don't --user install in --system_site_packages virtualenvs when a conflict is present in the virtualenv site.

[qwcode]

the sys.path ordering in a --system_site_packages virtualenv is this: virtualenv site, then usersite, then global site.
pip shouldn't allow a --user install if a conflict is present in the virtualenv site, because it won't have sys.path precedence and won't be importable.

this issue has been added to the --user fixes pull request plan as part5.
https://gist.github.com/gists/2822510

[qwcode]

here's the install scenario that can happen with the latest code, if this isn't fixed

1. pkg-1.3 installed in virtualenv site
2. pkg-1.1 installed in usersite
3. pip install --user pkg==1.2
4. pip.req detects a VersionConflict with pkg-1.3 (due to it having sys.path precedence) and it ignores it
5. pkg-1.2 gets installed alongside pkg-1.1 in the usersite

[pnasrat]

@qwcode Marcus - I noticed local failure on OS X when triaging other bugs - maybe loosen up the assert - also I note you're doing what looks like division not string concat - env.root_path / env.site_packages, we probably want os.path.join here

Do you want to fix up?

======================================================================
FAIL: tests.test_user_site.Tests_UserSite.test_install_user_in_global_virtualenv_with_conflict_fails
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/pnasrat/Development/pip/31/lib/python3.1/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/Users/pnasrat/Development/pip/tests/test_user_site.py", line 193, in test_install_user_in_global_virtualenv_with_conflict_fails
    %('INITools',env.root_path / env.site_packages)), result2.stdout
AssertionError: Will not install to the user site because it will lack sys.path precedence to INITools in /private/var/folders/00/0tcs0000h01000cxqpysvccm0039l4/T/tmppi_VNZ-piptest/.virtualenv/lib/python3.1/site-packages
Storing complete log in /var/folders/00/0tcs0000h01000cxqpysvccm0039l4/T/tmppi_VNZ-piptest/pip-log.txt

[pnasrat]

OK I see what is happening - real as /var is a symlink to private/var which is perfectly valid setup.

We probably should make the tests less brittle on the fs layout - maybe just assert contains error message, package and the env.site_packages. Other option is to enforce os.path.realpath in the test helper

actual=/private/var/folders/00/0tcs0000h01000cxqpysvccm0039l4/T/tmpCHPmJx-piptest/.virtualenv/lib/python3.1/site-packages
expected=/var/folders/00/0tcs0000h01000cxqpysvccm0039l4/T/tmpCHPmJx-piptest/.virtualenv/lib/python3.1/site-packages

[qwcode]

fixed in pulls #585, #592

[skyl]

what if I want to force the install to the PYTHONUSERBASE?

[qwcode]

@skyl you can do it in a different environment, that does not have the conflict scenario this issue deals with.

[skyl]

the thing is, I still want the rules about whether or not to skip based on the requirement being already satisfied. I could use the system pip and install with --user but then I redundantly install a bunch of stuff that should be skipped based on what is already in the virtualenv.

[qwcode]

yea, I can see possibly re-moving this restriction (and just logging warnings when "hidden" packages are installed). if you want to open a specific issue for that, please do. there's pros/cons either way.

but as for doing the --user install in another environment, you can target just the specific packages need using --no-deps if you're concerned about the redundancy.

[qwcode]

just logged #1443