Give imported packages lower precedence than standard library.

src/main/java/com/google/devtools/build/lib/bazel/rules/python/python_stub_template.txt

@@ -8,6 +8,11 @@ import sys
 import subprocess
 import zipfile
 
+# Stupid little utility to dedup entries in a list, while preserving order.
+def DedupList(lst):
+    seen = []
+    return [x for x in lst if not (x in seen or seen.append(x))]
+
 # Return True if running on Windows
 def IsWindows():
   return os.name == 'nt'
@@ -147,6 +152,17 @@ def RunfilesEnvvar(module_space):
 
   return (None, None)
 
+def BuildUserCustomizeModule(import_paths):
+  return """\
+# AUTOGENERATED FILE PRODUCED BY PYTHON LAUNCHER STUB SCRIPT. DO NOT EDIT.
+import sys
+
+import_paths = [
+%s
+]
+sys.path.extend(import_paths)
+""" % "\n".join('    "%s",' % path for path in import_paths)
+
 def Main():
   args = sys.argv[1:]
 
@@ -162,11 +178,19 @@ def Main():
   python_path_entries += GetRepositoriesImports(module_space, %import_all%)
 
   python_path_entries = [GetWindowsPathWithUNCPrefix(d) for d in python_path_entries]
+  python_path_entries = DedupList(python_path_entries)
+
+  # Write `usercustomize.py` file. This will be loaded during startup, and will append package
+  # directories to `sys.path`.
+  usercustomize_module = open("%s/usercustomize.py" % module_space, "w")
+  usercustomize_module.write(BuildUserCustomizeModule(python_path_entries))
+  usercustomize_module.close()
 
   old_python_path = os.environ.get('PYTHONPATH')
-  python_path = os.pathsep.join(python_path_entries)
   if old_python_path:
-    python_path += os.pathsep + old_python_path
+      python_path = old_python_path + os.pathsep + module_space
+  else:
+      python_path = module_space
 
   if IsWindows():
     python_path = python_path.replace("/", os.sep)

src/test/shell/bazel/python_version_test.sh

@@ -469,8 +469,8 @@ EOF
 # a separate suite.
 
 # Tests that a non-standard library module on the PYTHONPATH added by Bazel
-# can override the standard library. This behavior is not necessarily ideal, but
-# it is the current semantics; see #6532 about changing that.
+# cannot override the standard library. (See #6532 for discussion on this
+# behavior.)
 function test_source_file_does_not_override_standard_library() {
   mkdir -p test
 
@@ -506,8 +506,8 @@ EOF
 
   bazel run //test:main \
       &> $TEST_log || fail "bazel run failed"
-  # Indicates that the local module overrode the system one.
-  expect_log "I am lib!"
+  # Presence would indicate that the local module overrode the system one.
+  expect_not_log "I am lib!"
 }
 
 run_suite "Tests for how the Python rules handle Python 2 vs Python 3"