diff --git a/Lib/packaging/install.py b/Lib/packaging/install.py
index 4b82088a6ba..c5bda453074 100644
--- a/Lib/packaging/install.py
+++ b/Lib/packaging/install.py
@@ -13,7 +13,7 @@ import errno
 import shutil
 import logging
 import tempfile
-from sysconfig import get_config_var, get_path
+from sysconfig import get_config_var, get_path, is_python_build
 
 from packaging import logger
 from packaging.dist import Distribution
@@ -488,20 +488,31 @@ def install(project):
 
     Returns True on success, False on failure
     """
+    if is_python_build():
+        # Python would try to install into the site-packages directory under
+        # $PREFIX, but when running from an uninstalled code checkout we don't
+        # want to create directories under the installation root
+        message = ('installing third-party projects from an uninstalled '
+                   'Python is not supported')
+        logger.error(message)
+        return False
+
     logger.info('Checking the installation location...')
     purelib_path = get_path('purelib')
+
     # trying to write a file there
     try:
         with tempfile.NamedTemporaryFile(suffix=project,
                                          dir=purelib_path) as testfile:
             testfile.write(b'test')
     except OSError:
-        # was unable to write a file
+        # FIXME this should check the errno, or be removed altogether (race
+        # condition: the directory permissions could be changed between here
+        # and the actual install)
         logger.info('Unable to write in "%s". Do you have the permissions ?'
                     % purelib_path)
         return False
 
-
     logger.info('Getting information about %r...', project)
     try:
         info = get_infos(project)
@@ -520,7 +531,7 @@ def install(project):
 
     except InstallationConflict as e:
         if logger.isEnabledFor(logging.INFO):
-            projects = ['%s %s' % (p.name, p.version) for p in e.args[0]]
+            projects = ['%r %s' % (p.name, p.version) for p in e.args[0]]
             logger.info('%r conflicts with %s', project, ','.join(projects))
 
     return True
diff --git a/Lib/packaging/tests/test_command_install_distinfo.py b/Lib/packaging/tests/test_command_install_distinfo.py
index 6d40f66d92a..ade191c57a0 100644
--- a/Lib/packaging/tests/test_command_install_distinfo.py
+++ b/Lib/packaging/tests/test_command_install_distinfo.py
@@ -162,7 +162,7 @@ class InstallDistinfoTestCase(support.TempdirManager,
 
         expected = []
         for f in install.get_outputs():
-            if (f.endswith('.pyc') or f == os.path.join(
+            if (f.endswith(('.pyc', '.pyo')) or f == os.path.join(
                 install_dir, 'foo-1.0.dist-info', 'RECORD')):
                 expected.append([f, '', ''])
             else:
diff --git a/Lib/packaging/tests/test_dist.py b/Lib/packaging/tests/test_dist.py
index fb6d524fdc5..e1c5ff0a2d7 100644
--- a/Lib/packaging/tests/test_dist.py
+++ b/Lib/packaging/tests/test_dist.py
@@ -35,7 +35,7 @@ class DistributionTestCase(support.TempdirManager,
                            support.EnvironRestorer,
                            unittest.TestCase):
 
-    restore_environ = ['HOME']
+    restore_environ = ['HOME', 'PLAT']
 
     def setUp(self):
         super(DistributionTestCase, self).setUp()
diff --git a/Lib/packaging/tests/test_install.py b/Lib/packaging/tests/test_install.py
index c50a45e40e7..35733c8bf5d 100644
--- a/Lib/packaging/tests/test_install.py
+++ b/Lib/packaging/tests/test_install.py
@@ -1,5 +1,7 @@
 """Tests for the packaging.install module."""
 import os
+import logging
+from sysconfig import is_python_build
 from tempfile import mkstemp
 
 from packaging import install
@@ -357,9 +359,17 @@ class TestInstall(LoggingCatcher, TempdirManager, unittest.TestCase):
             install._install_dist = old_install_dist
 
     def test_install_permission_denied(self):
-        # if we don't have the access to the installation
-        # path, we should abort immediatly
+        # if we don't have access to the installation path, we should abort
+        # immediately
         project = os.path.join(os.path.dirname(__file__), 'package.tgz')
+
+        # when running from an uninstalled build, a warning is emitted and the
+        # installation is not attempted
+        if is_python_build():
+            self.assertFalse(install.install(project))
+            self.assertEqual(1, len(self.get_logs(logging.ERROR)))
+            return
+
         install_path = self.mkdtemp()
         old_get_path = install.get_path
         install.get_path = lambda path: install_path
diff --git a/Makefile.pre.in b/Makefile.pre.in
index 07486fde918..2ca89cf36a4 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -950,7 +950,7 @@ libinstall:	build_all $(srcdir)/Lib/$(PLATDIR)
 		else	true; \
 		fi; \
 	done
-	@for i in $(srcdir)/Lib/*.py $(srcdir)/Lib/*.egg-info $(srcdir)/Lib/*.cfg ; \
+	@for i in $(srcdir)/Lib/*.py $(srcdir)/Lib/*.cfg ; \
 	do \
 		if test -x $$i; then \
 			$(INSTALL_SCRIPT) $$i $(DESTDIR)$(LIBDEST); \
diff --git a/Misc/ACKS b/Misc/ACKS
index 3cf269bab29..8b3055c605b 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -550,6 +550,7 @@ Joerg Lehmann
 Robert Lehmann
 Petri Lehtinen
 Luke Kenneth Casson Leighton
+Tshepang Lekhonkhobe
 Marc-Andre Lemburg
 John Lenton
 Christopher Tur Lesniewski-Laas
diff --git a/Misc/NEWS b/Misc/NEWS
index c80fc6db76a..c4f1855503a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -189,6 +189,10 @@ Library
 
 - Issue #12009: Fixed regression in netrc file comment handling.
 
+- Issue #12246: Warn and fail when trying to install a third-party project from
+  an uninstalled Python (built in a source checkout).  Original patch by
+  Tshepang Lekhonkhobe.
+
 - Issue #10694: zipfile now ignores garbage at the end of a zipfile.
 
 - Issue #12283: Fixed regression in smtplib quoting of leading dots in DATA.