diff -r 7757e98a9f3b Lib/gzip.py
--- a/Lib/gzip.py	Tue Mar 17 06:56:11 2015 +0200
+++ b/Lib/gzip.py	Tue Mar 17 15:15:34 2015 +0100
@@ -336,15 +336,17 @@
 
         # Convert data type if called by io.BufferedWriter.
         if isinstance(data, memoryview):
-            data = data.tobytes()
+            length = data.nbytes
+        else:
+            length = len(data)
 
-        if len(data) > 0:
-            self.size = self.size + len(data)
+        if length > 0:
+            self.size = self.size + length
             self.crc = zlib.crc32(data, self.crc) & 0xffffffff
             self.fileobj.write( self.compress.compress(data) )
-            self.offset += len(data)
+            self.offset += length
 
-        return len(data)
+        return length
 
     def read(self, size=-1):
         self._check_closed()