Make sure --user and --prefix install flags conflict

pip/commands/install.py

@@ -215,6 +215,11 @@ class InstallCommand(RequirementCommand):
         options.src_dir = os.path.abspath(options.src_dir)
         install_options = options.install_options or []
         if options.use_user_site:
+            if options.prefix_path:
+                raise CommandError(
+                    "Can not combine '--user' and '--prefix' as they imply "
+                    "different installation locations"
+                )
             if virtualenv_no_global():
                 raise InstallationError(
                     "Can not perform a '--user' install. User site-packages "

pip/locations.py

@@ -184,11 +184,9 @@ def distutils_scheme(dist_name, user=False, home=None, root=None,
     # NOTE: setting user or home has the side-effect of creating the home dir
     # or user base for installations during finalize_options()
     # ideally, we'd prefer a scheme class that has no side-effects.
+    assert not (user and prefix), "Got user and prefix {}".format(user, prefix)
     i.user = user or i.user
-    if user:
-        i.prefix = ""
-    else:
-        i.prefix = prefix or i.prefix
+    i.prefix = prefix or i.prefix
     i.home = home or i.home
     i.root = root or i.root
     i.finalize_options()

tests/functional/test_install.py

@@ -1,5 +1,6 @@
 
 import os
+import sys
 import textwrap
 import glob
 
@@ -486,6 +487,41 @@ def test_install_package_with_root(script, data):
     assert root_path in result.files_created, str(result)
 
 
+def test_install_package_with_prefix(script, data):
+    """
+    Test installing a package using pip install --prefix
+    """
+    prefix_path = script.scratch_path / 'prefix'
+    result = script.pip(
+        'install', '--prefix', prefix_path, '-f', data.find_links,
+        '--no-index', 'simple==1.0',
+    )
+
+    if hasattr(sys, "pypy_version_info"):
+        path = script.scratch / 'prefix'
+    else:
+        path = script.scratch / 'prefix' / 'lib' / 'python{0}'.format(pyversion)  # noqa
+    install_path = (
+        path / 'site-packages' / 'simple-1.0-py{0}.egg-info'.format(pyversion)
+    )
+    assert install_path in result.files_created, str(result)
+
+
+def test_install_package_conflict_prefix_and_user(script, data):
+    """
+    Test installing a package using pip install --prefix --user errors out
+    """
+    prefix_path = script.scratch_path / 'prefix'
+    result = script.pip(
+        'install', '-f', data.find_links, '--no-index', '--user',
+        '--prefix', prefix_path, 'simple==1.0',
+        expect_error=True, quiet=True,
+    )
+    assert (
+        "Can not combine '--user' and '--prefix'" in result.stderr
+    )
+
+
 # skip on win/py3 for now, see issue #782
 @pytest.mark.skipif("sys.platform == 'win32' and sys.version_info >= (3,)")
 def test_install_package_that_emits_unicode(script, data):