PEP: 592
Title: Adding "Yank" Support to the Simple API
Author: Donald Stufft <donald@stufft.io>
BDFL-Delegate: Paul Moore <p.f.moore@gmail.com>
Discussions-To: https://discuss.python.org/t/1629
Status: Final
Type: Standards Track
Topic: Packaging
Created: 07-May-2019
Resolution: https://discuss.python.org/t/pep-592-support-for-yanked-files-in-the-simple-repository-api/1629/30


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).


Installers
----------

The desirable experience for users is that once a file is yanked, when
a human being is currently trying to directly install a yanked file, that
it fails as if that file had been deleted. However, when a human did that
a while ago, and now a computer is just continuing to mechanically follow
the original order to install the now yanked file, then it acts as if it
had not been yanked.

An installer **MUST** ignore yanked releases, if the selection constraints
can be satisfied with a non-yanked version, and **MAY** refuse to use a
yanked release even if it means that the request cannot be satisfied at all.
An implementation **SHOULD** choose a policy that follows the spirit of the
intention above, and that prevents "new" dependencies on yanked
releases/files.

What this means is left up to the specific installer, to decide how to best
fit into the overall usage of their installer. However, there are two
suggested approaches to take:

1. Yanked files are always ignored, unless they are the only file that
   matches a version specifier that "pins" to an exact version using
   either ``==`` (without any modifiers that make it a range, such as
   ``.*``) or ``===``. Matching this version specifier should otherwise
   be done as per :pep:`440` for things like local versions, zero padding,
   etc.
2. Yanked files are always ignored, unless they are the only file that
   matches what a lock file (such as ``Pipfile.lock`` or ``poetry.lock``)
   specifies to be installed. In this case, a yanked file **SHOULD** not
   be used when creating or updating a lock file from some input file or
   command.

Regardless of the specific strategy that an installer chooses for deciding
when to install yanked files, an installer **SHOULD** emit a warning when
it does decide to install a yanked file. That warning **MAY** utilize the
value of the ``data-yanked`` attribute (if it has a value) to provide more
specific feedback to the user about why that file had been yanked.


Mirrors
-------

Mirrors can generally treat yanked files one of two ways:

1. They may choose to omit them from their simple repository API completely,
   providing a view over the repository that shows only "active", unyanked
   files.
2. They may choose to include yanked files, and additionally mirror the
   ``data-yanked`` attribute as well.

Mirrors **MUST NOT** mirror a yanked file without also mirroring the
``data-yanked`` attribute for it.


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.


Journal Handling
----------------

Whenever a release has been yanked, an entry will be recorded in the
journal using one of the following string patterns:

* ``yank release``
* ``unyank release``


In both cases, the standard journal structure will indicate which release
of which project has been yanked or unyanked.


.. _`Simple Repository API`:
   https://packaging.python.org/specifications/simple-repository-api/


Copyright
=========

This document has been placed in the public domain.
