gh-56634: Gettext: raise descriptive ValueErrors instead of IndexErrors for incorrect .mo files metadata

Lib/gettext.py

@@ -407,10 +407,21 @@ def _parse(self, fp):
                     elif lastk:
                         self._info[lastk] += '\n' + item
                     if k == 'content-type':
-                        self._charset = v.split('charset=')[1]
+                        try:
+                            self._charset = v.split('charset=')[1]
+                        except IndexError:
+                            raise ValueError(
+                                f"expected 'charset=' in Content-Type metadata in {filename}, got {v!r}"
+                            ) from None
                     elif k == 'plural-forms':
                         v = v.split(';')
-                        plural = v[1].split('plural=')[1]
+                        try:
+                            plural = v[1].split('plural=')[1]
+                        except IndexError:
+                            raise ValueError(
+                                f"expected ';' and 'plural=' in Plural-Forms metadata in {filename}, "
+                                f"got {';'.join(v)!r}"
+                            ) from None
                         self.plural = c2py(plural)
             # Note: we unconditionally convert both msgids and msgstrs to
             # Unicode using the character encoding specified in the charset

Lib/test/test_gettext.py

@@ -1,6 +1,7 @@
 import os
 import base64
 import gettext
+import sys
 import unittest
 import unittest.mock
 from functools import partial
@@ -664,6 +665,70 @@ def test_ignore_comments_in_headers_issue36239(self):
             t = gettext.GNUTranslations(fp)
             self.assertEqual(t.info()["plural-forms"], "nplurals=2; plural=(n != 1);")
 
+    @property
+    def expected_filename(self):
+        if sys.platform == 'win32':
+            return r'xx\\LC_MESSAGES\\gettext.mo'
+        return 'xx/LC_MESSAGES/gettext.mo'
+
+    def test_raise_descriptive_error_for_incorrect_content_type(self):
+        with open(MOFILE, 'wb') as fp:
+            # below is msgfmt run on such a PO file:
+            # msgid ""
+            # msgstr ""
+            # "Content-Type: text/plain; charste=UTF-8\n"
+            fp.write(
+                b'\xde\x12\x04\x95\x00\x00\x00\x00\x01\x00\x00\x00\x1c\x00\x00\x00$\x00\x00\x00\x03\x00\x00\x00,\x00'
+                b'\x00\x00\x00\x00\x00\x008\x00\x00\x00(\x00\x00\x009\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00'
+                b'\x00\x00\x00\x00Content-Type: text/plain; charste=UTF-8\n\x00'
+            )
+        with self.assertRaisesRegex(
+            ValueError,
+            f"expected 'charset=' in Content-Type metadata in {self.expected_filename}, "
+            f"got 'text/plain; charste=UTF-8'"
+        ):
+            with open(MOFILE, 'rb') as fp:
+                gettext.GNUTranslations(fp)
+
+    def test_raise_descriptive_error_for_incorrect_plural_forms(self):
+        with open(MOFILE, 'wb') as fp:
+            # below is msgfmt run on such a PO file:
+            # msgid ""
+            # msgstr ""
+            # "Plural-Forms: \n"
+            fp.write(
+                b'\xde\x12\x04\x95\x00\x00\x00\x00\x01\x00\x00\x00\x1c\x00\x00\x00$\x00\x00\x00\x03\x00\x00\x00,\x00'
+                b'\x00\x00\x00\x00\x00\x008\x00\x00\x00\x0f\x00\x00\x009\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00'
+                b'\x00\x00\x00\x00\x00Plural-Forms: \n\x00'
+            )
+        with self.assertRaisesRegex(
+            ValueError,
+            f"expected ';' and 'plural=' in Plural-Forms metadata in {self.expected_filename}, got ''",
+        ):
+            with open(MOFILE, 'rb') as fp:
+                gettext.GNUTranslations(fp)
+
+
+    def test_raise_descriptive_error_for_incorrect_plural_forms_with_semicolon(self):
+        with open(MOFILE, 'wb') as fp:
+            # below is msgfmt run on such a PO file:
+            # msgid ""
+            # msgstr ""
+            # "Plural-Forms: nplurals=1; prulal=0;\n"
+            fp.write(
+                b'\xde\x12\x04\x95\x00\x00\x00\x00\x01\x00\x00\x00\x1c\x00\x00\x00$\x00\x00\x00\x03\x00\x00\x00,\x00'
+                b'\x00\x00\x00\x00\x00\x008\x00\x00\x00$\x00\x00\x009\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00'
+                b'\x00\x00\x00\x00Plural-Forms: nplurals=1; prulal=0;\n\x00'
+
+            )
+        with self.assertRaisesRegex(
+            ValueError,
+            f"expected ';' and 'plural=' in Plural-Forms metadata in {self.expected_filename}, "
+            "got 'nplurals=1; prulal=0;'"
+        ):
+            with open(MOFILE, 'rb') as fp:
+                gettext.GNUTranslations(fp)
+
 
 class UnicodeTranslationsTest(GettextBaseTest):
     def setUp(self):

Misc/NEWS.d/next/Library/2025-09-25-21-13-59.gh-issue-56634.8tySiu.rst

@@ -0,0 +1,2 @@
+Gettext now raises descriptive ``ValueError``\s instead of ``IndexError``\s for
+incorrect ``.mo`` files metadata.