Add a warning if pip installs packages outside of sys.path

What’s the problem this feature will solve?

Help users understand the issue.

Describe the solution you’d like

$ pip install foobar
  Downloading foobar-1.2.3-py3-none-any.whl (1.6 MB)
     |████████████████████████████████| 1.6 MB 31.9 MB/s
Installing collected packages: foobar
WARNING: pip is installing foobar to /home/foo/my-site-packages that is not present in sys.path
Successfully installed foobar-1.2.3

cf https://github.com/pypa/get-pip/issues/124

Alternative Solutions As an alternative (or in addition), it would be nice for pip debug to advertise the installation scheme

Additional context Of course, the warning would not appear when using pip with --target / --prefix / --root.

[pfmoore]

Ah yes, .pydistutils.cfg. So let’s go with a warning that says in effect “your system is badly configured, this is not pip’s fault” and adding the install scheme to pip debug. That sounds like the correct balance of being helpful without making it our problem, to me.

[pfmoore]

I can understand the benefit in helping people to debug an otherwise weird issue. But I’d make the warning significantly stronger, something like “WARNING: Your system is misconfigured so that pip installs to a location not on sys.path. If you did not change the configuration yourself, please report this issue to your Python distributor.” The key is I want people to know that it’s a system issue and not to report it to pip. Otherwise there’s no benefit, as they still come to us and we still spend time diagnosing and sending them on to the right place.

(Although I’m definitely losing patience with having to check even the most basic of assumptions because Debian violates them - I’d be quite happy to not include this check if it’s Debian-specific).