Fix GzipFile's handling of filenames given as bytes objects.

Lib/gzip.py

@@ -159,9 +159,8 @@ class GzipFile(io.BufferedIOBase):
         if fileobj is None:
             fileobj = self.myfileobj = builtins.open(filename, mode or 'rb')
         if filename is None:
-            if hasattr(fileobj, 'name') and isinstance(fileobj.name, str):
-                filename = fileobj.name
-            else:
+            filename = getattr(fileobj, 'name', '')
+            if not isinstance(filename, (str, bytes)):
                 filename = ''
         if mode is None:
             if hasattr(fileobj, 'mode'): mode = fileobj.mode
@@ -236,7 +235,8 @@ class GzipFile(io.BufferedIOBase):
             # RFC 1952 requires the FNAME field to be Latin-1. Do not
             # include filenames that cannot be represented that way.
             fname = os.path.basename(self.name)
-            fname = fname.encode('latin-1')
+            if not isinstance(fname, bytes):
+                fname = fname.encode('latin-1')
             if fname.endswith(b'.gz'):
                 fname = fname[:-3]
         except UnicodeEncodeError:

Lib/test/test_gzip.py

@@ -331,6 +331,20 @@ class TestGzip(unittest.TestCase):
             with gzip.GzipFile(fileobj=f, mode="w") as g:
                 pass
 
+    def test_bytes_filename(self):
+        str_filename = self.filename
+        try:
+            bytes_filename = str_filename.encode("ascii")
+        except UnicodeEncodeError:
+            self.skipTest("Temporary file name needs to be ASCII")
+        with gzip.GzipFile(bytes_filename, "wb") as f:
+            f.write(data1 * 50)
+        with gzip.GzipFile(bytes_filename, "rb") as f:
+            self.assertEqual(f.read(), data1 * 50)
+        # Sanity check that we are actually operating on the right file.
+        with gzip.GzipFile(str_filename, "rb") as f:
+            self.assertEqual(f.read(), data1 * 50)
+
     # Testing compress/decompress shortcut functions
 
     def test_compress(self):

Misc/NEWS

@@ -70,6 +70,8 @@ Core and Builtins
 Library
 -------
 
+- Fix GzipFile's handling of filenames given as bytes objects.
+
 - Issue #15101: Make pool finalizer avoid joining current thread.
 
 - Issue #15036: Mailbox no longer throws an error if a flush is done