Present install_commands instead of only install_command

[jaraco]

I present a feature request, inspired by an environment where I wish to install both Python packages and node packages, similar to the request in StackOverflow 37187753.

I propose that instead of having a single command for the install step, tox could expose install_commands, which would be more congruous with the commands syntax. It could still accept {opts} and {packages} substitutions such that one (or more) of the commands could honor those options.

I suspect install_command was originally meant as a way to provide a bridge for other Python installers, but if the concept could be generalized to provide more functionality for install, much like CI environments do in their install steps. Then, if one wanted to add a node environment to their environment, they could do something like this:

install_commands = 
    pip install {opts} {packages}
    nodeenv --python-virtualenv --prebuilt

[jaraco]

Fixing this would probably address #765 and #786.

[mulawamichal]

tox 3.4 introduced commands_pre and commands_post. Maybe the install_command option should be kept as is, but instead install_commands_pre and install_commands_post should be added?

[jaraco]

That could work too, but does feel a little artificially constrained... that you can only have multiple install steps in pre or post, and that the sole install command must stand alone and special. -0

[mulawamichal]

So maybe they shall all accept multiple values.

[mulawamichal]

one way or another - things get rough when you have multiple requirement files/index servers. my case: some of the requirements are on one devpi server, some are on different one. So my tox.ini looks simillar to below:

[tox]
indexserver =
    default = https://some.devpi.server/
    the_other_one= https://some.other.devpi.server/
[testenv]
deps = -I
    --no-cache-dir
    -r{toxinidir}/requirements_one.txt
    :the_other_one:-r{toxinidir}/requirements_other.txt

install_command = {toxinidir}/my_custom_script.sh

and the my_custom_script.sh looks like this:

#!/bin/bash
set -e
pip install "$@" 
some-fancy-command-that-depends-on-the-requirements_other-file

this results in the my_custom_script.sh being invoked twice - once for each of the requirements file. And in such a form it of course fails, as after installing requirements from the "requirements-one" file the second command fails.

(TLDR: ) so to sum up - any custom actions that depend on already installed packages (but not instaling packages themselves) should be executed in a separate step - thats why it would be nice to have this install_commands_post feature :)