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PEP 610 -- Recording the Direct URL Origin of installed distributions

PEP:610
Title:Recording the Direct URL Origin of installed distributions
Author:St├ęphane Bidoul <stephane.bidoul at acsone.eu>, Chris Jerdonek <chris.jerdonek at gmail.com>
Discussions-To:https://discuss.python.org/t/recording-the-source-url-of-an-installed-distribution/1535
Status:Draft
Type:Standards Track
Created:21-Apr-2019
Post-History:

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 [4]). 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).

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 [1]: 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 [2] 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 [4].

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.
  1. 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.

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 packages 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).

For the sake of clarity, it is important to note that this PEP is otherwise unrelated to editable installs.

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 [3]. They are also sufficient to reproduce PEP440 Direct References [4], as well as Pipfile and Pipfile.lock [5] 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 key-value 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.

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

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

This JSON MUST be a flat dictionary where all keys and values are of string type. For the sake of forward compatibility, tools SHOULD ignore values which are not of string type.

If present, it MUST contain at least one field with name url.

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-_]+\})?

When url refers to a VCS repository:

  • A vcs field MUST be present, containing the name of the VCS (i.e. one of git, hg, bzr, svn). Other VCS's SHOULD be registered by amending this PEP.
  • 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 field MAY be present naming a branch/tag/ref/commit/revision/etc (in a format compatible with the VCS) to install.
  • A commit_id field 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.
  • A tag field naming a tag MAY be present to indicate that a particular tag was installed.
  • A branch field naming a branch MAY be present to indicate that a particular branch was installed. If branch is present, tag MUST not be present.

When url refers to a source archive, a wheel, or a local directory:

  • A hash field 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'.

Note

When the requested URL 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 the vcs field) in direct_url.json.

A 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; and in some cases additional VCS-specific fields. Tools MAY support other VCS's although it is RECOMMENDED to register them by amending this PEP. 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).

branch field

If no requested_revision is provided and the remote repository has a default branch, this field can be used to record the default branch that was installed.

VCS-specific fields:

  • A git_ref field naming a Git ref (string beginning with refs/) MAY be present to indicate that a particular ref was installed (e.g. refs/pull/123/head).

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

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:

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

Git URL with tag and commit-hash:

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

Example pip commands and their effect on direct_url.json

Commands that generate a direct_url.json:

Commands that do not generate a direct_url.json

  • pip install app
  • pip install app --no-index --find-links https://example.com/
  • pip install --editable "git+https://example.com/repo/app.git#egg=app&subdirectory=setup"
  • pip install -e ./app

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.

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 [6].

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 [4].

It also lacks explicit support for environment variables in the user:password part of URLs [7].

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.

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.

Source: https://github.com/python/peps/blob/master/pep-0610.rst