catalog: Keep explicitly declared plural forms without a locale

babel/messages/catalog.py

@@ -551,7 +551,8 @@ def _get_mime_headers(self) -> list[tuple[str, str]]:
         if self.locale_identifier:
             headers.append(('Language', str(self.locale_identifier)))
         headers.append(('Language-Team', language_team))
-        if self.locale is not None:
+        if self.locale is not None or self._num_plurals is not None:
+            # Keep explicit plural forms even when no locale set.
             headers.append(('Plural-Forms', self.plural_forms))
         headers += [
             ('MIME-Version', '1.0'),
@@ -663,12 +664,11 @@ def num_plurals(self) -> int:
         >>> Catalog(locale='ga').num_plurals
         5
         """
-        if self._num_plurals is None:
-            num = 2
-            if self.locale:
-                num = get_plural(self.locale)[0]
-            self._num_plurals = num
-        return self._num_plurals
+        if self._num_plurals is not None:
+            return self._num_plurals
+        if self.locale:
+            return get_plural(self.locale)[0]
+        return 2
 
     @property
     def plural_expr(self) -> str:
@@ -681,12 +681,11 @@ def plural_expr(self) -> str:
         >>> Catalog(locale='ding').plural_expr  # unknown locale
         '(n != 1)'
         """
-        if self._plural_expr is None:
-            expr = '(n != 1)'
-            if self.locale:
-                expr = get_plural(self.locale)[1]
-            self._plural_expr = expr
-        return self._plural_expr
+        if self._plural_expr is not None:
+            return self._plural_expr
+        if self.locale:
+            return get_plural(self.locale)[1]
+        return '(n != 1)'
 
     @property
     def plural_forms(self) -> str:

tests/messages/test_pofile_write.py

@@ -11,7 +11,7 @@
 # history and logs, available at https://github.com/python-babel/babel/commits/master/.
 
 from datetime import datetime
-from io import BytesIO
+from io import BytesIO, StringIO
 
 from babel.messages import Catalog, Message, pofile
 
@@ -439,3 +439,39 @@ def test_wrap_with_enclosed_file_locations():
 #: \xe2\x81\xa8test utils.py\xe2\x81\xa9:3
 msgid "foo"
 msgstr ""'''
+
+
+def test_explicit_plural_forms_survive_without_a_locale():
+    """Plural forms declared in the header must not need a locale to survive."""
+    catalog = pofile.read_po(StringIO(
+        'msgid ""\n'
+        'msgstr ""\n'
+        '"Content-Type: text/plain; charset=utf-8\\n"\n'
+        '"Plural-Forms: nplurals=3; plural=(n % 9 == 2 ? 2 : n % 21 == 4 ? 0 : 1)\\n"\n'
+        '\n'
+        'msgid "file"\n'
+        'msgid_plural "files"\n'
+        'msgstr[0] "flerb"\n'
+        'msgstr[1] "flöäarb"\n'
+        'msgstr[2] "flirblurb"\n',
+    ))
+    assert catalog.locale is None
+    assert catalog.num_plurals == 3
+
+    buf = BytesIO()
+    pofile.write_po(buf, catalog)
+    content = buf.getvalue().decode()
+    assert 'Plural-Forms: nplurals=3' in content
+    assert 'msgstr[2] "flirblurb"' in content
+    assert pofile.read_po(StringIO(content)).num_plurals == 3  # roundtrip test
+
+
+def test_a_template_declares_no_plural_forms():
+    """Reading `num_plurals` must not make a locale-less catalog claim plural forms."""
+    catalog = Catalog()
+    catalog.add(('file', 'files'))
+    assert catalog.num_plurals == 2
+
+    buf = BytesIO()
+    pofile.write_po(buf, catalog)
+    assert 'Plural-Forms' not in buf.getvalue().decode()