Revisiting `tox`'s requirement that `build-system` is defined in `pyproject.toml`

[di]

Currently tox requires that a build-system table is defined in pyproject.toml:

$ echo -e '[tox]\nisolated_build = true' > tox.ini

$ touch pyproject.toml

$ python -m tox -e py
ERROR: build-system section missing inside /tmp/sampleproject/pyproject.toml

This was previously discussed in #1482 by @nuno-andre and eventually closed, but I think the intention there was generally correct: PEP 518 is explicit that the build-system table is not required:

Tools should not require the existence of the [build-system] table... If the file exists but is lacking the [build-system] table then the default values as specified above should be used.

Furthermore, most other tools do not have this requirement. For example, build supports omitting the build-system table -- when building with the above example:

$ python -m build
* Creating virtualenv isolated environment...
* Installing packages in isolated environment... (setuptools >= 40.8.0, wheel)
* Getting dependencies for sdist...
running egg_info
creating UNKNOWN.egg-info
writing UNKNOWN.egg-info/PKG-INFO
writing dependency_links to UNKNOWN.egg-info/dependency_links.txt
writing top-level names to UNKNOWN.egg-info/top_level.txt
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
reading manifest file 'UNKNOWN.egg-info/SOURCES.txt'
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
* Building sdist...
running sdist
running egg_info
writing UNKNOWN.egg-info/PKG-INFO
writing dependency_links to UNKNOWN.egg-info/dependency_links.txt
writing top-level names to UNKNOWN.egg-info/top_level.txt
reading manifest file 'UNKNOWN.egg-info/SOURCES.txt'
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
warning: sdist: standard file not found: should have one of README, README.rst, README.txt, README.md

running check
warning: check: missing required meta-data: name

creating UNKNOWN-0.0.0
creating UNKNOWN-0.0.0/UNKNOWN.egg-info
copying files to UNKNOWN-0.0.0...
copying pyproject.toml -> UNKNOWN-0.0.0
copying UNKNOWN.egg-info/PKG-INFO -> UNKNOWN-0.0.0/UNKNOWN.egg-info
copying UNKNOWN.egg-info/SOURCES.txt -> UNKNOWN-0.0.0/UNKNOWN.egg-info
copying UNKNOWN.egg-info/dependency_links.txt -> UNKNOWN-0.0.0/UNKNOWN.egg-info
copying UNKNOWN.egg-info/top_level.txt -> UNKNOWN-0.0.0/UNKNOWN.egg-info
Writing UNKNOWN-0.0.0/setup.cfg
Creating tar archive
removing 'UNKNOWN-0.0.0' (and everything under it)
* Building wheel from sdist
* Creating virtualenv isolated environment...
* Installing packages in isolated environment... (setuptools >= 40.8.0, wheel)
* Getting dependencies for wheel...
running egg_info
writing UNKNOWN.egg-info/PKG-INFO
writing dependency_links to UNKNOWN.egg-info/dependency_links.txt
writing top-level names to UNKNOWN.egg-info/top_level.txt
reading manifest file 'UNKNOWN.egg-info/SOURCES.txt'
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
* Installing packages in isolated environment... (wheel)
* Building wheel...
running bdist_wheel
running build
installing to build/bdist.linux-x86_64/wheel
running install
running install_egg_info
running egg_info
writing UNKNOWN.egg-info/PKG-INFO
writing dependency_links to UNKNOWN.egg-info/dependency_links.txt
writing top-level names to UNKNOWN.egg-info/top_level.txt
reading manifest file 'UNKNOWN.egg-info/SOURCES.txt'
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
Copying UNKNOWN.egg-info to build/bdist.linux-x86_64/wheel/UNKNOWN-0.0.0-py3.9.egg-info
running install_scripts
creating build/bdist.linux-x86_64/wheel/UNKNOWN-0.0.0.dist-info/WHEEL
creating '/tmp/sampleproject/dist/tmpkm28yh9l/UNKNOWN-0.0.0-py3-none-any.whl' and adding 'build/bdist.linux-x86_64/wheel' to it
adding 'UNKNOWN-0.0.0.dist-info/METADATA'
adding 'UNKNOWN-0.0.0.dist-info/WHEEL'
adding 'UNKNOWN-0.0.0.dist-info/top_level.txt'
adding 'UNKNOWN-0.0.0.dist-info/RECORD'
removing build/bdist.linux-x86_64/wheel
Successfully built UNKNOWN-0.0.0.tar.gz and UNKNOWN-0.0.0-py3-none-any.whl

Additionally, pip does support omitting it as well:

$ pip install .
Defaulting to user installation because normal site-packages is not writeable
Processing /tmp/sampleproject
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Preparing metadata (pyproject.toml) ... done
Building wheels for collected packages: UNKNOWN
  Building wheel for UNKNOWN (pyproject.toml) ... done
  Created wheel for UNKNOWN: filename=UNKNOWN-0.0.0-py3-none-any.whl size=920 sha256=df0aa573f73a6d411f548e7064cf1566e4121560cca4e4d4475bd4705aad4da1
  Stored in directory: /tmp/pip-ephem-wheel-cache-4yph8vy7/wheels/43/b3/41/c546307af7091a98924f94fc7774884cc49fc15f0211c49065
Successfully built UNKNOWN
Installing collected packages: UNKNOWN
Successfully installed UNKNOWN-0.0.0

I think tox should do what these tools do and provide defaults when the build-system table is omitted, rather than erroring.

[gaborbernat]

This has been fixed in tox v4. If anyone wants to fix it in v3 they're invited to put in a PR for it.

[gaborbernat]

Furthermore, most other tools do not have this requirement. For example, build supports omitting the build-system table -- when building with the above example:

Additionally, pip does support omitting it as well:

Arguably those tools support it, however, it's still not considered best practice to use the default. You know, explicit better than implicit? Considering tox is encouraging users to follow best practices one can make the argument that tox shouldn't support defaulting...

[di]

This has been fixed in tox v4.

Thanks, I forgot to check against pre-releases! Feel free to close this as that's sufficient for me. (As an aside, is there an issue I can track for the tox4 release? Or is https://github.com/tox-dev/tox/milestone/7 what I should use?)

Arguably those tools support it, however, it's still not considered best practice to use the default. You know, explicit better than implicit? Considering tox is encouraging users to follow best practices one can make the argument that tox shouldn't support defaulting...

I think it's fine for tox to make additional recommendations on declaring default fields, but my original issue was just with the error when it's not present, which is explicitly prohibited by the PEP 518 spec 😉

[gaborbernat]

Version 4 is now the main one, so closing this.