gh-159422: Always close GzipFile in __del__ even if the warning fails

Lib/gzip.py

@@ -467,12 +467,13 @@ def readline(self, size=-1):
         return self._buffer.readline(size)
 
     def __del__(self):
-        if self.mode == WRITE and not self.closed:
-            import warnings
-            warnings.warn("unclosed GzipFile",
-                          ResourceWarning, source=self, stacklevel=2)
-
-        super().__del__()
+        try:
+            if self.mode == WRITE and not self.closed:
+                import warnings
+                warnings.warn("unclosed GzipFile",
+                              ResourceWarning, source=self, stacklevel=2)
+        finally:
+            super().__del__()
 
 def _read_exact(fp, n):
     '''Read exactly *n* bytes from `fp`

Lib/test/test_gzip.py

@@ -11,7 +11,7 @@
 import unittest
 from subprocess import PIPE, Popen
 from unittest import mock
-from test.support import catch_unraisable_exception
+from test.support import catch_unraisable_exception, gc_collect
 from test.support import force_not_colorized_test_class, import_helper
 from test.support import os_helper
 from test.support import _4G, bigmemtest, requires_subprocess
@@ -965,6 +965,19 @@ def test_refloop_unraisable(self):
                 gc.collect()
                 self.assertIsNone(cm.unraisable)
 
+    def test_del_closes_if_warning_fails(self):
+        # The file must still be closed if emitting the ResourceWarning
+        # raises, as can happen at interpreter shutdown.
+        b = io.BytesIO()
+        f = gzip.GzipFile(fileobj=b, mode="wb")
+        f.write(data1)
+        with (mock.patch("warnings.warn", side_effect=RuntimeError),
+              catch_unraisable_exception() as cm):
+            del f
+            gc_collect()
+            self.assertIs(cm.unraisable.exc_type, RuntimeError)
+        self.assertEqual(gzip.decompress(b.getvalue()), data1)
+
 
 class TestOpen(BaseTest):
     def test_binary_modes(self):

Misc/NEWS.d/next/Library/2026-10-14-15-50-59.gh-issue-159422.16mSqQ.rst

@@ -0,0 +1,3 @@
+Fix truncated output when a writable :class:`gzip.GzipFile` that was never
+closed is finalized and emitting its :exc:`ResourceWarning` fails, for
+example at interpreter shutdown. The file is now always closed.