encouraged also to output
  the ``tag`` value if present, as it has immutable semantics.
  Tools MAY choose another approach, depending on the needs of their users.

  Note the initial iteration of this PEP does not attempt to make environments
  that include editable installs or installs from local directories
  reproducible, but it does attempt to make them readily identifiable. By
  locating the local project directory via the ``url`` and ``dir_info`` fields
  of this specification, tools can implement any strategy that fits their use
  cases.

Backwards Compatibility
=======================

Since this PEP specifies a new file in the ``.dist-info`` directory,
there are no backwards compatibility implications.

Alternatives
============

PEP426 source_url
-----------------

The now withdrawn PEP 426 specifies a ``source_url`` metadata entry.
It is also implemented in `distlib`_.

It was intended for a slightly different purpose, for use in sdists.

This format lacks support for the ``subdirectory`` option of pip requirement
URLs. The same limitation is present in `PEP440 Direct References`_.

It also lacks explicit support for `environment variables in the user:password
part of URLs`_.

The introduction of a key/value extensibility mechanism and support
for environment variables for user:password in PEP 440, would be necessary
for use in this PEP.

revision vs ref
---------------

The ``requested_revision`` key was retained over ``requested_ref`` as it is a more generic term
across various VCS and ``ref`` has a specific meaning for ``git``.


References
==========

.. _`pip issue #609`: https://github.com/pypa/pip/issues/609
.. _`thread on discuss.python.org`:  https://discuss.python.org/t/pip-freeze-vcs-urls-and-pep-517-feat-editable-installs/1473
.. _PEP440: http://www.python.org/dev/peps/pep-0440
.. _`VCS URLs supported by pip`: https://pip.pypa.io/en/stable/reference/pip_install/#vcs-support
.. _`PEP440 Direct References`: https://www.python.org/dev/peps/pep-0440/#direct-references
.. _`Pipfile and Pipfile.lock`: https://github.com/pypa/pipfile
.. _distlib: https://distlib.readthedocs.io
.. _`environment variables in the user:password part of URLs`: https://pip.pypa.io/en/stable/reference/pip_install/#id10
.. _`RFC 8259`: https://tools.ietf.org/html/rfc8259
.. _`RFC 8089`: https://tools.ietf.org/html/rfc8089
.. _`Recording the Direct URL Origin of installed distributions`: https://packaging.python.org/specifications/direct-url

Acknowledgements
================

Various people helped make this PEP a reality. Paul F. Moore provided the
essence of the abstract. Nick Coghlan suggested the direct_url name.

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.


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