don't allow --user installs in no-global virtualenvs

pip/commands/install.py

@@ -5,7 +5,7 @@
 from pip.req import InstallRequirement, RequirementSet
 from pip.req import parse_requirements
 from pip.log import logger
-from pip.locations import build_prefix, src_prefix
+from pip.locations import build_prefix, src_prefix, virtualenv_no_global
 from pip.basecommand import Command
 from pip.index import PackageFinder
 from pip.exceptions import InstallationError, CommandError
@@ -190,6 +190,8 @@ def run(self, options, args):
         options.src_dir = os.path.abspath(options.src_dir)
         install_options = options.install_options or []
         if options.use_user_site:
+            if virtualenv_no_global():
+                raise InstallationError("Can not perform a '--user' install. User site-packages are not visible in this virtualenv.")
             install_options.append('--user')
         if options.target_dir:
             options.ignore_installed = True

pip/locations.py

@@ -1,6 +1,7 @@
 """Locations where we look for configs, install stuff, etc"""
 
 import sys
+import site
 import os
 import tempfile
 from pip.backwardcompat import get_python_lib
@@ -13,6 +14,16 @@ def running_under_virtualenv():
     """
     return hasattr(sys, 'real_prefix')
 
+def virtualenv_no_global():
+    """
+    Return True if in a venv and no system site packages.
+    """
+    #this mirrors the logic in virtualenv.py for locating the no-global-site-packages.txt file
+    site_mod_dir = os.path.dirname(os.path.abspath(site.__file__))
+    no_global_file = os.path.join(site_mod_dir,'no-global-site-packages.txt')
+    if running_under_virtualenv() and os.path.isfile(no_global_file):
+        return True
+
 
 if running_under_virtualenv():
     ## FIXME: is build/ a good name?

tests/test_user_site.py

@@ -83,3 +83,14 @@ def test_install_curdir_usersite(self):
         assert fspkg_folder in result.files_created, str(result.stdout)
 
         assert egg_info_folder in result.files_created, str(result)
+
+
+    def test_install_user_venv_nositepkgs_fails(self):
+        """
+        user install in virtualenv (with no system packages) fails with message
+        """
+        env = reset_env()
+        run_from = abspath(join(here, 'packages', 'FSPkg'))
+        result = run_pip('install', '--user', curdir, cwd=run_from, expect_error=True)
+        assert "Can not perform a '--user' install. User site-packages are not visible in this virtualenv." in result.stdout
+