Per-command dependencies

[Kentzo]

It's not unusual for a setuptools command to have dependencies. The most common example is tests_require, but there are other applications. Often this is resolved by introspecting name of the command at runtime and constructing list for setup_requires accordingly.

With installation requirements there is more or less standard practice of having common dependencies expressed via install_requires and optional via extras_require.

I propose the similar hierarchy for setuptools commands:

• setup_requires should be used for common dependencies
• setup_extra_requires should be used similarly to extra_requires where keys are commands like test, install, or any other string allowed as a command name

Backward compatibility

• tests_require should be an alias to setup_extra_requires['test']; if both are specified error should be issued
• install_requires is independent from setup_extra_requires['install'] because it expresses package requirements, not setuptools command execution requirements

[Kentzo]

Given that PEP-518 was accepted, can (should?) this feature be expressed via pyproject.toml?

[jaraco]

Yes, this issue is obsolete, and setuptools has embarked on the path of discouraging commands altogether, and has deprecated setup_requires/tests_require (#2823).