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Expectations that projects provide ever more wheels

Most users expect pip install project-name to work on all platforms. When building from source is difficult, that expectation translates into requests to projects to provide more wheels. Building many wheels can result in a lot of maintenance work and load on CI systems.

Current state

See this summary for the combination of operating systems, Python interpreters, CPU architectures and libc flavors which are supported by wheel specs/tags. The cibuildwheel matrix adds up to 70 combinations - and that doesn't include multiple manylinux versions, universal2 for macOS, i686 (32-bit) Linux, or some architectures that were added in PEP 599 (armv7l, ppc64). Nor does it include tags that are likely to be added (WASM), or for which tags exist but aren't supported by PyPI (Pyston, AIX).

In practice, the less popular platforms are not supported by most projects and the number of wheels they upload to PyPI is in the 20-40 range. Examples:

That is still a large amount of wheels, and maintaining support for them is often problematic.

A key ingredient for building wheels is availability of CI systems which support the target platforms. Most projects use 3-4 different CI systems, and/or make use of cross-compilation when a target platform isn't natively supported (e.g., macOS arm64 CI runners were unavailable for 2 years, and are still only available on Cirrus CI as of Dec 2022, and PowerPC (ppc64le) and IBM Z (s390x) are only available on Travis CI). cibuildwheel has lowered the effort to build wheels compared to multibuild, however the CI system requirements haven't changed. Each CI system requires maintenance - and self-hosted CI runners are even worse in this respect. That maintenance cost can be shared much more easily for packaging systems with centralized builds; for building wheels they have to be paid by every project (see Lack of a build farm for PyPI for more details).


The primary problem is the large amount of maintenance effort on CI systems for wheel building. This is particularly painful because:

  • there are usually one a couple of maintainers (perhaps even a single person) who are responsible for or have expertise in wheel building,
  • issues that show up are often specific to the platform and hence cannot be debugged locally on the maintainer's development machine,
  • there are a lot of "layers" to wade through: CI system, cibuildwheel, pip, build backend, backend, build system.

Another issue is that a lot of discussion needed each time there is a request to any project to support a new platform.



Relevant resources

Links to key issues, forum discussions, PEPs, blog posts, etc.

Related to platform support:

Potential solutions or mitigations

  • Reducing the number of wheels needed. HPy is probably the most promising way of achieving this.
  • Defining a "supported platforms" policy that can be used across projects to make and document decisions around this topic.
  • A build farm for PyPI (see this meta topic).
  • Having PyPI integrate better with other packaging systems, so users can obtain pure Python packages from PyPI in combination with hard-to-build packages from a package manager with support for the platform of interest.
  • Manage user expectations better, and be honest about limitations of PyPI/wheels. Users are often better served by a distribution that is built and tested in a consistent fashion than by downloading a standalone interpreter and then installing packages from PyPI. This is often not even presented on install pages as an option, let alone recommended.

Last update: December 20, 2022
Created: December 20, 2022