Wheel violates PEP517 Build requirements

[jameshilliard]

Specifically the build requirements state:

Project build requirements will define a directed graph of requirements (project A needs B to build, B needs C and D, etc.) This graph MUST NOT contain cycles. If (due to lack of co-ordination between projects, for example) a cycle is present, front ends MAY refuse to build the project.

However front ends MAY have modes where they do not consider wheels when locating build requirements, and so projects MUST NOT assume that publishing wheels is sufficient to break a requirement cycle.

This specific PEP517 Build requirements violation can be identified by the dependency cycle validator in build that I'm working on.

This flags the problem:

ERROR Failed to validate `build-system` in pyproject.toml, dependency cycle detected: `wheel` -> `setuptools.build_meta` -> `wheel`

[agronholm]

Does this mean wheel cannot ever have a standard pyproject.toml file?

[pfmoore]

In theory, it probably means that either wheel or setuptools should somehow use the in-tree backend mechanism in PEP 517 to break the loop.

In practice, though, people seem mostly happy with just providing wheels to break the build loop - the people who were focused on building everything from source (which was the motivating case for that feature) don't seem to have followed up by pushing build backends to self-host. Flit has a bootstrapping process that doesn't use that mechanism, I believe.

[layday]

Flit will be bundling tomli to break the loop from next version.

[agronholm]

But setuptools already uses the in-tree backend mechanism, here? Just what is wheel expected to do here?

[agronholm]

I've backed out of PEP 517 build metadata in pyproject.toml until a solution can be devised. I've left the tooling configuration there as it shouldn't pose a problem.

[layday]

Per PEP 518 a pyproject.toml without a build-system table is assumed to use setuptools and wheel. In pip, the presence of pyproject.toml is sufficient to trigger a PEP 517 build with the PEP 518 defaults.

[layday]

$ python -m pip list
Package    Version
---------- -------
pip        21.3.1
setuptools 60.1.1
$ python -m pip -v install --no-build-isolation https://github.com/pypa/wheel/archive/refs/heads/main.zip
Using pip 21.3.1 from /[...]/venv/lib/python3.10/site-packages/pip (python 3.10)
Collecting https://github.com/pypa/wheel/archive/refs/heads/main.zip
  Using cached https://github.com/pypa/wheel/archive/refs/heads/main.zip
  Running command /[...]/venv/bin/python /[...]/venv/lib/python3.10/site-packages/pip/_vendor/pep517/in_process/_in_process.py prepare_metadata_for_build_wheel /tmp/tmpgwsjdnm5
  running dist_info
  creating /private/tmp/pip-modern-metadata-ifnp9e4_/wheel.egg-info
  writing manifest file '/private/tmp/pip-modern-metadata-ifnp9e4_/wheel.egg-info/SOURCES.txt'
  warning: no files found matching '*.dynlib' under directory 'tests'
  no previously-included directories found matching 'tests/testdata/*/build'
  no previously-included directories found matching 'tests/testdata/*/dist'
  no previously-included directories found matching 'tests/testdata/*/*.egg-info'
  writing manifest file '/private/tmp/pip-modern-metadata-ifnp9e4_/wheel.egg-info/SOURCES.txt'
  error: invalid command 'bdist_wheel'
  Preparing metadata (pyproject.toml) ... error
WARNING: Discarding https://github.com/pypa/wheel/archive/refs/heads/main.zip. Command errored out with exit status 1: /[...]/venv/bin/python /[...]/venv/lib/python3.10/site-packages/pip/_vendor/pep517/in_process/_in_process.py prepare_metadata_for_build_wheel /tmp/tmpgwsjdnm5 Check the logs for full command output.
ERROR: Command errored out with exit status 1: /[...]/venv/bin/python /[...]/venv/lib/python3.10/site-packages/pip/_vendor/pep517/in_process/_in_process.py prepare_metadata_for_build_wheel /tmp/tmpgwsjdnm5 Check the logs for full command output.

Installing with isolation currently works because there's no published release with pyproject.toml but expect no binary installs to break.

[agronholm]

Per PEP 518 a pyproject.toml without a build-system table is assumed to use setuptools and wheel. In pip, the presence of pyproject.toml is sufficient to trigger a PEP 517 build with the PEP 518 defaults.

Wasn't that bug fixed quite a while ago?

[layday]

It's not a bug, it's intentional.

[agronholm]

So what you're saying that I cannot make a release before doing away with pyproject.toml entirely?

[layday]

You don't have to delete it I suppose so long as you exclude it from the sdist (and nobody tries to pip install from git).

[agronholm]

To play it safe, I've gone and removed it entirely. I hope I can put it back one of these days. Right now I just don't see how.