Fix: In babel/messages/catalog.py Catalog._set_mime_headers(), the...

babel/messages/catalog.py

@@ -579,8 +579,15 @@ def _set_mime_headers(self, headers: Iterable[tuple[str, str]]) -> None:
                     self.charset = params['charset'].lower()
             elif name == 'plural-forms':
                 params = parse_separated_header(f" ;{value}")
-                self._num_plurals = int(params.get('nplurals', 2))
-                self._plural_expr = params.get('plural', '(n != 1)')
+                # nplurals/plural values may be left as the untranslated
+                # xgettext placeholders (e.g. in a fresh POT template);
+                # fall back to the defaults in that case instead of
+                # crashing on int(), mirroring the po-revision-date guard
+                # for the analogous 'YEAR' placeholder below.
+                num_plurals = params.get('nplurals', 2)
+                self._num_plurals = 2 if num_plurals == 'INTEGER' else int(num_plurals)
+                plural_expr = params.get('plural', '(n != 1)')
+                self._plural_expr = '(n != 1)' if plural_expr == 'EXPRESSION' else plural_expr
             elif name == 'pot-creation-date':
                 self.creation_date = _parse_datetime_header(value)
             elif name == 'po-revision-date':

tests/messages/test_catalog.py

@@ -462,6 +462,22 @@ def test_catalog_mime_headers_set_locale():
     ]
 
 
+def test_catalog_mime_headers_xgettext_plural_forms_placeholder():
+    """
+    A freshly-generated POT template from xgettext may leave the
+    Plural-Forms header with unexpanded placeholder tokens
+    (``nplurals=INTEGER; plural=EXPRESSION;``) instead of real values.
+    Parsing such a header should not raise, and should fall back to the
+    same defaults used elsewhere in the catalog.
+    """
+    cat = catalog.Catalog()
+    cat.mime_headers = [
+        ('Plural-Forms', 'nplurals=INTEGER; plural=EXPRESSION;'),
+    ]
+    assert cat.num_plurals == 2
+    assert cat.plural_expr == '(n != 1)'
+
+
 def test_catalog_mime_headers_type_coercion():
     """
     Test that mime headers' keys and values are coerced to strings

tests/messages/test_pofile.py

@@ -146,6 +146,20 @@ def test_issue_1087():
     assert pofile.read_po(buf).locale is None
 
 
+def test_issue_1154():
+    # A freshly-generated POT template from xgettext may leave the
+    # Plural-Forms header with unexpanded placeholder tokens instead of
+    # real values; parsing it should not raise.
+    buf = StringIO(r'''
+msgid ""
+msgstr ""
+"Plural-Forms: nplurals=INTEGER; plural=EXPRESSION;\n"
+''')
+    cat = pofile.read_po(buf)
+    assert cat.num_plurals == 2
+    assert cat.plural_expr == '(n != 1)'
+
+
 @pytest.mark.parametrize("case", ['msgid "foo"', 'msgid "foo"\nmsgid_plural "foos"'])
 @pytest.mark.parametrize("abort_invalid", [False, True])
 def test_issue_1134(case: str, abort_invalid: bool):