Forgetting to source a virtualenv is more of an issue since pip 20

What’s the problem this feature will solve?

Since pip 20, doing a --user install when the main site-packages directory is not writeable and user site-packages are enabled is the default.

When working with virtualenv, this makes the common (for me) mistake of forgetting to source the virtualenv much more of an issue. I had this happen a couple of times since upgrading. It took me quite some time to figure out the issue, and some time to clean up my system. Before, when mistakenly running pip install outside the virtualenv, it would fail because of the required root privileges. Now it succeeds and modifies the user site-packages. I understand this is the expected behavior, since the user forgot to source.

Describe the solution you’d like

I agree with the rationale behind #1668. I am not too sure about the best solution here. My own workflow almost never requires to use pip outside of a virtualenv, but I am unsure about how common this is. I believe, for example, that this behavior would achieve consistency regarding the installation site:

pip install something              # virtualenv only
pip install --user something       # user site-packages
pip install --root-user something  # main site-packages

I understand this would be yet again a breaking change, so doing nothing might work too 😉 Thanks for the great work on pip!

[uranusjr]

I wonder if I should use this issue as a cue to bring up the conversation to completely disable pip installing into the global Python installation 😃 There are very old issues for this, but tl;dr

pip install foo  # This only works inside a virtual environment.
pip install --user foo  # This installs to the user base, not changed.
pip install --global foo  # This installs to the installation base.

[uranusjr]

One wrinkle to this is whether --global should automatically switch to --user if the global prefix is unavailble. This would also require a rethought on #2081 to be designed properly. (e.g. can you --global in a virtual environment? What about setting global = true in a config file? Environment variable?)