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Python Enhancement Proposals

PEP 438 – Transitioning to release-file hosting on PyPI

Holger Krekel <holger at>, Carl Meyer <carl at>
Richard Jones <richard at>
Distutils-SIG list
Distutils-SIG message

Table of Contents


This PEP proposes a backward-compatible two-phase transition process to speed up, simplify and robustify installing from the (PyPI) package index. To ease the transition and minimize client-side friction, no changes to distutils or existing installation tools are required in order to benefit from the first transition phase, which will result in faster, more reliable installs for most existing packages.

The first transition phase implements easy and explicit means for a package maintainer to control which release file links are served to present-day installation tools. The first phase also includes the implementation of analysis tools for present-day packages, to support communication with package maintainers and the automated setting of default modes for controlling release file links. The first phase also will default newly-registered projects on PyPI to only serve links to release files which were uploaded to PyPI.

The second transition phase concerns end-user installation tools, which shall default to only install release files that are hosted on PyPI and tell the user if external release files exist, offering a choice to automatically use those external files. External release files shall in the future be registered together with a checksum hash so that installation tools can verify the integrity of the eventual download (PyPI-hosted release files always carry such a checksum).

Alternative PyPI server implementations should implement the new simple index serving behaviour of transition phase 1 to avoid installation tools treating their release links as external ones in phase 2.


History and motivations for external hosting

When PyPI went online, it offered release registration but had no facility to host release files itself. When hosting was added, no automated downloading tool existed yet. When Phillip Eby implemented automated downloading (through setuptools), he made the choice to allow people to use download hosts of their choice. The finding of externally-hosted packages was implemented as follows:

  1. The PyPI simple/ index for a package contains all links found by scraping them from that package’s long_description metadata for any release. Links in the “Download-URL” and “Home-page” metadata fields are given rel=download and rel=homepage attributes, respectively.
  2. Any of these links whose target is a file whose name appears to be in the form of an installable source or binary distribution, with name in the form “packagename-version.ARCHIVEEXT”, is considered a potential installation candidate by installation tools.
  3. Similarly, any links suffixed with an “#egg=packagename-version” fragment are considered an installation candidate.
  4. Additionally, the rel=homepage and rel=download links are crawled by installation tools and, if HTML, are themselves scraped for release-file links in the above formats.

See the easy_install documentation for a complete description of this behavior. [1]

Today, most packages indexed on PyPI host their release files on PyPI. Out of 29,117 total projects on PyPI, only 2,581 (less than 10%) include any links to installable files that are available only off-PyPI. [2]

There are many reasons [3] why people have chosen external hosting. To cite just a few:

  • release processes and scripts have been developed already and upload to external sites
  • it takes too long to upload large files from some places in the world
  • export restrictions e.g. for crypto-related software
  • company policies which require offering open source packages through own sites
  • problems with integrating uploading to PyPI into one’s release process (because of release policies)
  • desiring download statistics different from those maintained by PyPI
  • perceived bad reliability of PyPI
  • not aware that PyPI offers file-hosting

Irrespective of the present-day validity of these reasons, there clearly is a history why people choose to host files externally and it even was for some time the only way you could do things. This PEP takes the position that there remain some valid reasons for external hosting even today.


Today, python package installers (pip, easy_install, buildout, and others) often need to query many non-PyPI URLs even if there are no externally hosted files. Apart from querying’s simple index pages, also all homepages and download pages ever specified with any release of a package are crawled by an installer. The need for installers to crawl external sites slows down installation and makes for a brittle and unreliable installation process. Those sites and packages also don’t take part in the PEP 381 mirroring infrastructure, further decreasing reliability and speed of automated installation processes around the world.

Most packages are hosted directly on [2]. Even for these packages, installers still crawl their homepage and download-url, if specified. Many package uploaders are not aware that specifying the “homepage” or “download-url” in their package metadata will needlessly slow down the installation process for all users.

Relying on third party sites also opens up more attack vectors for injecting malicious packages into sites using automated installs. A simple attack might just involve getting hold of an old now-unused homepage domain and placing malicious packages there. Moreover, performing a Man-in-The-Middle (MITM) attack between an installation site and any of the download sites can inject malicious packages on the installation site. As many homepages and download locations are using HTTP and not HTTPS, such attacks are not hard to launch. Such MITM attacks can easily happen even for packages which never intended to host files externally as their homepages are contacted by installers anyway.

