Prevent distutils option error target prefix conflict

news/6008.bugfix

@@ -0,0 +1 @@
+Prevent DistutilsOptionError when prefix is indicated in the global environment and `--target` is used.

src/pip/_internal/locations.py

@@ -156,8 +156,9 @@ def distutils_scheme(dist_name, user=False, home=None, root=None,
     # 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), "user={} prefix={}".format(user, prefix)
+    assert not (home and prefix), "home={} prefix={}".format(home, prefix)
     i.user = user or i.user
-    if user:
+    if user or home:
         i.prefix = ""
     i.prefix = prefix or i.prefix
     i.home = home or i.home

tests/functional/test_install.py

@@ -1473,3 +1473,22 @@ def test_install_conflict_warning_can_be_suppressed(script, data):
         'install', '--no-index', pkgB_path, '--no-warn-conflicts'
     )
     assert "Successfully installed pkgB-2.0" in result2.stdout, str(result2)
+
+
+def test_target_install_ignores_distutils_config_install_prefix(script):
+    prefix = script.scratch_path / 'prefix'
+    distutils_config = Path(os.path.expanduser('~'),
+                            'pydistutils.cfg' if sys.platform == 'win32'
+                            else '.pydistutils.cfg')
+    distutils_config.write(textwrap.dedent(
+        '''
+        [install]
+        prefix=%s
+        ''' % str(prefix)))
+    target = script.scratch_path / 'target'
+    result = script.pip_install_local('simplewheel', '-t', target)
+    assert (
+        "Successfully installed simplewheel" in result.stdout and
+        (target - script.base_path) in result.files_created and
+        (prefix - script.base_path) not in result.files_created
+    ), str(result)