Index: Lib/zipfile.py
===================================================================
--- Lib/zipfile.py	(revision 82847)
+++ Lib/zipfile.py	(working copy)
@@ -660,7 +660,7 @@
         self.compression = compression  # Method of compression
         self.mode = key = mode.replace('b', '')[0]
         self.pwd = None
-        self.comment = ''
+        self.comment = self._original_comment = ''
 
         # Check if we were passed a file-like object
         if isinstance(file, basestring):
@@ -725,6 +725,7 @@
         size_cd = endrec[_ECD_SIZE]             # bytes in central directory
         offset_cd = endrec[_ECD_OFFSET]         # offset of central directory
         self.comment = endrec[_ECD_COMMENT]     # archive comment
+        self._original_comment = self.comment
 
         # "concat" is zero, unless zip was concatenated to another file
         concat = endrec[_ECD_LOCATION] - size_cd - offset_cd
@@ -777,7 +778,6 @@
             if self.debug > 2:
                 print "total", total
 
-
     def namelist(self):
         """Return a list of file names in the archive."""
         l = []
@@ -1116,6 +1116,9 @@
         if self.fp is None:
             return
 
+        if self.comment != self._original_comment:
+            self._didModify = True
+
         if self.mode in ("w", "a") and self._didModify: # write ending records
             count = 0
             pos1 = self.fp.tell()
@@ -1210,11 +1213,13 @@
                 self.comment = self.comment[:ZIP_MAX_COMMENT]
 
             endrec = struct.pack(structEndArchive, stringEndArchive,
-                                 0, 0, centDirCount, centDirCount,
-                                 centDirSize, centDirOffset, len(self.comment))
+                                 0, 0, centDirCount, centDirCount, centDirSize,
+                                 centDirOffset, len(self.comment))
             self.fp.write(endrec)
             self.fp.write(self.comment)
             self.fp.flush()
+            if len(self.comment) < len(self._original_comment):
+                self.fp.truncate()
 
         if not self._filePassed:
             self.fp.close()
Index: Lib/test/test_zipfile.py
===================================================================
--- Lib/test/test_zipfile.py	(revision 82847)
+++ Lib/test/test_zipfile.py	(working copy)
@@ -864,6 +864,24 @@
         with zipfile.ZipFile(TESTFN, mode="r") as zipf:
             self.assertEqual(zipf.comment, comment2)
 
+        # check that comments are correctly modified in append mode
+        with zipfile.ZipFile(TESTFN,mode="w") as zipf:
+            zipf.comment = "original comment"
+            zipf.writestr("foo.txt", "O, for a Muse of Fire!")
+        with zipfile.ZipFile(TESTFN,mode="a") as zipf:
+            zipf.comment = "an updated comment"
+        with zipfile.ZipFile(TESTFN,mode="r") as zipf:
+            self.assertEqual(zipf.comment, "an updated comment")
+
+        # check that comments are correctly shortened in append mode
+        with zipfile.ZipFile(TESTFN,mode="w") as zipf:
+            zipf.comment = "original comment that's longer"
+            zipf.writestr("foo.txt", "O, for a Muse of Fire!")
+        with zipfile.ZipFile(TESTFN,mode="a") as zipf:
+            zipf.comment = "shorter comment"
+        with zipfile.ZipFile(TESTFN,mode="r") as zipf:
+            self.assertEqual(zipf.comment, "shorter comment")
+
     def tearDown(self):
         unlink(TESTFN)
         unlink(TESTFN2)