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Python Enhancement Proposals

PEP 470 – Removing External Hosting Support on PyPI

Donald Stufft <donald at>
Paul Moore <p.f.moore at>
Distutils-SIG list
14-May-2014, 05-Jun-2014, 03-Oct-2014, 13-Oct-2014, 26-Aug-2015
Distutils-SIG message

Table of Contents


This PEP proposes the deprecation and removal of support for hosting files externally to PyPI as well as the deprecation and removal of the functionality added by PEP 438, particularly rel information to classify different types of links and the meta-tag to indicate API version.


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 index.

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 oftentimes do not realize who they need to contact in order to get this fixed or what their remediation steps are.

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.

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.

Key User Experience Expectations

  1. Easily allow external hosting to “just work” when appropriately configured at the system, user or virtual environment level.
  2. 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).
  3. 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.
  4. 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 that 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 install.

This UX failure exists for several reasons.

  1. 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.
  2. 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.
  3. 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”.
  4. 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 implementation.

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.

The Python packaging guide MUST be updated with a section detailing the options for setting up their own repository so that any project that wishes to not host on PyPI in the future can reference that documentation. This should include the suggestion that projects relying on hosting their own repositories should document in their project description how to install their project.

Summary of Changes

Repository side

  1. Deprecate and remove the hosting modes as defined by PEP 438.
  2. Restrict simple API to only list the files that are contained within the repository.

Client side

  1. Implement multiple repository support.
  2. Implement some mechanism for removing/disabling the default repository.
  3. Deprecate / Remove PEP 438


To determine impact, we’ve looked at all projects using a method of searching PyPI which is similar to what pip and setuptools use and searched for all files available on PyPI, safely linked from PyPI, unsafely linked from PyPI, and finally unsafely available outside of PyPI. When the same file was found in multiple locations it was deduplicated and only counted it in one location based on the following preferences: PyPI > Safely Off PyPI > Unsafely Off PyPI. This gives us the broadest possible definition of impact, it means that any single file for this project may no longer be visible by default, however that file could be years old, or it could be a binary file while there is a sdist available on PyPI. This means that the real impact will likely be much smaller, but in an attempt not to miscount we take the broadest possible definition.

At the time of this writing there are 65,232 projects hosted on PyPI and of those, 59 of them rely on external files that are safely hosted outside of PyPI and 931 of them rely on external files which are unsafely hosted outside of PyPI. This shows us that 1.5% of projects will be affected in some way by this change while 98.5% will continue to function as they always have. In addition, only 5% of the projects affected are using the features provided by PEP 438 to safely host outside of PyPI while 95% of them are exposing their users to Remote Code Execution via a Man In The Middle attack.

Frequently Asked Questions

I can’t host my project on PyPI because of <X>, what should I do?

First you should decide if <X> is something inherent to PyPI, or if PyPI could grow a feature to solve <X> for you. If PyPI can add a feature to enable you to host your project on PyPI then you should propose that feature. However, if <X> is something inherent to PyPI, such as wanting to maintain control over your own files, then you should setup your own package repository and instruct your users in your project’s description to add it to the list of repositories their installer of choice will use.

My users have a worse experience with this PEP than before, how do I explain that?

Part of this answer is going to be specific to each individual project, you’ll need to explain to your users what caused you to decide to host in your own repository instead of utilizing one that they already have in their installer’s default list of repositories. However, part of this answer will also be explaining that the previous behavior of transparently including external links was both a security hazard (given that in most cases it allowed a MITM to execute arbitrary Python code on the end users machine) and a reliability concern and that PEP 438 attempted to resolve this by making them explicitly opt in, but that PEP 438 brought along with it a number of serious usability issues. PEP 470 represents a simplification of the model to a model that many users will be familiar with, which is common amongst Linux distributions.

Switching to a repository structure breaks my workflow or isn’t allowed by my host?

There are a number of cheap or free hosts that would gladly support what is required for a repository. In particular you don’t actually need to upload your files anywhere differently as long as you can generate a host with the correct structure that points to where your files are actually located. Many of these hosts provide free HTTPS using a shared domain name, and free HTTPS certificates can be gotten from StartSSL, or in the near future LetsEncrypt or they may be gotten cheap from any number of providers.

Why don’t you provide <X>?

The answer here will depend on what <X> is, however the answers typically are one of:

  • We hadn’t been thought of it and nobody had suggested it before.
  • We don’t have sufficient experience with <X> to properly design a solution for it and would welcome a domain expert to help us provide it.
  • We’re an open source project and nobody has decided to volunteer to design and implement <X> yet.

Additional PEPs to propose additional features are always welcome, however they would need someone with the time and expertise to accurately design <X>. This particular PEP is intended to focus on getting us to a point where the capabilities of PyPI are straightforward with an easily understood baseline that is similar to existing models such as Linux distribution repositories.

Why should I register on PyPI if I’m running my own repository anyways?

PyPI serves two critical functions for the Python ecosystem. One of those is as a central repository for the actual files that get downloaded and installed by pip or another package manager and it is this function that this PEP is concerned with and that you’d be replacing if you’re running your own repository. However, it also provides a central registry of who owns what name in order to prevent naming collisions, think of it sort of as DNS but for Python packages. In addition to making sure that names are handed out in a first-come, first-served manner it also provides a single place for users to go to look search for and discover new projects. So the simple answer is, you should still register your project with PyPI to avoid naming collisions and to make it so people can still easily discover your project.

Rejected Proposals

Allow easier discovery of externally hosted indexes

A previous version of this PEP included a new feature added to both PyPI and installers that would allow project authors to enter into PyPI a list of URLs that would instruct installers to ignore any files uploaded to PyPI and instead return an error telling the end user about these extra URLs that they can add to their installer to make the installation work.

This feature has been removed from the scope of the PEP because it proved too difficult to develop a solution that avoided UX issues similar to those that caused so many problems with the PEP 438 solution. If needed, a future PEP could revisit this idea.

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 hosted.
  • 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 oftentimes 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.


Last modified: 2023-09-09 17:39:29 GMT