[Distutils] PEP 470 - Once More, with Feeling

Donald Stufft donald at stufft.io
Mon Oct 13 20:23:33 CEST 2014

Alright, here's yet another go at PEP 470.

See it online at www.python.org/dev/peps/pep-0470/ or reproduced in full down
below. The diff between this version and the last is available at

Important Notes:

* Continue to use a ``<meta>`` link instead of an href to prevent older
  installers from silently picking up insecure hosting URLs.

* Reduce the overall impact by dropping the special case for PIL and instead
  scan all projects for URLs which add installable files and move them to
  the new external repository feature.

* Reduce the overall impact by explicitly stating that PyPI will add the
  location of any external repository in the UI for people using installers
  which have not implemented the discovery feature.

* Explicitly call out the key user experience requirements of a solution to
  the general problem.

* Simplify the ``<meta>`` tag a bit, and also add explicit repository vs
  find-links types as well as include an example of the ``find-links`` type.

* Allow a project to both host files on PyPI and register external repositories,
  these can be used for things which cannot be hosted on PyPI such as data
  files or Linux Wheels while still using PyPI as the repository for "regular"

* Mandate that the discovery mechanism must exist in a released pip prior to
  starting the deprecation process (with the exception of ``pypi-only`` for new
  projects) and call this out using an important admonition.

* Explicitly call out the fact that 99.5% of the users of the deprecated
  features are doing so unsafely.

* Explicitly reject the idea of preserving the existing links indefinentely.

* Removed all examples which used the ``--extra-index-url`` feature of pip to
  remove the distraction of the discussion of how that currently works and in
  what scenarios it's safe or unsafe.


I've thought it over and gone back and forth on it to myself and others. I
cannot justify an attempt to preserve backwards compatability when that
backwards compatability is almost entirely unsafe to begin with. What I have
done is remove the special case of PIL and essentially apply that to all
projects. This should mean that all projects should have the correct metadata
immediately without any need for interaction by the authors of said projects.
I've also explicitly included adding the new metadata to the PyPI web UI to
improve the discoverability for users of installers which don't have the
discovery features.



This PEP proposes a mechanism for project authors to register with PyPI an
external repository where their project's downloads can be located. This
information can than be included as part of the simple API so that installers
can use it to tell users where the item they are attempting to install is
located and what they need to do to enable this additional repository. In
addition to adding discovery information to make explicit multiple repositories
easy to use, this PEP also deprecates and removes the implicit multiple
repository support which currently functions through directly or indirectly
linking off site via the simple API. Finally this PEP also proposes deprecating
and removing the functionality added by PEP 438, particularly the additional
rel information and the meta tag to indicate the API version.

This PEP *does* not propose mandating that all authors upload their projects to
PyPI in order to exist in the index nor does it propose any change to the human
facing elements of PyPI.


Historically PyPI did not have any method of hosting files nor any method of
automatically retrieving installables, it was instead focused on providing a
central registry of names, to prevent naming collisions, and as a means of
discovery for finding projects to use. In the course of time setuptools began
to scrape these human facing pages, as well as pages linked from those pages,
looking for things it could automatically download and install. Eventually this
became the "Simple" API which used a similar URL structure however it
eliminated any of the extraneous links and information to make the API more
efficient. Additionally PyPI grew the ability for a project to upload release
files directly to PyPI enabling PyPI to act as a repository in addition to an

This gives PyPI two equally important roles that it plays in the Python
ecosystem, that of index to enable easy discovery of Python projects and
central repository to enable easy hosting, download, and installation of Python
projects. Due to the history behind PyPI and the very organic growth it has
experienced the lines between these two roles are blurry, and this blurring has
caused confusion for the end users of both of these roles and this has in turn
caused ire between people attempting to use PyPI in different capacities, most
often when end users want to use PyPI as a repository but the author wants to
use PyPI solely as an index.

This confusion comes down to end users of projects not realizing if a project
is hosted on PyPI or if it relies on an external service. This often manifests
itself when the external service is down but PyPI is not. People will see that
PyPI works, and other projects works, but this one specific one does not. They
often times do not realize who they need to contact in order to get this fixed
or what their remediation steps are.

