builds proceed with zero configuration

Hello, my understanding is that the build_meta:__legacy__ backend should behave like python setup.py ... calls.

However, I’ve realized that it proceeds fine without any setup.py / setup.cfg files:

$ mkdir empty
$ cd empty/
$ pip list
Package    Version
---------- -------
packaging  20.4
pip        20.2.2
pyparsing  2.4.7
setuptools 49.6.0   # also happens with 41.6.0 and probably anything in between
six        1.15.0
wheel      0.35.0
$ python
>>> from setuptools.build_meta import __legacy__
>>> __legacy__.build_wheel('.')
running bdist_wheel
running build
installing to build/bdist.linux-x86_64/wheel
running install
running install_egg_info
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'
Copying UNKNOWN.egg-info to build/bdist.linux-x86_64/wheel/UNKNOWN-0.0.0-py3.8.egg-info
running install_scripts
creating build/bdist.linux-x86_64/wheel/UNKNOWN-0.0.0.dist-info/WHEEL
creating '/home/churchyard/tmp/empty/tmpy86e5sx4/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
'UNKNOWN-0.0.0-py3-none-any.whl'
>>> ^D
[empty]$ ls
build  UNKNOWN-0.0.0-py3-none-any.whl  UNKNOWN.egg-info

This might be a problem in scripted environments. I would have expect an error here. Is this behavior deliberate or accidental?

[henryiii]

FWIW, name is required in PEP 621. Also, I think it’s fine to not specify name / version, etc, immediately, but by the time you are ready to make the output, if nothing has added it, it doesn’t seem to make sense to build without name and versions, since they are part of the output file name.

[pradyunsg]

Can the legacy backend enforce that a setup.py file must exist?

The language in PEP 517 seems to imply that the expectation is for the file to exist, in projects built with the legacy backend:

If the pyproject.toml file is absent, or the build-backend key is missing, the source tree is not using this specification, and tools should revert to the legacy behaviour of running setup.py (either directly, or by implicitly invoking the setuptools.build_meta:__legacy__ backend).