[3.15] gh-161086: Honor --fast and --best for file arguments in gzip CLI (GH-161087) (#161118)

The file branch of gzip.main() did not pass the compression level to
gzip.open(), so only stdin input was affected by --fast/--best.

(cherry picked from commit 8a316ff)

Author: gpshead

Lib/gzip.py

@@ -762,7 +762,7 @@ def main():
                              compresslevel=compresslevel)
             else:
                 f = builtins.open(arg, "rb")
-                g = open(arg + ".gz", "wb")
+                g = open(arg + ".gz", "wb", compresslevel=compresslevel)
         while True:
             chunk = f.read(READ_BUFFER_SIZE)
             if not chunk:

Lib/test/test_gzip.py

@@ -1218,7 +1218,8 @@ def test_compress_infile_outfile_default(self):
 
     @create_and_remove_directory(TEMPDIR)
     def test_compress_infile_outfile(self):
-        for compress_level in ('--fast', '--best'):
+        # The XFL header byte records the compression level used.
+        for compress_level, xfl in (('--fast', 4), ('--best', 2)):
             with self.subTest(compress_level=compress_level):
                 local_testgzip = os.path.join(TEMPDIR, 'testgzip')
                 gzipname = local_testgzip + '.gz'
@@ -1232,6 +1233,8 @@ def test_compress_infile_outfile(self):
                 self.assertTrue(os.path.exists(gzipname))
                 self.assertEqual(out, b'')
                 self.assertEqual(err, b'')
+                with open(gzipname, 'rb') as fp:
+                    self.assertEqual(fp.read()[8], xfl)
                 os.remove(gzipname)
                 self.assertFalse(os.path.exists(gzipname))
 

Misc/NEWS.d/next/Library/2026-10-22-05-24-34.gh-issue-161086.FGPjqA.rst

@@ -0,0 +1,2 @@
+Fix the :mod:`gzip` command-line interface ignoring :option:`gzip --fast`
+and :option:`gzip --best` when compressing files given as arguments.