By moving to using explicit multiple repositories we can make the lines between
these two roles much more explicit and remove the "hidden" surprises caused by
the current implementation of handling people who do not want to use PyPI as a
repository. However simply moving to explicit multiple repositories is a
regression in discoverability, and for that reason this PEP adds an extension
to the current simple API which will enable easy discovery of the specific
repository that a project can be found in.

PEP 438 attempted to solve this issue by allowing projects to explicitly
declare if they were using the repository features or not, and if they were
not, it had the installers classify the links it found as either "internal",
"verifiable external" or "unverifiable external". PEP 438 was accepted and
implemented in pip 1.4 (released on Jul 23, 2013) with the final transition
implemented in pip 1.5 (released on Jan 2, 2014).

PEP 438 was successful in bringing about more people to utilize PyPI's
repository features, an altogether good thing given the global CDN powering
PyPI providing speed ups for a lot of people, however it did so by introducing
a new point of confusion and pain for both the end users and the authors.

Key User Experience Expectations

#. Easily allow external hosting to "just work" when appropriately configured
   at the system, user or virtual environment level.
#. Easily allow package authors to tell PyPI "my releases are hosted <here>"
   and have that advertised in such a way that tools can clearly communicate it
   to users, without silently introducing unexpected dependencies on third
   party services.
#. Eliminate any and all references to the confusing "verifiable external" and
   "unverifiable external" distinction from the user experience (both when
   installing and when releasing packages).
#. The repository aspects of PyPI should become *just* the default package
   hosting location (i.e. the only one that is treated as opt-out rather than
   opt-in by most client tools in their default configuration). Aside from that
   aspect, hosting on PyPI should not otherwise provide an enhanced user
   experience over hosting your own package repository.
#. Do all of the above while providing default behaviour that is secure against
   most attackers below the nation state adversary level.

Why Additional Repositories?

The two common installer tools, pip and easy_install/setuptools, both support
the concept of additional locations to search for files to satisfy the
installation requirements and have done so for many years. This means that
there is no need to "phase" in a new flag or concept and the solution to
installing a project from a repository other than PyPI will function regardless
of how old (within reason) the end user's installer is. Not only has this
concept existed in the Python tooling for some time, but it is a concept that
exists across languages and even extending to the OS level with OS package
tools almost universally using multiple repository support making it extremely
likely that someone is already familiar with the concept.

Additionally, the multiple repository approach is a concept that is useful
outside of the narrow scope of allowing projects which wish to be included on
the index portion of PyPI but do not wish to utilize the repository portion of
PyPI. This includes places where a company may wish to host a repository that
contains their internal packages or where a project may wish to have multiple
"channels" of releases, such as alpha, beta, release candidate, and final
release. This could also be used for projects wishing to host files which
cannot be uploaded to PyPI, such as multi-gigabyte data files or, currently at
least, Linux Wheels.

Why Not PEP 438 or Similar?

While the additional search location support has existed in pip and setuptools
for quite some time support for PEP 438 has only existed in pip since the 1.4
version, and still has yet to be implemented in setuptools. The design of
PEP 438 did mean that users still benefited for projects which did not require
external files even with older installers, however for projects which *did*
require external files, users are still silently being given either potentially
unreliable or, even worse, unsafe files to download. This system is also unique
to Python as it arises out of the history of PyPI, this means that it is almost
certain that this concept will be foreign to most, if not all users, until they
encounter it while attempting to use the Python toolchain.

Additionally, the classification system proposed by PEP 438 has, in practice,
turned out to be extremely confusing to end users, so much so that it is a
position of this PEP that the situation as it stands is completely untenable.
The common pattern for a user with this system is to attempt to install a
project possibly get an error message (or maybe not if the project ever
uploaded something to PyPI but later switched without removing old files), see
that the error message suggests ``--allow-external``, they reissue the command
adding that flag most likely getting another error message, see that this time
the error message suggests also adding ``--allow-unverified``, and again issue
the command a third time, this time finally getting the thing they wish to

This UX failure exists for several reasons.

