Don't set LANG during locale coercion

It turns out setting onlyLC_CTYPE is sufficient, which improves cross-platform consistency and reduces the chance of unintended side effects.
ncoghlan · Jun 11, 2017 · Mar 5, 2017 · Mar 11, 2017 · Mar 11, 2017 · Mar 13, 2017
commit 939ba0a77d4b52a04315c129f9db89b90c0532cd
diff --git a/Lib/test/test_c_locale_coercion.py b/Lib/test/test_c_locale_coercion.py
@@ -16,12 +16,7 @@
 
 # In order to get the warning messages to match up as expected, the candidate
 # order here must much the target locale order in Python/pylifecycle.c
-_C_UTF8_LOCALES = (
-    # Entries: (Target locale, expected env var updates)
-    ("C.UTF-8", "LC_CTYPE & LANG"),
-    ("C.utf8", "LC_CTYPE & LANG"),
-    ("UTF-8", "LC_CTYPE"),
-)
+_C_UTF8_LOCALES = ("C.UTF-8", "C.utf8", "UTF-8")
 
 # There's no reliable cross-platform way of checking locale alias
 # lists, so the only way of knowing which of these locales will work
@@ -144,7 +139,7 @@ def test_library_c_locale_warning(self):
 
 # Details of the CLI locale coercion warning emitted at runtime
 CLI_COERCION_WARNING_FMT = (
-    "Python detected LC_CTYPE=C: {} coerced to {} (set another locale "
+    "Python detected LC_CTYPE=C: LC_CTYPE coerced to {} (set another locale "
     "or PYTHONCOERCECLOCALE=0 to disable this locale coercion behavior)."
 )
 
@@ -156,21 +151,19 @@ class _LocaleCoercionTargetsTestCase(_ChildProcessEncodingTestCase):
 
     @classmethod
     def setUpClass(cls):
-        first_target_locale = first_env_updates = None
+        first_target_locale = None
         available_targets = cls.available_targets
         # Find the target locales available in the current system
-        for target_locale, env_updates in _C_UTF8_LOCALES:
+        for target_locale in _C_UTF8_LOCALES:
             if _set_locale_in_subprocess(target_locale):
                 available_targets.append(target_locale)
                 if first_target_locale is None:
                     first_target_locale = target_locale
-                    first_env_updates = env_updates
         if cls.targets_required and not available_targets:
             raise unittest.SkipTest("No C-with-UTF-8 locale available")
         # Expect coercion to use the first available locale
-        cls.EXPECTED_COERCION_WARNING = CLI_COERCION_WARNING_FMT.format(
-            first_env_updates, first_target_locale
-        )
+        warning_msg = CLI_COERCION_WARNING_FMT.format(first_target_locale)
+        cls.EXPECTED_COERCION_WARNING = warning_msg
 
 
 class LocaleConfigurationTests(_LocaleCoercionTargetsTestCase):

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
@@ -357,14 +357,13 @@ _Py_LegacyLocaleDetected(void)
 
 typedef struct _CandidateLocale {
     const char *locale_name; /* The locale to try as a coercion target */
-    int set_LANG;            /* Whether to set LANG in addition to LC_CTYPE */
 } _LocaleCoercionTarget;
 
 static _LocaleCoercionTarget _TARGET_LOCALES[] = {
-    { "C.UTF-8", 1 },
-    { "C.utf8",  1},
-    { "UTF-8", 0 },
-    { NULL, 0 }
+    {"C.UTF-8"},
+    {"C.utf8"},
+    {"UTF-8"},
+    {NULL}
 };
 
 static char *
@@ -392,33 +391,24 @@ get_default_standard_stream_error_handler(void)
 
 #ifdef PY_COERCE_C_LOCALE
 static const char *_C_LOCALE_COERCION_WARNING =
-    "Python detected LC_CTYPE=C: %.20s coerced to %.20s (set another locale "
+    "Python detected LC_CTYPE=C: LC_CTYPE coerced to %.20s (set another locale "
     "or PYTHONCOERCECLOCALE=0 to disable this locale coercion behavior).\n";
 
 static void
 _coerce_default_locale_settings(const _LocaleCoercionTarget *target)
 {
-    const char *env_vars_updated = "LC_CTYPE";
     const char *newloc = target->locale_name;
 
     /* Reset locale back to currently configured defaults */
     setlocale(LC_ALL, "");
 
-    /* Set the relevant locale environment variables */
+    /* Set the relevant locale environment variable */
     if (setenv("LC_CTYPE", newloc, 1)) {
         fprintf(stderr,
                 "Error setting LC_CTYPE, skipping C locale coercion\n");
         return;
     }
-    if (target->set_LANG) {
-        if (setenv("LANG", newloc, 1) == 0) {
-            env_vars_updated = "LC_CTYPE & LANG";
-        } else {
-            fprintf(stderr,
-                    "Error setting LANG during C locale coercion\n");
-        }
-    }
-    fprintf(stderr, _C_LOCALE_COERCION_WARNING, env_vars_updated, newloc);
+    fprintf(stderr, _C_LOCALE_COERCION_WARNING, newloc);
 
     /* Reconfigure with the overridden environment variables */
     setlocale(LC_ALL, "");