Make the Python implementation stop apptempting to pointlessly copy s…

…trings and (somewhat pointedly) copy mappings, like the C implementation.

src/zope/i18nmessageid/message.py

@@ -37,11 +37,11 @@ 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__):
-            self.domain = ustr.domain[:] if ustr.domain else None
-            self.default = ustr.default[:] if ustr.default else None
-            self.mapping = ustr.mapping.copy() if ustr.mapping is not None else None
-            self.msgid_plural = ustr.msgid_plural[:] if ustr.msgid_plural else None
-            self.default_plural = ustr.default_plural[:] if ustr.default_plural else None
+            self.domain = ustr.domain
+            self.default = ustr.default
+            self.mapping = ustr.mapping
+            self.msgid_plural = ustr.msgid_plural
+            self.default_plural = ustr.default_plural
             self.number = ustr.number
         else:
             self.domain = None

src/zope/i18nmessageid/tests.py

@@ -113,6 +113,19 @@ def test_copy(self):
         self.assertEqual(message.msgid_plural, 'testings')
         self.assertEqual(message.default_plural, 'defaults')
         self.assertEqual(message.number, 0)
+
+        # Besides just being equal, they maintain their identity
+        for attr in (
+                'domain',
+                'default',
+                'mapping',
+                'msgid_plural',
+                'default_plural',
+                'number',
+        ):
+            self.assertIs(getattr(source, attr),
+                          getattr(message, attr))
+
         if self._TEST_READONLY:
             self.assertTrue(message._readonly)
 
@@ -156,7 +169,6 @@ def test_copy_no_overrides(self):
             self.assertIsNone(getattr(pref_msg, attr))
             self.assertIsNone(getattr(copy, attr))
 
-
     def test_domain_immutable(self):
         message = self._makeOne('testing')
         with self.assertRaises((TypeError, AttributeError)):