Abstract

This PEP proposes adding the ability to mark a particular file download on a simple repository as “yanked”. Yanking a file allows authors to effectively delete a file, without breaking things for people who have pinned to exactly a specific version.

It also changes to the canonical source for the simple repository API to the Simple Repository API reference document.

Motivation

Whenever a project detects that a particular release on PyPI might be broken, they oftentimes will want to prevent further users from inadvertently using that version. However, the obvious solution of deleting the existing file from a repository will break users who have followed best practices and pinned to a specific version of the project.

This leaves projects in a catch-22 situation where new projects may be pulling down this known broken version, but if they do anything to prevent that they’ll break projects that are already using it.

By allowing the ability to “yank” a file, but still make it available for users who are explicitly asking for it, this allows projects to mitigate the worst of the breakage while still keeping things working for projects who have otherwise worked around or didn’t hit the underlying issues.

One of the main scenarios where this may happen, is when dropping support for a particular version of Python. The python-requires metadata allows for dropping support for a version of Python in a way that is not disruptive to users who are still using that Python. However, a common mistake is to either omit or forget to update that bit of metadata. When that mistake has been made, a project really only has three options:

• Prevent that version from being installed through some mechanism (currently, the only mechanism is by deleting the release entirely).
• Re-release the version that worked as a higher version number, and then re-release the version that dropped support as an even higher version number with the correct metadata.
• Do nothing, and document that people using that older Python have to manually exclude that release.

With this PEP, projects can choose the first option, but with a mechanism that is less likely to break the world for people who are currently successfully using said project.

Specification

Links in the simple repository MAY have a data-yanked attribute which may have no value, or may have an arbitrary string as a value. The presence of a data-yanked attribute SHOULD be interpreted as indicating that the file pointed to by this particular link has been “Yanked”, and should not generally be selected by an installer, except under specific scenarios.

The value of the data-yanked attribute, if present, is an arbitrary string that represents the reason for why the file has been yanked. Tools that process the simple repository API MAY surface this string to end users.

The yanked attribute is not immutable once set, and may be rescinded in the future (and once rescinded, may be reset as well). Thus API users MUST be able to cope with a yanked file being “unyanked” (and even yanked again).

Rejected Ideas

A previous, undocumented, version of the simple repository API had version specific pages, like /simple/<project>/<version>/. If we were to add those back, the yanked files could only appear on those pages and not on the version-less page at all. However this would drastically reduce the cache-ability of the simple API and would directly impact our ability to scale it out to handle all of the incoming traffic.

A previous iteration of this PEP had the data-yanked attribute act as a boolean value. However it was decided that allowing a string both simplified the implementation, and provided additional generalized functionality to allow projects to provide a mechanism to indicate why they were yanking a release.

Another suggestion was to reserve some syntax in the arbitrary string to allow us to evolve the standard in the future if we ever need to. However, given we can add additional attributes in the future, this idea has been rejected, favoring instead to use additional attributes if the need ever arose.

Warehouse/PyPI Implementation Notes

While this PEP implements yanking at the file level, that is largely due to the shape the simple repository API takes, not a specific decision made by this PEP.

In Warehouse, the user experience will be implemented in terms of yanking or unyanking an entire release, rather than as an operation on individual files, which will then be exposed via the API as individual files being yanked.

Other repository implementations may choose to expose this capability in a different way, or not expose it at all.

Copyright

This document has been placed in the public domain.