diff --git a/Lib/test/test_importlib/test_api.py b/Lib/test/test_importlib/test_api.py
--- a/Lib/test/test_importlib/test_api.py
+++ b/Lib/test/test_importlib/test_api.py
@@ -4,9 +4,11 @@
 frozen_util, source_util = util.import_importlib('importlib.util')
 frozen_machinery, source_machinery = util.import_importlib('importlib.machinery')
 
+import os
 import os.path
 import sys
 from test import support
+import tempfile
 import types
 import unittest
 import warnings
@@ -333,6 +335,52 @@
                     self.assertEqual(loader.path, init_path)
                     self.assertEqual(ns, expected)
 
+    def _add_namespace_portion(self, sub, rootdir, content='eggs = None'):
+        name, _, modname = sub.rpartition('.')
+        # Set up the portion dir.
+        pathentry = '{}-{}'.format(tempfile.mkdtemp(dir=rootdir), modname)
+        portionpath = os.path.join(rootdir, pathentry, *name.split('.'))
+        os.makedirs(portionpath)
+        # Set up the module.
+        filename = os.path.join(portionpath, '{}.py'.format(modname))
+        with open(filename, 'w') as pyfile:
+            pyfile.write(content)
+        return pathentry, portionpath
+
+    def test_reload_namespace_additions(self):
+        name = 'spam'
+        subnames = ['{}.{}'.format(name, sub)
+                    for sub in ['ham', 'eggs']]
+        extra = '{}.{}'.format(name, 'cheddar')
+        with support.temp_cwd(None) as cwd:
+            with util.uncache(name, *subnames):
+                pathentries = []
+                for sub in subnames:
+                    pe, _ = self._add_namespace_portion(sub, cwd)
+                    pathentries.append(pe)
+                with support.DirsOnSysPath(*pathentries):
+                    self.init.invalidate_caches()
+                    module = self.init.import_module(name)
+                    before = module.__path__
+                    for sub in subnames:
+                        self.init.import_module(sub)
+                    with self.assertRaises(ImportError):
+                        self.init.import_module(extra)
+
+                    # Add more portions
+                    pe, extrapath = self._add_namespace_portion(extra, cwd)
+                    with support.DirsOnSysPath(pe):
+                        #self.init.invalidate_caches()
+                        reloaded = self.init.reload(module)
+                        after = reloaded.__path__
+                        for sub in subnames:
+                            self.init.import_module(sub)
+                        self.init.import_module(extra)
+                        diff = set(after) - set(before)
+                        empty = set(before) - set(after)
+                        self.assertEqual(empty, set())
+                        self.assertEqual(diff, {extrapath})
+
     def test_reload_submodule(self):
         # See #19851.
         name = 'spam'