Issue #13781: Fix GzipFile to work with os.fdopen()'d file objects.

Lib/gzip.py

@@ -156,8 +156,10 @@ 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'): filename = fileobj.name
-            else: filename = ''
+            if hasattr(fileobj, 'name') and isinstance(fileobj.name, str):
+                filename = fileobj.name
+            else:
+                filename = ''
         if mode is None:
             if hasattr(fileobj, 'mode'): mode = fileobj.mode
             else: mode = 'rb'

Lib/test/test_gzip.py

@@ -323,6 +323,14 @@ class TestGzip(unittest.TestCase):
             self.assertEqual(f.read(100), b'')
             self.assertEqual(nread, len(uncompressed))
 
+    def test_fileobj_from_fdopen(self):
+        # Issue #13781: Opening a GzipFile for writing fails when using a
+        # fileobj created with os.fdopen().
+        fd = os.open(self.filename, os.O_WRONLY | os.O_CREAT)
+        with os.fdopen(fd, "wb") as f:
+            with gzip.GzipFile(fileobj=f, mode="w") as g:
+                pass
+
     # Testing compress/decompress shortcut functions
 
     def test_compress(self):

Misc/NEWS

@@ -100,6 +100,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #13781: Fix GzipFile bug that caused an exception to be raised when
+  opening for writing using a fileobj returned by os.fdopen().
+
 - Issue #13803: Under Solaris, distutils doesn't include bitness
   in the directory name.