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Metadata handling on PyPI

Metadata for sdists and wheels on PyPI is contained in those sdists and wheels themselves, in a form as prescribed by the Core metadata specifications. That metadata format is typically produced by build backends. The source of that metadata is information that package authors maintain within their project. It historically lived in, and may still live there today - however it now preferably should be specified in pyproject.toml (see PEP 621 - Storing project metadata in pyproject.toml). Specifying that metadata is relatively straightforward, and is mostly in decent shape (even though there are some loose ends1) - with the exception of native dependencies, as discussed on this key issue page.

Can package metadata be queried for PyPI?

From this Discourse thread (Dec 2022):

dist-info-metadata is available in the simple API for wheels (and sdists if they follow PEP 643). But PyPI doesn’t expose that data (yet).

The PyPI JSON API includes metadata, but it is unreliable as it is at the project release level, so it doesn’t take into account the possibility of different wheels having different metadata. But it’s the nearest you can get right now.

See also the Project Metadata Table in BigQuery.

Similarly, metadata about the package can be queried. For example, provides an easy to use overview of projects including reverse dependencies. And there are a number of ways of querying package download statistics, see for example for a UI with quick numbers, and the Analyzing PyPI package downloads page of the Python Packaging User Guide which lists a number of tools to enable more in-depth analysis.

While specifying metadata for a package is relatively straightforward in most cases2, the same cannot be said for the workflows around dealing with problems in metadata.

Current state

By design, metadata in artifacts on PyPI is (a) immutable and (b) contained within the artifact itself rather than available separately. Both of those design aspects can be problematic.

Impact of immutable metadata

When a package author discovers an issue with their release or with a particular artifact in a release, there are good solutions. In the former case, a release can be yanked. In the latter case, a new artifact with a higher build number can be uploaded. This blog post by Brett Cannon (2021) explains what to do in those cases in detail. Where immutability becomes a problem is when the issue is in the metadata - in particular, when a build or runtime dependency changes something that breaks a package. It's a problem because:

  1. Doing a new release for complex packages with native code is very expensive. It may be days of full-time work (builds may have to run on multiple CI systems, and those configs tend to degrade pretty quickly for older releases), and therefore it may not be feasible to do on short notice.
  2. Having affected users deal with the situation by themselves is also very expensive. The most popular packages have hundreds of thousands or even millions of users, so even if only 1% of users3 are affected by a problem with a dependency, that is still an unacceptably large amount of work (and probably lots of complaints on the issue tracker).

(2) is often advocated for by Python packaging experts, in particular by having users apply post-hoc constraints through a constraints file. (2) is the worst solution though in the case of large-scale breakage, both because of the large numbers of users that each need to take action and because users are, more often than not, not developers. Instead, they're (data) scientists, engineers, business analysts and so on. They don't want to, and shouldn't need to, understand things like constraints files. If the metadata needs patching, the far better solution would be to patch them on PyPI. And this is not possible, because artifacts are immutable.

Depending on the situation, these are the most common ways that an issue with a dependency gets dealt with:

  • Bite the bullet and do a new release of the affected package,
  • Convince the authors of the dependency that broke things to unbreak them again (e.g., undo removal of a deprecated or private API),
  • Or even temporarily yank the dependency that broke things.

Because this kind of situation happens frequently, it may also be a good idea to add upper bounds on version specifiers of dependencies. No one likes upper bounds, because they result in incompatibilities and make dependency resolution more difficult. A lot of effort has been spent discussing the issues with upper bounds (e.g., see this blog post and this Discourse thread); package authors are caught between a rock and a hard place though - the problem is immutability of metadata.

On upper bounds - Matthias Bussonnier

I echo many sentiments here that 1) I hate that some projects have to put an upper bound [in their metadata], but 2) they do it because removing the upper bound is worse.

Experience with other package managers that are able to patch metadata shows that this is a much nicer experience. For example, conda-forge uses "repo data patching", while Spack and Nix build from source (with a binary cache providing many common build configs) and the Spack repo and Nixpgs repo contain metadata for all packages and can therefore be updated via a PR. As a result, upper bounds that are present on PyPI can typically be left out safely in these package managers; applying new constraints later is cheap.

Managing the necessary upper bounds itself is an exercise that may have to be repeated for each release, and takes time and effort. See for example this part of the SciPy developer guide.

Metadata contained within artifacts

Each wheel has its own metadata contained within the artifact. It can be different metadata than that for the sdist for which it came - and this is more likely to happen for packages with native code. Wheels for packages with native code also tend to be larger -from tens of MBs for the likes of NumPy, SciPy, Pandas and PyArrow to many hundreds of MBs for deep learning packages.

Downloading such large packages in order to access the metadata is clearly suboptimal. Especially if that metadata then shows a conflict and Pip has to backtrack. Also during debugging install issues this is a significant problem - when one wants to go through a number of wheels and compare differences with for example the METADATA or RECORD files, the current process is slow and bandwidth-intensive.

The solution seems obvious: make metadata separately accessible from wheels. Luckily, the solution for this is currently in progress:

PEP 568 - Serve Distribution Metadata in the Simple Repository API

PEP 568 (accepted) proposes to make the metadata file in the .dist-info directory of a wheel separately available. This should solve the problems identified in this section. Support is already implemented in pip. Implementation in PyPI is still pending, see warehouse#8254.

There are also issues around packages who don't yet use static metadata in pyproject.toml, and reliable metadata for sdists being only relatively recently available (PEP 643, Nov 2020). With dynamic metadata or usage, sdists have to be built in order to obtain the metadata. This is a general packaging issue however, not specific to packages with native code, and not nearly as much of a problem as the other issues discussed higher up. See, e.g., pip#1884 and this thread for details.


The most important problem is the need to add upper bounds on version specifications of dependencies.


TODO: add more history

Relevant resources


Potential solutions or mitigations

  • Making metadata editable. This would require a PEP and be a large effort.
  • The impact of issues with build dependencies, and hence the need to add upper bounds, would be much less if Pip did not install from sdist by default, as discussed in Unsuspecting users getting failing from-source builds.
  • Fix issues in installers. E.g., Poetry and PDM should not propagate upper bounds the way they currently do, as discussed in this thread. Pip also needs to continue reducing the amount of excessive backtracking, and use the separate metadata available soon with PEP 568 to reduce the impact of that backtracking. See Possible ways to reduce backtracking in the Pip docs for current mitigation options available to users.

  1. Example of a not unimportant loose end: PEP 639 - Improving License Clarity with Better Package Metadata is still in Draft status and not supported by PyPI as of Dec 2022. 

  2. See Example: Using the NumPy C API on this page for a case where getting the right metadata into a wheel is very difficult. 

  3. The number of users on platforms without wheel support on PyPI is on the order of 1%, and that is a set of users that is frequently affected by issues with build dependencies.