Very confusing. This file appears to be unused by ZODB, but

imported from the Zope3 tests.  Since ZODB doesn't use it,
running the ZODB tests didn't reveal that its testGhostChanged()
got broken (deliberately -- "it's a feature") in ZODB 3.6.

src/persistent/tests/persistenttestbase.py

@@ -16,6 +16,9 @@ import unittest
 from persistent import Persistent
 from persistent.interfaces import IPersistent
 
+# Confusing:  ZODB doesn't use this file.  It appears to be used only
+# by Zope3, where it's imported by zope/app/schema/tests/test_wrapper.py.
+
 try:
     import zope.interface
 except ImportError:
@@ -115,8 +118,10 @@ class Test(unittest.TestCase):
         self.assertEqual(dm.called, 1)
 
     def testGhostChanged(self):
-        # An object is a ghost, and it's _p_changed it set to True.
-        # This assignment should have no effect.
+        # If an object is a ghost and its _p_changed is set to True (any
+        # true value),i t should activate (unghostify) the object.  This
+        # behavior is new in ZODB 3.6; before then, an attempt to do
+        # "ghost._p_changed = True" was ignored.
         p = self.klass()
         p._p_oid = 1
         dm = DM()
@@ -124,7 +129,7 @@ class Test(unittest.TestCase):
         p._p_deactivate()
         self.assertEqual(p._p_changed, None)
         p._p_changed = True
-        self.assertEqual(p._p_changed, None)
+        self.assertEqual(p._p_changed, 1)
 
     def testRegistrationFailure(self):
         p = self.klass()