pyproject.toml uses the incorrect "legacy" setuptools backend and unnecessary wheel dep

[Michał Górny]

The pyproject.toml file lists the following:

[build-system]
requires = ['setuptools>=40.8.0', 'wheel']
build-backend = 'setuptools.build_meta:__legacy__'

The wheel dependency is redundant and discouraged here. Setuptools adds this dependency via the backend automatically since day one. It was historically included in the documentation but it was a mistake. See: ​https://github.com/pypa/setuptools/commit/f7d30a9529378cf69054b5176249e5457aaf640a

The legacy backend was never supposed to be used in pyproject.toml. It is only an "internal" fallback that is used by tools like pip when pyproject.toml is not present at all. The regular backend must always be used in pyproject.toml. See: ​https://github.com/pypa/setuptools/issues/1689

[Mariusz Felisiak]

Thanks for the report.

[Florian Apolloner]

The wheel dependency is redundant and discouraged here. Setuptools adds this dependency via the backend automatically since day one. It was historically included in the documentation but it was a mistake. See: ​​https://github.com/pypa/setuptools/commit/f7d30a9529378cf69054b5176249e5457aaf640a

Mhm, I can remember that some frontends did need it, not 100% sure though nowadays

The legacy backend was never supposed to be used in pyproject.toml. It is only an "internal" fallback that is used by tools like pip when pyproject.toml is not present at all. The regular backend must always be used in pyproject.toml. See: ​​https://github.com/pypa/setuptools/issues/1689

This is a little bit more tricky. The problem here is that the presence of pyproject.toml changes behavior. Hence our fix was using the __legacy__ backend. Most importantly pip install -e wouldn't work otherwise.

[Carlton Gibson]

​Not explicitly declaring this was raised and rejected on the original PR. Unless something has changed this would be wontfix? 🤔

[Michał Górny]

Well, unless I'm testing wrong, after changing pyproject.toml to:

[build-system]
requires = ['setuptools>=40.8.0']
build-backend = 'setuptools.build_meta'

[tool.black]
target-version = ['py38']
extend-exclude = 'tests/test_runner_apps/tagged/tests_syntax_error.py'

editable install just works™:

$ pip install -e .
Obtaining file:///tmp/django
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build wheel ... done
  Installing backend dependencies ... done
  Preparing metadata (pyproject.toml) ... done
Collecting sqlparse>=0.2.2
  Using cached sqlparse-0.4.2-py3-none-any.whl (42 kB)
Collecting asgiref<4,>=3.4.1
  Using cached asgiref-3.5.2-py3-none-any.whl (22 kB)
Installing collected packages: sqlparse, asgiref, Django
  Running setup.py develop for Django
Successfully installed Django-4.0.6.dev20220613071132 asgiref-3.5.2 sqlparse-0.4.2

This is with pip-22.0.4 (as injected into venv by Python).

Note that ​PEP 660 explicitly provisions for editable installs.

[Florian Apolloner]

Hi, it is great to hear that this works nowadays. What happens with an older pip like on ubuntu 18.04 or 20.04? Though I guess it would be okay to say that editable installs are not supported there.

[Michał Górny]

I've tested a bunch of old pip version with these changes (and the hack from setup.py removed) and FWICS:

• pip 10.0.1 (Apr 2018) complains about the missing wheel dependency because it "does not implement PEP 517 so it cannot build a wheel without 'setuptools' and 'wheel'" but I don't think that's very relevant
• I was able to pip install -e . correctly with all versions down to 9.0 (Nov 2016); older versions seem to be broken with Python 3.9

Just in case, I've also tried downgrading setuptools and managed to get pip install -e . working down to setuptools 41.1.0 (Aug 2019); older versions seem to be broken with Python 3.9.

[Rui]

Replying to Michał Górny:
hello，I'm new to package program, what means to used incorrect legacy backend?how to fix it? Besides,just need to remove duplicated 'wheel' requires?

The pyproject.toml file lists the following:

[build-system]
requires = ['setuptools>=40.8.0', 'wheel']
build-backend = 'setuptools.build_meta:__legacy__'

The wheel dependency is redundant and discouraged here. Setuptools adds this dependency via the backend automatically since day one. It was historically included in the documentation but it was a mistake. See: ​https://github.com/pypa/setuptools/commit/f7d30a9529378cf69054b5176249e5457aaf640a

The legacy backend was never supposed to be used in pyproject.toml. It is only an "internal" fallback that is used by tools like pip when pyproject.toml is not present at all. The regular backend must always be used in pyproject.toml. See: ​https://github.com/pypa/setuptools/issues/1689

[Florian Apolloner]

Replying to Michał Górny:

I've tested a bunch of old pip version with these changes (and the hack from setup.py removed) and FWICS …

Ok, that sounds good, I guess we can really change that for the main branch then. After all we require rather new pythons nowadays :)

[Adam Johnson]

Thank you for reporting this, I've updated ​my maintained packages as a result.

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 247e2589:

Fixed #33778 -- Updated build-system configuration.

This removes unnecessary wheel dependency and legacy fallback.