Validate single-form plurals against plural source placeholders

babel/messages/checkers.py

@@ -56,7 +56,11 @@ def python_format(catalog: Catalog | None, message: Message) -> None:
         msgstrs = (msgstrs,)
 
     if msgstrs[0]:
-        _validate_format(msgids[0], msgstrs[0])
+        msgid = msgids[0]
+        # A single plural form must also support the plural source's placeholders.
+        if message.pluralizable and catalog is not None and catalog.num_plurals == 1:
+            msgid = msgids[1]
+        _validate_format(msgid, msgstrs[0])
     if message.pluralizable:
         for msgstr in msgstrs[1:]:
             if msgstr:

tests/messages/test_checkers.py

@@ -19,7 +19,7 @@
 from babel.core import Locale, UnknownLocaleError
 from babel.dates import format_datetime
 from babel.messages import Message, checkers
-from babel.messages.catalog import TranslationError
+from babel.messages.catalog import Catalog, TranslationError
 from babel.messages.checkers import _validate_format, python_format
 from babel.messages.plurals import PLURALS
 from babel.messages.pofile import read_po
@@ -416,3 +416,34 @@ def test__validate_format_invalid(msgid, msgstr, error):
 )
 def test__validate_format_valid(msgid, msgstr):
     _validate_format(msgid, msgstr)
+
+
+@pytest.mark.parametrize('locale', ['zh', 'ja', 'ko'])
+def test_python_format_single_plural_uses_plural_placeholders(locale):
+    message = Message(
+        ('One file for %(owner)s', '%(num)d files for %(owner)s'),
+        ('%(owner)s: %(num)d',),
+    )
+    python_format(Catalog(locale), message)
+
+
+@pytest.mark.parametrize(('translation', 'error'), [
+    ('%(owner)s: %(num)s', "incompatible format for placeholder 'num'"),
+    ('%(other)s: %(num)d', "unknown named placeholder 'other'"),
+])
+def test_python_format_single_plural_rejects_invalid_placeholders(translation, error):
+    message = Message(
+        ('One file for %(owner)s', '%(num)d files for %(owner)s'),
+        (translation,),
+    )
+    with pytest.raises(TranslationError, match=error):
+        python_format(Catalog('zh'), message)
+
+
+def test_python_format_multiple_plurals_checks_singular_placeholders():
+    message = Message(
+        ('One file for %(owner)s', '%(num)d files for %(owner)s'),
+        ('%(owner)s: %(num)d', '%(owner)s: %(num)d'),
+    )
+    with pytest.raises(TranslationError, match="unknown named placeholder 'num'"):
+        python_format(Catalog('en'), message)