Fix override with None.

src/zope/i18nmessageid/message.py

@@ -14,6 +14,7 @@
 """I18n Messages and factories.
 """
 __docformat__ = "reStructuredText"
+_marker = object()
 
 import six
 
@@ -32,21 +33,29 @@ class Message(six.text_type):
         'domain', 'default', 'mapping', '_readonly',
         'msgid_plural', 'default_plural', 'number')
 
-    def __new__(cls, ustr, domain=None, default=None, mapping=None,
-                msgid_plural=None, default_plural=None, number=None):
+    def __new__(cls, ustr, domain=_marker, default=_marker, mapping=_marker,
+                msgid_plural=_marker, default_plural=_marker, number=_marker):
         self = six.text_type.__new__(cls, ustr)
         if isinstance(ustr, self.__class__):
-            domain = domain if ustr.domain is None else ustr.domain[:]
-            default = default if ustr.default is None else ustr.default[:]
-            mapping = mapping if ustr.mapping is None else ustr.mapping.copy()
+            domain = ustr.domain[:] if domain is _marker else domain
+            default = ustr.default[:] if default is _marker else default
+            mapping = ustr.mapping.copy() if mapping is _marker else mapping
             msgid_plural = (
-                msgid_plural if ustr.msgid_plural is None else
-                ustr.msgid_plural[:])
+                ustr.msgid_plural[:] if msgid_plural is _marker else
+                msgid_plural)
             default_plural = (
-                default_plural if ustr.default_plural is None else
-                ustr.default_plural[:])
-            number = number if ustr.number is None else ustr.number
+                ustr.default_plural[:] if default_plural is _marker else
+                default_plural)
+            number = ustr.number if number is _marker else number
             ustr = six.text_type(ustr)
+        else:
+            domain = None if domain is _marker else domain
+            default = None if default is _marker else default
+            mapping = None if mapping is _marker else mapping
+            msgid_plural = None if msgid_plural is _marker else msgid_plural
+            default_plural = (None if default_plural is _marker else
+                              default_plural)
+            number = None if number is _marker else number
 
         self.domain = domain
         self.default = default

src/zope/i18nmessageid/tests.py

@@ -118,17 +118,18 @@ def test_copy(self):
 
     def test_copy_with_overrides(self):
         mapping = {'key': 'value'}
-        source = self._makeOne('testing')
+        source = self._makeOne(
+            'testing', 'domain', default='other', mapping=mapping,
+            msgid_plural='workings', default_plural='others', number=3)
         message = self._makeOne(
-            source, 'domain', 'default', mapping,
-            msgid_plural='testings', default_plural="defaults", number=2)
+            source, mapping=None, msgid_plural='override', number=0)
         self.assertEqual(message, 'testing')
         self.assertEqual(message.domain, 'domain')
-        self.assertEqual(message.default, 'default')
-        self.assertEqual(message.mapping, mapping)
-        self.assertEqual(message.msgid_plural, 'testings')
-        self.assertEqual(message.default_plural, 'defaults')
-        self.assertEqual(message.number, 2)
+        self.assertEqual(message.default, 'other')
+        self.assertEqual(message.mapping, None)
+        self.assertEqual(message.msgid_plural, 'override')
+        self.assertEqual(message.default_plural, 'others')
+        self.assertEqual(message.number, 0)
         if self._TEST_READONLY:
             self.assertTrue(message._readonly)