gh-151316: Prefer UTF-8 for the en_IN locale alias

sankalpsthakur · cursoragent · sankalpsthakur · commit ffd9b4122748 · 2026-08-11T16:23:34.000+05:30
X11 locale.alias mapped en_IN to ISO8859-1, so getlocale() invented a
codeset that does not exist on modern UTF-8-only systems and broke
setlocale(getlocale()) round-trips. Prefer en_IN.UTF-8 (mirroring other
modernized aliases) and cover the round-trip in tests.

Signed-off-by: Sankalp Thakur &lt;sankalphimself@gmail.com&gt;
Co-authored-by: Cursor &lt;cursoragent@cursor.com&gt;
diff --git a/Lib/locale.py b/Lib/locale.py
@@ -921,6 +921,11 @@ def getpreferredencoding(do_setlocale=True):
 #
 #    removed 'el_gr@euro'
 #    removed 'uz_uz@cyrillic'
+#
+# gh-151316:
+# Prefer UTF-8 for en_IN.  X11 locale.alias maps en_IN to ISO8859-1 and that
+# entry overrides glibc's en_IN/UTF-8 during makelocalealias regeneration,
+# so hardcode the modern codeset (mirroring the c.utf8 carve-out).
 
 locale_alias = {
     'a3':                                   'az_AZ.KOI8-C',
@@ -1066,7 +1071,7 @@ def getpreferredencoding(do_setlocale=True):
     'en_hk':                                'en_HK.ISO8859-1',
     'en_ie':                                'en_IE.ISO8859-1',
     'en_il':                                'en_IL.ISO8859-1',
-    'en_in':                                'en_IN.ISO8859-1',
+    'en_in':                                'en_IN.UTF-8',
     'en_ng':                                'en_NG.UTF-8',
     'en_nz':                                'en_NZ.ISO8859-1',
     'en_ph':                                'en_PH.ISO8859-1',
diff --git a/Lib/test/test_locale.py b/Lib/test/test_locale.py
@@ -417,6 +417,13 @@ def test_english(self):
         self.check('english', 'en_EN.ISO8859-1')
         self.check('english_uk.ascii', 'en_GB.ISO8859-1')
 
+    def test_en_in_utf8(self):
+        # gh-151316: en_IN is UTF-8 on modern glibc; do not invent ISO8859-1.
+        self.check('en_IN', 'en_IN.UTF-8')
+        self.check('en_in', 'en_IN.UTF-8')
+        self.assertEqual(locale._parse_localename('en_IN'), ('en_IN', 'UTF-8'))
+        self.assertEqual(locale._parse_localename('en_in'), ('en_IN', 'UTF-8'))
+
     def test_hyphenated_encoding(self):
         self.check('az_AZ.iso88599e', 'az_AZ.ISO8859-9E')
         self.check('az_AZ.ISO8859-9E', 'az_AZ.ISO8859-9E')
@@ -642,6 +649,38 @@ def test_getlocale_with_modifier(self, localename, localetuple):
         self.assertEqual(locale.getlocale(locale.LC_CTYPE), localetuple)
 
 
+class TestEnINLocale(unittest.TestCase):
+    """gh-151316: en_IN must round-trip without inventing ISO8859-1."""
+
+    def setUp(self):
+        self.oldlocale = locale.setlocale(locale.LC_CTYPE)
+        self.addCleanup(locale.setlocale, locale.LC_CTYPE, self.oldlocale)
+
+    def test_getlocale_setlocale_roundtrip(self):
+        try:
+            locale.setlocale(locale.LC_CTYPE, 'en_IN')
+        except locale.Error as exc:
+            self.skipTest(str(exc))
+        loc = locale.getlocale(locale.LC_CTYPE)
+        self.assertEqual(loc[0], 'en_IN')
+        self.assertNotEqual(loc[1], 'ISO8859-1')
+        locale.setlocale(locale.LC_CTYPE, loc)
+        self.assertEqual(locale.getlocale(locale.LC_CTYPE), loc)
+
+    def test_setlocale_from_getlocale_tuple(self):
+        # Reproduces the issue report: setlocale(LC_*, getlocale()).
+        try:
+            locale.setlocale(locale.LC_CTYPE, 'en_IN.UTF-8')
+        except locale.Error:
+            try:
+                locale.setlocale(locale.LC_CTYPE, 'en_IN')
+            except locale.Error as exc:
+                self.skipTest(str(exc))
+        loc = locale.getlocale(locale.LC_CTYPE)
+        locale.setlocale(locale.LC_CTYPE, loc)
+        self.assertEqual(locale.getlocale(locale.LC_CTYPE)[0], 'en_IN')
+
+
 class TestMiscellaneous(unittest.TestCase):
     def test_defaults_UTF8(self):
         # Issue #18378: on (at least) macOS setting LC_CTYPE to "UTF-8" is
diff --git a/Misc/NEWS.d/next/Library/2026-08-11-09-47-40.gh-issue-151316.M7ANwQ.rst b/Misc/NEWS.d/next/Library/2026-08-11-09-47-40.gh-issue-151316.M7ANwQ.rst
@@ -0,0 +1,4 @@
+:mod:`locale` now maps the ``en_IN`` locale alias to ``en_IN.UTF-8``
+instead of the obsolete X11 ``ISO8859-1`` codeset.  This restores
+``setlocale(getlocale())`` round-trips on modern glibc systems where
+``en_IN`` is UTF-8-only.
diff --git a/Tools/i18n/makelocalealias.py b/Tools/i18n/makelocalealias.py
@@ -154,6 +154,9 @@ def check(data):
     # Hardcode 'c.utf8' -> 'C.UTF-8' because 'en_US.UTF-8' does not exist
     # on all platforms.
     data['c.utf8'] = 'C.UTF-8'
+    # Hardcode 'en_in' -> 'en_IN.UTF-8'.  X11 locale.alias still maps en_IN to
+    # ISO8859-1 and would otherwise override glibc's en_IN/UTF-8 (gh-151316).
+    data['en_in'] = 'en_IN.UTF-8'
     while True:
         # Repeat optimization while the size is decreased.
         n = len(data)