#. If pip can locate files at all for a project on the Simple API it will
   simply use that instead of attempting to locate more. This is generally the
   right thing to do as attempting to locate more would erase a large part of
   the benefit of PEP 438. This means that if a project *ever* uploaded a file
   that matches what the user has requested for install that will be used
   regardless of how old it is.
#. PEP 438 makes an implicit assumption that most projects would either upload
   themselves to PyPI or would update themselves to directly linking to release
   files. While a large number of projects did ultimately decide to upload to
   PyPI, some of them did so only because the UX around what PEP 438 was so bad
   that they felt forced to do so. More concerning however, is the fact that
   very few projects have opted to directly and safely link to files and
   instead they still simply link to pages which must be scraped in order to
   find the actual files, thus rendering the safe variant
   (``--allow-external``) largely useless.
#. Even if an author wishes to directly link to their files, doing so safely is
   non-obvious. It requires the inclusion of a MD5 hash (for historical
   reasons) in the hash of the URL. If they do not include this then their
   files will be considered "unverified".
#. PEP 438 takes a security centric view and disallows any form of a global opt
   in for unverified projects. While this is generally a good thing, it creates
   extremely verbose and repetitive command invocations such as::

      $ pip install --allow-external myproject --allow-unverified myproject myproject
      $ pip install --allow-all-external --allow-unverified myproject myproject

Multiple Repository/Index Support

Installers SHOULD implement or continue to offer, the ability to point the
installer at multiple URL locations. The exact mechanisms for a user to
indicate they wish to use an additional location is left up to each individual

Additionally the mechanism discovering an installation candidate when multiple
repositories are being used is also up to each individual implementation,
however once configured an implementation should not discourage, warn, or
otherwise cast a negative light upon the use of a repository simply because it
is not the default repository.

Currently both pip and setuptools implement multiple repository support by
using the best installation candidate it can find from either repository,
essentially treating it as if it were one large repository.

Installers SHOULD also implement some mechanism for removing or otherwise
disabling use of the default repository. The exact specifics of how that is
achieved is up to each individual implementation.

Installers SHOULD also implement some mechanism for whitelisting and
blacklisting which projects a user wishes to install from a particular
repository. The exact specifics of how that is achieved is up to each
individual implementation.

External Index Discovery

One of the problems with using an additional index is one of discovery. Users
will not generally be aware that an additional index is required at all much
less where that index can be found. Projects can attempt to convey this
information using their description on the PyPI page however that excludes
people who discover their project organically through ``pip search``.

To support projects that wish to externally host their files and to enable
users to easily discover what additional indexes are required, PyPI will gain
the ability for projects to register external index URLs along with an
associated comment for each. These URLs will be made available on the simple
page however they will not be linked or provided in a form that older
installers will automatically search them.

This ability will take the form of a ``<meta>`` tag. The name of this tag must
be set to ``repository`` or ``find-link`` and the content will be a link to the
location of the repository. An optional data-description attribute will convey
any comments or description that the author has provided.

An example would look something like::

    <meta name="repository" content="https://index.example.com/" data-description="Primary Repository">
    <meta name="repository" content="https://index.example.com/Ubuntu-14.04/" data-description="Wheels built for Ubuntu 14.04">
    <meta name="find-link" content="https://links.example.com/find-links/" data-description="A flat index for find links">

When an installer fetches the simple page for a project, if it finds this
additional meta-data then it should use this data to tell the user how to add
one or more of the additional URLs to search in. This message should include
any comments that the project has included to enable them to communicate to the
user and provide hints as to which URL they might want (e.g. if some are only
useful or compatible with certain platforms or situations). When the installer
has implemented the auto discovery mechanisms they should also deprecate any of
the mechanisms added for PEP 438 (such as ``--allow-external``) for removal at
the end of the deprecation period proposed by the PEP.

