DEPS: clarifying setuptools's status as a dependency

[fangchenli]

In install.rst, we list setuptools as a runtime dependency. If that is the case, we don't need to vendor packaging #41199 since it's vendored by pkg_resources, which is part of setuptools. And we need to update setup.cfg and README.md to reflect this.

If setuptools is not a runtime dependency, vendoring packaging is necessary. And, we need to list it as a test and optional dependency because we are using pkg_resources in pandas/plotting/_core.py and pandas/tests/plotting/test_backend.py #41503. An alternative of pkg_resources for accessing entry points is importlib.metadata, which is only available after Python 3.8. And importlib.metadata is the suggested way to do this according to https://setuptools.readthedocs.io/en/latest/pkg_resources.html.

[jreback]

setuptools is needed and we should list it

[jorisvandenbossche]

Setuptools (pkg_resources) is not a required dependency, only an optional one when using plotting backends (it's not even needed when using the default matplotlib backend).
So I don't think we should list it as required dependency.

An alternative of pkg_resources for accessing entry points is importlib.metadata, which is only available after Python 3.8. And importlib.metadata is the suggested way to do this according to https://setuptools.readthedocs.io/en/latest/pkg_resources.html.

Somewhat a separate issue from listing setuptools as dependency or not, but I think we should indeed look into replacing pkg_resources (probably easier to do once we only support Python 3.8+ #41678)

[simonjayhawkins]

Setuptools (pkg_resources) is not a required dependency, only an optional one when using plotting backends (it's not even needed when using the default matplotlib backend).
So I don't think we should list it as required dependency.

An alternative of pkg_resources for accessing entry points is importlib.metadata, which is only available after Python 3.8. And importlib.metadata is the suggested way to do this according to https://setuptools.readthedocs.io/en/latest/pkg_resources.html.

Somewhat a separate issue from listing setuptools as dependency or not, but I think we should indeed look into replacing pkg_resources (probably easier to do once we only support Python 3.8+ #41678)

maybe open a new issue and we can close this one now that #41818 is merged.

[jreback]

setup tools is an undeclared dependency now

it should be an official runtime one

[jorisvandenbossche]

There is a difference between "optional" and "required" dependencies. We have lots of optional dependencies that are not declared in install_requires. And in practice, setuptools always has been an optional dependency (not required for importing pandas / normal usage, only required when actually using one of the non-default plotting backends).

It's true that we don't provide a proper error message about "pkg_resources" (setuptools) being required for using a plotting backend (as we do for other optional dependencies), but give a plain import error instead. This certainly gives the impression we assume the library is always present, but giving a more informative import error message is also something we can easily fix.

[simonjayhawkins]

removing the blocker label since #42006 fixed the import issue.

[simonjayhawkins]

but giving a more informative import error message is also something we can easily fix.

is that now the only outstanding actionable item here? moving to 1.3.1

[jreback]

actually i think prob ok for 1.3.x to close this (as #42410) is going to remove the need for this in 3.8

[jreback]

we merged the update for 1.4 to remove the need for setuptools entirely, so not merging this to 1.3.x unless strong desire (as efffectively by not merging we are not changing 1.3 vs 1.2 at all w.r.t. requirements).

[jorisvandenbossche]

Sounds good!