Issue #20875: Prevent possible gzip "'read' is not defined" NameError.

Patch by Claudiu Popa.

Lib/gzip.py

@@ -99,7 +99,7 @@ class _PaddedFile:
             self._read -= len(prepend)
             return
         else:
-            self._buffer = self._buffer[read:] + prepend
+            self._buffer = self._buffer[self._read:] + prepend
         self._length = len(self._buffer)
         self._read = 0
 

Lib/test/test_gzip.py

@@ -396,6 +396,13 @@ class TestGzip(BaseTest):
         with gzip.GzipFile(fileobj=io.BytesIO(gzdata)) as f:
             self.assertEqual(f.read(), b'Test')
 
+    def test_prepend_error(self):
+        # See issue #20875
+        with gzip.open(self.filename, "wb") as f:
+            f.write(data1)
+        with gzip.open(self.filename, "rb") as f:
+            f.fileobj.prepend()
+
 class TestOpen(BaseTest):
     def test_binary_modes(self):
         uncompressed = data1 * 50

Misc/NEWS

@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #20875: Prevent possible gzip "'read' is not defined" NameError.
+  Patch by Claudiu Popa.
+
 - Issue #20283: RE pattern methods now accept the string keyword parameters
   as documented.  The pattern and source keyword parameters are left as
   deprecated aliases.