Add option for global installation warning

[sujaldev]

This PR implements a new (opt-in) option --global-install-warning to warn the user if they're installing without a virtual environment activated, this does not conflict with the existing option --require-virtualenv which gives an error when enabled and a virtualenv is not detected. If both --require-virtualenv and --global-install-warning are given then it will behave as if only --require-virtualenv was passed.

Example:

$ pip config set install.require-virtualenv true
Writing to /home/<user>/.config/pip/pip.conf

$ pip config set install.global-install-warning true
Writing to /home/<user>/.config/pip/pip.conf

$ pip install -r requirements.txt
ERROR: Could not find an activated virtualenv (required).

$ pip config unset install.require-virtualenv
Writing to /home/<user>/.config/pip/pip.conf

$ pip install -r requirements.txt
WARNING: Could not find an activated virtualenv.
Proceed (y/N)? 

fixes #11752.

[pfmoore]

I’m -1 on this. Just use —require-virtualenv for this. We don’t need 2 options that do extremely similar things.

[sujaldev]

In my opinion, the UX with --require-virtualenv is a bit frustrating. What if that is changed to what I'm proposing with --global-install-warning?

[notatallshaw]

My two cents as a user of Pip is that I don't have an opinion on --global-install-warning vs. --require-virtualenv but I don't like the use of ask as part of pip install here, I'd rather pip just failed.

At a quick glance it seems Pip doesn't use ask unless it's to do with authentication or deleting files, otherwise pip tends to succeed or fail. Which means when pip is called in a subprocess (e.g. as part of CI scripts or in conda requirements) you can generally expect it to succeed or fail.

Currently you have to set the environmental variable PIP_NO_INPUT to throw an error instead, I think if ask is used in more places it should behave like other global settings and be able to be set as any of environmental variable, configuration, or command line flag. Or even change this to behave like conda and have flags to provide yes or no to every prompt.

[pfmoore]

I don't like the use of ask as part of pip install here, I'd rather pip just failed.

Thanks @notatallshaw, that's articulated my reservation far better than I managed. I agree, pip install should not prompt the user like this, it has the potential to break workflows that run pip install in non-interactive environments. And without the interactive prompt. there's basically no difference between --global-install-warning and --require-virtualenv except that one requires you to opt in and the other to opt out.

And we're looking at making --require-virtualenv the default behaviour sometime in the near future anyway, at which point --global-install-warning becomes pointless.

[sujaldev]

I was thinking that the use of a prompt could be implemented inside --require-virtualenv itself to be an opt-in feature, which when not explicitly enabled, raise an error as it currently does otherwise a warning and a prompt. This approach will not break any existing scripts that run pip in a non-interactive environment, satisfy those who prefer an error and simultaneously those who prefer a prompt.

Personally, I'd love to have a prompt and a warning but seems like everyone prefers an error. Should I close this PR then?

[uranusjr]

Maybe alternatively we could deprecate --require-virtualenv and turn it into something like --allow-global-site=always|ask|never. The always choice is the default, never is identical to --require-virtualenv, and ask would ask if there’s a terminal, or fall back to never otherwise.

But this could be an overkill, idk.