There is currently no way for package maintainers to avoid external-link crawling, other than removing all homepage/download url metadata for all historic releases. While a script [4] has been written to perform this action, it is not a good general solution because it removes useful metadata from PyPI releases.

Even if the sites referenced by “Homepage” and “Download-URL” links were not scraped for further links, there is no obvious way under the current system for a package owner to link to an installable file from a long_description metadata field (which is shown as package documentation on /pypi/PKG) without installation tools automatically considering that file a candidate for installation. Conversely, there is no way to explicitly register multiple external release files without putting them in metadata fields.


These are the goals to be achieved by implementation of this PEP:

  • Package owners should be able to explicitly control which files are presented by PyPI to installer tools as installation candidates. Installation should not be slowed and made less reliable by extensive and unnecessary crawling of links that package owners did not explicitly nominate as installation files.
  • It should remain possible for package owners to choose to host their release files on their own hosting, external to PyPI. It should be easy for a user to request the installation of such releases using automated installer tools, especially if the external release files were registered together with a checksum hash.
  • Automated installer tools should not install externally-hosted packages by default, but require explicit authorization to do so by the user. When tools refuse to install such a package by default, they should tell the user exactly which external link(s) the installer needs to follow, and what option(s) the user can provide to authorize the tool to follow those links. PyPI should provide all necessary metadata for installer tools to implement this easily and within a single request/reply interaction.
  • Migration from the status quo to the above points should be gradual and minimize breakage. This includes tooling that makes it easy for package owners with an existing release process that uploads to non-PyPI hosting to also upload those release files to PyPI.

Solution / two transition phases

The first transition phase introduces a “hosting-mode” field for each project on PyPI, allowing package owners explicit control of which release file links are served to present-day installation tools in the machine-readable simple/ index. The first transition will, after successful hosting-mode manipulations by individual early-adopters, set a default hosting mode for existing packages, based on automated analysis. Maintainers will be notified one month ahead of any such automated change. At completion of the first transition phase, all present-day existing release and installation processes and tools are expected to continue working. Any remaining errors or problems are expected to only relate to installation of individual packages and can be easily corrected by package maintainers or PyPI admins if maintainers are not reachable.

Also in the first phase, each link served in the simple/ index will be explicitly marked as rel="internal" if it is hosted by the index itself (even if on a separate domain, which may be the case if the index uses a CDN for file-serving). Any link not so marked will be considered an external link.

In the second transition phase, PyPI client installation tools shall be updated to default to only install rel="internal" packages unless a user specifies option(s) to permit installing from external links. See second transition phase for details on how installers should behave.

Maintainers of packages which currently host release files on non-PyPI sites shall receive instructions and tools to ease “re-hosting” of their historic and future package release files. This re-hosting tool MUST be available before automated hosting-mode changes are announced to package maintainers.


Hosting modes

The foundation of the first transition phase is the introduction of three “modes” of PyPI hosting for a package, affecting which links are generated for the simple/ index. These modes are implemented without requiring changes to installation tools via changes to the algorithm for generating the machine-readable simple/ index.

The modes are:

  • pypi-scrape-crawl: no change from the current situation of generating machine-readable links for installation tools, as outlined in the history.
  • pypi-scrape: for a package in this mode, links to be added to the simple/ index are still scraped from package metadata. However, the “Home-page” and “Download-url” links are given rel=ext-homepage and rel=ext-download attributes instead of rel=homepage and rel=download. The effect of this (with no change in installation tools necessary) is that these links will not be followed and scraped for further candidate links by present-day installation tools: only installable files directly hosted from PyPI or linked directly from PyPI metadata will be considered for installation. Installation tools MAY evolve to offer an option to use the new rel-attribution to crawl external pages but MUST NOT default to it.
  • pypi-explicit: for a package in this mode, only links to release files uploaded to PyPI, and external links to release files explicitly nominated by the package owner, will be added to the simple/ index. PyPI will provide a new interface for package owners to supply external release-file URLs. These URLs MUST include a URL fragment in the form “#hashtype=hashvalue” specifying a hash of the externally-linked file which installer tools MUST use to validate that they have downloaded the intended file.

Thus the hope is that eventually all projects on PyPI can be migrated to the pypi-explicit mode, while preserving the ability to install release files hosted externally via installer tools. Deprecation of hosting modes to eventually only allow the pypi-explicit mode is NOT REGULATED by this PEP but is expected to become feasible some time after successful implementation of the transition phases described in this PEP. It is expected that deprecation requires a new process to deal with abandoned packages because of unreachable maintainers for still popular packages.

