Adopting "working" scheme for every run

Just carrying over my idea in https://github.com/pypa/pip/issues/1056#issuecomment-218130183, for proper dedicated discussion.

AFAIK, there are 3 possible schemes for packages:

• “system” - for system/global packages (--system)
• “user” - for packages installed in user space (--user)
• “local” - for virtualenv packages (--local)

pip enforces a “working” scheme on every run. Outside a virtualenv, the default working scheme would be “system” (It should really be “user”, that’s another issue #1668). Inside a virtualenv, the default working scheme should be “local”. Passing --system or --user or --local overrides the working scheme.

Only packages in the same scheme as the working scheme can be modified. By modifying, I mean installing or uninstalling a package. Trying to modify a package in a different scheme is not allowed and pip would print a message and error out.

So, modifying a package in system scheme with a “user” working scheme is not allowed. Nor is modifing a package in user scheme with a “system” working scheme. Niether are the other permutations with “local”.

I think this results in a pretty simple behaviour model.

As @ncoghlan pointed out, this would need some logic to understand that user installs shadow system ones. Also, I would slightly change this to spell it like --scheme {global,user,venv} because I like how this signifies exclusiveness of the behaviours better than plain flags do and is consistent with some internal configuration stuff.

Additionally, while doing this:

• preparatory work of #1668 can also be done, meaning that defaulting to user would just be flipping the default (something like what we did for --upgrade-strategy).
• addresses #1056 by requiring the user to specify the scheme that they are working with.

/ping @dstufft @pfmoore

[piotr-dobrogost]

How does --target mode of operation relate to schemes? It feels like --target essentially is another type of scheme and treating it as such would potentially solve many bugs pertaining to --target option.

[nanonyme]

I personally think that fallback logic is too complicated. If you’re operating under a badly configured distro, use virtualenv.