Fix Catalog.is_identical() for context-specific messages

babel/messages/catalog.py

@@ -1046,8 +1046,13 @@ def is_identical(self, other: Catalog) -> bool:
         """
         assert isinstance(other, Catalog)
         for key in self._messages.keys() | other._messages.keys():
-            message_1 = self.get(key)
-            message_2 = other.get(key)
+            # Index ``_messages`` directly: these are already internal keys, and
+            # passing one back through ``get`` would re-run ``_key_for`` on it.
+            # For a context-specific message that key is a ``(msgid, msgctxt)``
+            # tuple, which ``_key_for`` mistakes for a pluralizable message's id
+            # and reduces to ``msgid`` -- a key no message is stored under.
+            message_1 = self._messages.get(key)
+            message_2 = other._messages.get(key)
             if message_1 is None or message_2 is None or not message_1.is_identical(message_2):
                 return False
         return dict(self.mime_headers) == dict(other.mime_headers)

tests/messages/test_catalog.py

@@ -596,3 +596,43 @@ def test_catalog_tz_pickleable():
 msgstr ""
 "POT-Creation-Date: 2007-04-01 15:30+0200\n"
     """))))
+
+
+def test_catalog_is_identical_with_context():
+    """A catalog holding a context-specific message compares equal to itself.
+
+    ``is_identical`` iterates the internal ``_messages`` keys, and the key of a
+    context-specific message is a ``(msgid, msgctxt)`` tuple. Looking that key
+    up through ``Catalog.get`` re-ran ``_key_for`` on it, which took it for a
+    pluralizable message's id and reduced it to ``msgid`` -- so the lookup found
+    nothing and every such catalog compared as changed. That made
+    ``pybabel update --check`` report any catalog using ``pgettext`` or
+    ``npgettext`` as out of date, on every run.
+    """
+    cat = catalog.Catalog(locale='en')
+    cat.add('Guide', 'Guide', context='navigation')
+    assert cat.is_identical(cat)
+
+    same = catalog.Catalog(locale='en')
+    same.add('Guide', 'Guide', context='navigation')
+    assert cat.is_identical(same)
+
+    translated_differently = catalog.Catalog(locale='en')
+    translated_differently.add('Guide', 'Manual', context='navigation')
+    assert not cat.is_identical(translated_differently)
+
+    # The same msgid under a different context is a different message.
+    other_context = catalog.Catalog(locale='en')
+    other_context.add('Guide', 'Guide', context='footer')
+    assert not cat.is_identical(other_context)
+
+
+def test_catalog_is_identical_with_pluralizable_context():
+    """The same, for a context-specific *pluralizable* message."""
+    cat = catalog.Catalog(locale='en')
+    cat.add(('%d page', '%d pages'), ('%d page', '%d pages'), context='report')
+    assert cat.is_identical(cat)
+
+    same = catalog.Catalog(locale='en')
+    same.add(('%d page', '%d pages'), ('%d page', '%d pages'), context='report')
+    assert cat.is_identical(same)