First transition phase (PyPI)

The proposed solution consists of multiple implementation and communication steps:

  1. Implement in PyPI the three modes described above, with an interface for package owners to select the mode for each package and register explicit external file URLs.
  2. For packages in all modes, label links in the simple/ index to index-hosted files with rel="internal", to make it easier for client tools to distinguish these links in the second phase.
  3. Add an HTML tag <meta name="api-version" value="2"> to all simple/ index pages, to allow clients to distinguish between indexes providing the rel="internal" metadata and older ones that do not.
  4. Default all newly-registered packages to pypi-explicit mode (package owners can still switch to the other modes as desired).
  5. Determine (via automated analysis [2]) which packages have all installable files available on PyPI itself (group A), which have all installable files on PyPI or linked directly from PyPI metadata (group B), and which have installable versions available that are linked only from external homepage/download HTML pages (group C).
  6. Send mail to maintainers of projects in group A that their project will be automatically configured to pypi-explicit mode in one month, and similarly to maintainers of projects in group B that their project will be automatically configured to pypi-scrape mode. Inform them that this change is not expected to affect installability of their project at all, but will result in faster and safer installs for their users. Encourage them to set this mode themselves sooner to benefit their users.
  7. Send mail to maintainers of packages in group C that their package hosting mode is pypi-scrape-crawl, list the URLs which currently are crawled, and suggest that they either re-host their packages directly on PyPI and switch to pypi-explicit, or at least provide direct links to release files in PyPI metadata and switch to pypi-scrape. Provide instructions and tools to help with these transitions.

Second transition phase (installer tools)

For the second transition phase, maintainers of installation tools are asked to release two updates.

The first update shall provide clear warnings if externally-hosted release files (that is, files whose link does not include rel="internal") are selected for download, for which projects and URLs exactly this happens, and warn that in future versions externally-hosted downloads will be disabled by default.

The second update should change the default mode to allow only installation of rel="internal" package files, and allow installation of externally-hosted packages only when the user supplies an option.

The installer should distinguish between verifiable and non-verifiable external links. A verifiable external link is a direct link to an installable file from the PyPI simple/ index that includes a hash in the URL fragment (“#hashtype=hashvalue”) which can be used to verify the integrity of the downloaded file. A non-verifiable external link is any link (other than those explicitly supplied by the user of an installer tool) without a hash, scraped from external HTML, or injected into the search via some other non-PyPI source (e.g. setuptools’ dependency_links feature).

Installers should provide a blanket option to allow installing any verifiable external link. Non-verifiable external links should only be installed if the user-provided option specifies exactly which external domains can be used or for which specific package names external links can be used.

When download of an externally-hosted package is disallowed by the default configuration, the user should be notified, with instructions for how to make the install succeed and warnings about the implication (that a file will be downloaded from a site that is not part of the package index). The warning given for non-verifiable links should clearly state that the installer cannot verify the integrity of the downloaded file. The warning given for verifiable external links should simply note that the file will be downloaded from an external URL, but that the file integrity can be verified by checksum.

Alternative PyPI-compatible index implementations should upgrade to begin providing the rel="internal" metadata and the <meta name="api-version" value="2"> tag as soon as possible. For alternative indexes which do not yet provide the meta tag in their simple/ pages, installation tools should provide backwards-compatible fallback behavior (treat links as internal as in pre-PEP times and provide a warning).

API For Submitting External Distribution URLs

New distribution URLs may be submitted by performing a HTTP POST to the URL:

With the following form-encoded data:

Name Value
:action The string “urls”
name The package name as a string
version The release version as a string
new-url The new URL to store
submit_new_url The string “yes”

The POST must be accompanied by an HTTP Basic Auth header encoding the username and password of the user authorized to maintain the package on PyPI.

The HTTP response to this request will be one of:

Code Meaning URL submission implications
200 OK Everything worked just fine
400 Bad request Data provided for submission was malformed
401 Unauthorised The username or password supplied were incorrect
403 Forbidden User does not have permission to update the package information (not Owner or Maintainer)



Phillip Eby for precise information and the basic ideas to implement the transition via server-side changes only.

Donald Stufft for pushing away from external hosting and offering to implement both a Pull Request for the necessary PyPI changes and the analysis tool to drive the transition phase 1.

Marc-Andre Lemburg, Alyssa Coghlan and catalog-sig in general for thinking through issues regarding getting rid of “external hosting”.


Last modified: 2023-10-11 12:05:51 GMT