Issue #23016: A warning no longer produces an AttributeError when sys.stderr

is None.

Lib/test/test_warnings.py

@@ -560,6 +560,15 @@ class _WarningsTests(BaseTest):
         finally:
             globals_dict['__file__'] = oldfile
 
+    def test_stderr_none(self):
+        rc, stdout, stderr = assert_python_ok("-c",
+            "import sys; sys.stderr = None; "
+            "import warnings; warnings.simplefilter('always'); "
+            "warnings.warn('Warning!')")
+        self.assertEqual(stdout, b'')
+        self.assertNotIn(b'Warning!', stderr)
+        self.assertNotIn(b'Error', stderr)
+
 
 class WarningsDisplayTests(unittest.TestCase):
 

Lib/warnings.py

@@ -26,6 +26,9 @@ def _show_warning(message, category, filename, lineno, file=None, line=None):
     """Hook to write a warning to a file; replace if you like."""
     if file is None:
         file = sys.stderr
+        if file is None:
+            # sys.stderr is None - warnings get lost
+            return
     try:
         file.write(formatwarning(message, category, filename, lineno, line))
     except IOError:

Misc/NEWS

@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #23016: A warning no longer produces an AttributeError when sys.stderr
+  is None.
+
 - Issue #14099: ZipFile.open() no longer reopen the underlying file.  Objects
   returned by ZipFile.open() can now operate independently of the ZipFile even
   if the ZipFile was created by passing in a file-like object as the first