Use `locale.getpreferredencoding(False)` if possible (most systems) Fix #436 Fix #479 (#528)

* Handle impossibility to set locale in tests
  (docker container scenario)
* `detect_encoding` start from non-destructive behavior
  (do not change locale)
  and restore exact locale in destructive

Co-authored-by: Aleksei Stepanov <alekseis@nvidia.com>

2 files changed, 39 insertions, 13 deletions

diff --git a/urwid/tests/test_util.py b/urwid/tests/test_util.py
index abe7aa5..eeae2f9 100644
--- a/urwid/tests/test_util.py
+++ b/urwid/tests/test_util.py
@@ -193,6 +193,7 @@ class RleTest(unittest.TestCase):
         self.assertListEqual(rle3, [('A', 10), ('B', 20)])
         self.assertListEqual(rle4, [('A', 10), ('B', 15), ('K', 1)])
 
+
 class PortabilityTest(unittest.TestCase):
     def test_locale(self):
         initial = locale.getlocale()
@@ -201,8 +202,28 @@ class PortabilityTest(unittest.TestCase):
         util.detect_encoding()
         self.assertEqual(locale.getlocale(), (None, None))
 
-        locale.setlocale(locale.LC_ALL, ('en_US', 'UTF-8'))
+        try:
+            locale.setlocale(locale.LC_ALL, ('en_US', 'UTF-8'))
+        except locale.Error as exc:
+            if "unsupported locale setting" not in str(exc):
+                raise
+
+            print(
+                f"Locale change impossible, probably locale not supported by system (libc ignores this error).\n"
+                f"{exc}"
+            )
+            return
+
         util.detect_encoding()
         self.assertEqual(locale.getlocale(), ('en_US', 'UTF-8'))
 
-        locale.setlocale(locale.LC_ALL, initial)
+        try:
+            locale.setlocale(locale.LC_ALL, initial)
+        except locale.Error as exc:
+            if "unsupported locale setting" not in str(exc):
+                raise
+
+            print(
+                f"Locale restore impossible, probably locale not supported by system (libc ignores this error).\n"
+                f"{exc}"
+            )
diff --git a/urwid/util.py b/urwid/util.py
index 3ac4657..9def762 100644
--- a/urwid/util.py
+++ b/urwid/util.py
@@ -37,28 +37,33 @@ move_prev_char = str_util.move_prev_char
 within_double_byte = str_util.within_double_byte
 
 
-def detect_encoding():
+def detect_encoding() -> str:
     # Try to determine if using a supported double-byte encoding
     import locale
-    initial = locale.getlocale()
+
+    no_set_locale = locale.getpreferredencoding(False)
+
+    if no_set_locale != "ascii":
+        # ascii is fallback locale in case of detect failed
+
+        return no_set_locale
+
+    # Use actual `getpreferredencoding` with public API only
+    old_loc = locale.setlocale(locale.LC_CTYPE)  # == getlocale, but not mangle data
     try:
         try:
-            locale.setlocale(locale.LC_ALL, "")
+            locale.setlocale(locale.LC_CTYPE, "")
         except locale.Error:
             pass
-        return locale.getlocale()[1] or ""
-    except ValueError as e:
-        # with invalid LANG value python will throw ValueError
-        if e.args and e.args[0].startswith("unknown locale"):
-            return ""
-        else:
-            raise
+        # internally call private `_get_locale_encoding`
+        return locale.getpreferredencoding(False)
     finally:
         try:
-            locale.setlocale(locale.LC_ALL, initial)
+            locale.setlocale(locale.LC_CTYPE, old_loc)
         except locale.Error:
             pass
 
+
 if 'detected_encoding' not in locals():
     detected_encoding = detect_encoding()
 else: