PEP 658 – Serve Distribution Metadata in the Simple Repository API

PEP: 658
Title: Serve Distribution Metadata in the Simple Repository API
Author: Tzu-ping Chung <uranusjr at gmail.com>
Sponsor: Brett Cannon <brett at python.org>
PEP-Delegate: Donald Stufft <donald at stufft.io>
Discussions-To: https://discuss.python.org/t/8651
Status: Accepted
Type: Standards Track
Created: 10-May-2021
Post-History: 10-May-2021
Resolution: https://discuss.python.org/t/8651/48

Abstract
Motivation
Rationale
Specification
Backwards Compatibility
Rejected Ideas
References
Copyright

Abstract

This PEP proposes adding an anchor tag to expose the METADATA file from distributions in the PEP 503 “simple” repository API. A data-dist-info-metadata attribute is introduced to indicate that the file from a given distribution can be independently fetched.

Motivation

Package management workflows made popular by recent tooling increase the need to inspect distribution metadata without intending to install the distribution, and download multiple distributions of a project to choose from based on their metadata. This means they end up discarding much downloaded data, which is inefficient and results in a bad user experience.

Rationale

Tools have been exploring methods to reduce the download size by partially downloading wheels with HTTP range requests. This, however, adds additional run-time requirements to the repository server. It also still adds additional overhead, since a separate request is needed to fetch the wheel’s file listing to find the correct offset to fetch the metadata file. It is therefore desired to make the server extract the metadata file in advance, and serve it as an independent file to avoid the need to perform additional requests and ZIP inspection.

The metadata file defined by the Core Metadata Specification [core-metadata] will be served directly by repositories since it contains the necessary information for common use cases. The metadata must only be served for standards-compliant distributions such as wheels [wheel] and sdists [sdist], and must be identical to the distribution’s canonical metadata file, such as a wheel’s METADATA file in the .dist-info directory [dist-info].

An HTML attribute on the distribution file’s anchor link is needed to indicate whether a client is able to choose the separately served metadata file. The attribute is also used to provide the metadata content’s hash for client-side verification. The attribute’s absence indicates that a separate metadata entry is not available for the distribution, either because of the distribution’s content, or lack of repository support.

Specification

In a simple repository’s project page, each anchor tag pointing to a distribution MAY have a data-dist-info-metadata attribute. The presence of the attribute indicates the distribution represented by the anchor tag MUST contain a Core Metadata file that will not be modified when the distribution is processed and/or installed.

If a data-dist-info-metadata attribute is present, the repository MUST serve the distribution’s Core Metadata file alongside the distribution with a .metadata appended to the distribution’s file name. For example, the Core Metadata of a distribution served at /files/distribution-1.0-py3.none.any.whl would be located at /files/distribution-1.0-py3.none.any.whl.metadata. This is similar to how PEP 503 specifies the GPG signature file’s location.

The repository SHOULD provide the hash of the Core Metadata file as the data-dist-info-metadata attribute’s value using the syntax <hashname>=<hashvalue>, where <hashname> is the lower cased name of the hash function used, and <hashvalue> is the hex encoded digest. The repository MAY use true as the attribute’s value if a hash is unavailable.

Backwards Compatibility

If an anchor tag lacks the data-dist-info-metadata attribute, tools are expected to revert to their current behaviour of downloading the distribution to inspect the metadata.

Older tools not supporting the new data-dist-info-metadata attribute are expected to ignore the attribute and maintain their current behaviour of downloading the distribution to inspect the metadata. This is similar to how prior data- attribute additions expect existing tools to operate.

Rejected Ideas

Put metadata content on the project page

Since tools generally only need dependency information from a distribution in addition to what’s already available on the project page, it was proposed that repositories may directly include the information on the project page, like the data-requires-python attribute specified in PEP 503.

This approach was abandoned since a distribution may contain arbitrarily long lists of dependencies (including required and optional), and it is unclear whether including the information for every distribution in a project would result in net savings since the information for most distributions generally ends up unneeded. By serving the metadata separately, performance can be better estimated since data usage will be more proportional to the number of distributions inspected.

Expose more files in the distribution

It was proposed to provide the entire .dist-info directory as a separate part, instead of only the metadata file. However, searving multiple files in one entity through HTTP requires re-archiving them separately after they are extracted from the original distribution by the repository server, and there are no current use cases for files other than METADATA when the distribution itself is not going to be installed.

It should also be noted that the approach taken here does not preclude other files from being introduced in the future, whether we want to serve them together or individually.

Explicitly specify the metadata file’s URL on the project page

An early version of this draft proposed putting the metadata file’s URL in the data-dist-info-metadata attribute. But people feel it is better for discoverability to require the repository to serve the metadata file at a determined location instead. The current approach also has an additional benefit of making the project page smaller.