Should setuptools be in the setup_requires entry of setup.cfg files?

The importlib_resources backport for Python < 3.7 of the importlib.resources standard library module has the following section in the setup.cfg file:

[options]
python_requires = >=2.7,!=3.0,!=3.1,!=3.2,!=3.3
setup_requires =
    setuptools
    wheel
install_requires =
    pathlib2; python_version < '3'
    typing; python_version < '3.5'
packages = find:

Why does setup_requires include setuptools? This does not seem to make sense since:

the first line of the setup.py file imports setuptools, so by the time the setup function is called and reads the setup.cfg file that instructs to install setuptools it is already too late to install setuptools:
```
from setuptools import setup
setup()
```

setuptools is already installed on any fresh Python installation (well, only tested on Windows 10 and MacOS 10.15 with Python 3.8.0):

$ python -V
Python 3.8.0
$ pip list
Package    Version
---------- -------
pip        19.2.3
setuptools 41.2.0
WARNING: You are using pip version 19.2.3, however version 19.3.1 is available.
You should consider upgrading via the 'python -m pip install --upgrade pip' command.

Solution

No, setuptools should not be included in setup_requires, according to PEP 518 (bold emphasis mine):

Setuptools tried to solve this with a setup_requires argument to its setup() function [3]. This solution has a number of issues, such as:

No tooling (besides setuptools itself) can access this information without executing the setup.py, but setup.py can't be executed without having these items installed.

While setuptools itself will install anything listed in this, they won't be installed until during the execution of the setup() function, which means that the only way to actually use anything added here is through increasingly complex machinations that delay the import and usage of these modules until later on in the execution of the setup() function.

This cannot include setuptools itself nor can it include a replacement to setuptools, which means that projects such as numpy.distutils are largely incapable of utilizing it and projects cannot take advantage of newer setuptools features until their users naturally upgrade the version of setuptools to a newer one.

The items listed in setup_requires get implicitly installed whenever you execute the setup.py but one of the common ways that the setup.py is executed is via another tool, such as pip, who is already managing dependencies. This means that a command like pip install spam might end up having both pip and setuptools downloading and installing packages and end users needing to configure both tools (and for setuptools without being in control of the invocation) to change settings like which repository it installs from. It also means that users need to be aware of the discovery rules for both tools, as one may support different package formats or determine the latest version differently.