In addition to the API for programtic access to the registered external
repositories, PyPI will also prevent these URLs in the UI so that users with
an installer that does not implement the discovery mechanism can still easily
discover what repository the project is using to host itself.

This feature **MUST** be added to PyPI and be contained in a released version
of pip prior to starting the deprecation and removal process for the implicit
offsite hosting functionality.

Deprecation and Removal of Link Spidering

.. important:: The deprecation specified in this section **MUST** not start to
    until after the discovery mechanisms have been implemented and released in

    The only exception to this is the addition of the ``pypi-only`` mode and
    defaulting new projects to it without abilility to switch to a different

A new hosting mode will be added to PyPI. This hosting mode will be called
``pypi-only`` and will be in addition to the three that PEP 438 has already
given us which are ``pypi-explicit``, ``pypi-scrape``, ``pypi-scrape-crawl``.
This new hosting mode will modify a project's simple api page so that it only
lists the files which are directly hosted on PyPI and will not link to anything

Upon acceptance of this PEP and the addition of the ``pypi-only`` mode, all new
projects will be defaulted to the PyPI only mode and they will be locked to
this mode and unable to change this particular setting. ``pypi-only`` projects
will still be able to register external index URLs as described above - the
"pypi-only" refers only to the download links that are published directly on

An email will then be sent out to all of the projects which are hosted only on
PyPI informing them that in one month their project will be automatically
converted to the ``pypi-only`` mode. A month after these emails have been sent
any of those projects which were emailed, which still are hosted only on PyPI
will have their mode set to ``pypi-only``.

After that switch, an email will be sent to projects which rely on hosting
external to PyPI. This email will warn these projects that externally hosted
files have been deprecated on PyPI and that in 6 months from the time of that
email that all external links will be removed from the installer APIs. This
email **MUST** include instructions for converting their projects to be hosted
on PyPI and **MUST** include links to a script or package that will enable them
to enter their PyPI credentials and package name and have it automatically
download and re-host all of their files on PyPI. This email **MUST** also
include instructions for setting up their own index page and registering that
with PyPI, including the fact that they can use pythonhosted.org as a host for
an index page without requiring them to host any additional infrastructure or
purchase a TLS certificate. This email must also contain a link to the Terms of
Service for PyPI as many users may have signed up a long time ago and may not
recall what those terms are. Finally this email must also contain a list of
the links registered with PyPI where we were able to detect an installable file
was located.

Five months after the initial email, another email must be sent to any projects
still relying on external hosting. This email will include all of the same
information that the first email contained, except that the removal date will
be one month away instead of six.

Finally a month later all projects will be switched to the ``pypi-only`` mode
and PyPI will be modified to remove the externally linked files functionality,
when switching these projects to the ``pypi-only`` mode we will move any links
which are able to be used for discovering other projects automatically to as
an external repository.

Summary of Changes

Repository side

#. Implement simple API changes to allow the addition of an external
#. *(Optional, Mandatory on PyPI)* Deprecate and remove the hosting modes as
   defined by PEP 438.
#. *(Optional, Mandatory on PyPI)* Restrict simple API to only list the files
   that are contained within the repository and the external repository

Client side

#. Implement multiple repository support.
#. Implement some mechanism for removing/disabling the default repository.
#. Implement the discovery mechanism.
#. *(Optional)* Deprecate / Remove PEP 438


The large impact of this PEP will be that for users of older installation
clients they will not get a discovery mechanism built into the install command.
This will require them to browse to the PyPI web UI and discover the repository
there. Since any URLs required to instal a project will be automatically
migrated to the new format, the biggest change to users will be requiring a new
option to install these projects.

Looking at the numbers the actual impact should be quite low, with it affecting
just 3.8% of projects which host any files only externally or 2.2% which have
their latest version hosted only externally.

6674 unique IP addresses have accessed the Simple API for these 3.8% of
projects in a single day (2014-09-30). Of those, 99.5% of them installed
something which could not be verified, and thus they were open to a Remote Code
Execution via a Man-In-The-Middle attack, while 7.9% installed something which
could be verified and only 0.4% only installed things which could be verified.

