Expose pip's cache dir

[woodruffw]

pypa/pip-audit#75 tries to re-use the pip cache whenever possible.

The pip cache's location is normally system defined (but consistent), but it can probably be overridden.

Proposed API: pip_api.cache_dir() -> Path, corresponding to the pip cache dir command.

[di]

A reminder that the goal of this library is to providing a drop-in replacement for existing uses of pip's internal API by wrapping command-line calls to pip, so any new API added here needs to have the equivalent:

1. accessible from some internal pip API (or combination of internal APIs)
2. accessible from some command-line calls (or combination of command-line calls)

This one's somewhat complicated: if I wanted to get the cache dir from pip's internal API, I'd need to do something like:

>>> from pip._internal.cli.base_command import Command
>>> c = Command('throwaway', 'throwaway')
>>> options, args = c.parse_args([])
>>> options.cache_dir
'/root/.cache/pip'

So it seems like an API to get all the options available would be preferable, but there's no CLI to get this value as far as I can tell.

We might want to think about a more general API that provides various default configuration values like cache_dir, and combines multiple CLI commands like pip cache dir instead.

[woodruffw]

So it seems like an API to get all the options available would be preferable, but there's no CLI to get this value as far as I can tell.

Does pip cache dir not work for you? It does for me, on macOS:

work:~ william$ pip cache dir
/Users/william/Library/Caches/pip

Edit: It looks like pip cache dir was added in 20.1, so that explains why it might be absent by default with a distro-provided pip.

[di]

So it seems like an API to get all the options available would be preferable, but there's no CLI to get this value as far as I can tell.

Emphasis on "all the options", I think an API to just get the cache directory is probably too fine-grained here given how many options there are.

[woodruffw]

Emphasis on "all the options", I think an API to just get the cache directory is probably too fine-grained here given how many options there are.

Gotcha, I understand now! Yeah, there's nothing that does all config options via the pip CLI that I'm aware of.

[woodruffw]

(There's pip config list, but AFAICT it doesn't dump any default settings that aren't explicitly overridden.)