gh-159422: Eagerly import warnings module to failure of GzipFile.__del__ during finalization

Lib/gzip.py

@@ -11,6 +11,7 @@
 import struct
 import sys
 import time
+import warnings  # must stay eager to support finalization
 import weakref
 import zlib
 from compression._common import _streams
@@ -229,7 +230,6 @@ def __init__(self, filename=None, mode=None,
 
             elif mode.startswith(('w', 'a', 'x')):
                 if origmode is None:
-                    import warnings
                     warnings.warn(
                         "GzipFile was opened for writing, but this will "
                         "change in future Python releases.  "
@@ -466,13 +466,15 @@ def readline(self, size=-1):
         self._check_not_closed()
         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__()
+    def __del__(self, _warn=warnings.warn):
+        try:
+            # Wrap within a try to ensure the file is closed even if the
+            # warning raises, e.g. when ResourceWarning is made an error.
+            if self.mode == WRITE and not self.closed:
+                _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

@@ -9,9 +9,10 @@
 import struct
 import sys
 import unittest
+import warnings
 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 +966,19 @@ def test_refloop_unraisable(self):
                 gc.collect()
                 self.assertIsNone(cm.unraisable)
 
+    def test_del_closes_if_warning_fails(self):
+        # ResourceWarning escalated to an error is reported as unraisable,
+        # and the file is still closed.
+        b = io.BytesIO()
+        f = gzip.GzipFile(fileobj=b, mode="wb")
+        f.write(data1)
+        with (warnings.catch_warnings(action="error", category=ResourceWarning),
+              catch_unraisable_exception() as cm):
+            del f
+            gc_collect()
+            self.assertIs(cm.unraisable.exc_type, ResourceWarning)
+        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 +1,2 @@
+Fix :class:`gzip.GzipFile` not being closed at interpreter shutdown when
+emitting its :exc:`ResourceWarning` failed with an :exc:`ImportError`.