This means that 99.5% users of these features, both new and old, are doing
something unsafe, and for anything using an older copy of pip or using
setuptools at all they are silently unsafe.

Projects Which Rely on Externally Hosted files

This is determined by crawling the simple index and looking for installable
files using a similar detection method as pip and setuptools use. The "latest"
version is determined using ``pkg_resources.parse_version`` sort order and it
is used to show whether or not the latest version is hosted externally or only
old versions are.

============ ======= ================ =================== =======
\             PyPI    External (old)   External (latest)   Total
============ ======= ================ =================== =======
 **Safe**     43313   16               39                  43368
 **Unsafe**   0       756              1092                1848
 **Total**    43313   772              1131                45216
============ ======= ================ =================== =======

Top Externally Hosted Projects by Requests

This is determined by looking at the number of requests the
``/simple/<project>/`` page had gotten in a single day. The total number of
requests during that day was 10,623,831.

============================== ========
Project                        Requests
============================== ========
PIL                            63869
Pygame                         2681
mysql-connector-python         1562
pyodbc                         724
elementtree                    635
salesforce-python-toolkit      316
wxPython                       295
PyXML                          251
RBTools                        235
python-graph-core              123
cElementTree                   121
============================== ========

Top Externally Hosted Projects by Unique IPs

This is determined by looking at the IP addresses of requests the
``/simple/<project>/`` page had gotten in a single day. The total number of
unique IP addresses during that day was 124,604.

============================== ==========
Project                        Unique IPs
============================== ==========
PIL                            4553
mysql-connector-python         462
Pygame                         202
pyodbc                         181
elementtree                    166
wxPython                       126
RBTools                        114
PyXML                          87
salesforce-python-toolkit      76
pyDes                          76
============================== ==========

Rejected Proposals

Keep the current classification system but adjust the options

This PEP rejects several related proposals which attempt to fix some of the
usability problems with the current system but while still keeping the general
gist of PEP 438.

This includes:

* Default to allowing safely externally hosted files, but disallow unsafely

* Default to disallowing safely externally hosted files with only a global flag
  to enable them, but disallow unsafely hosted.

* Continue on the suggested path of PEP 438 and remove the option to unsafely
  host externally but continue to allow the option to safely host externally.

These proposals are rejected because:

* The classification system introduced in PEP 438 in an entirely unique concept
  to PyPI which is not generically applicable even in the context of Python
  packaging. Adding additional concepts comes at a cost.

* The classification system itself is non-obvious to explain and to
  pre-determine what classification of link a project will require entails
  inspecting the project's ``/simple/<project>/`` page, and possibly any URLs
  linked from that page.

* The ability to host externally while still being linked for automatic
  discovery is mostly a historic relic which causes a fair amount of pain and
  complexity for little reward.

* The installer's ability to optimize or clean up the user interface is limited
  due to the nature of the implicit link scraping which would need to be done.
  This extends to the ``--allow-*`` options as well as the inability to
  determine if a link is expected to fail or not.

* The mechanism paints a very broad brush when enabling an option, while
  PEP 438 attempts to limit this with per package options. However a project
  that has existed for an extended period of time may often times have several
  different URLs listed in their simple index. It is not unusual for at least
  one of these to no longer be under control of the project. While an
  unregistered domain will sit there relatively harmless most of the time, pip
  will continue to attempt to install from it on every discovery phase. This
  means that an attacker simply needs to look at projects which rely on unsafe
  external URLs and register expired domains to attack users.

Implement this PEP, but Do Not Remove the Existing Links

This is essentially the backwards compatible version of this PEP. It attempts
to allow people using older clients, or clients which do not implement this
PEP to continue on as if nothing had changed. This proposal is rejected because
the vast bulk of those scenarios are unsafe uses of the deprecated features. It
is the opinion of this PEP that silently allowing unsafe actions to take place
on behalf of end users is simply not an acceptable solution.

Donald Stufft
PGP: 7C6B 7C5D 5E2B 6356 A926 F04F 6E3C BCE9 3372 DCFA

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