don't write orig_nbformat to disk in v3

IPython/nbformat/v3/nbjson.py

@@ -1,72 +1,76 b''
 """Read and write notebooks in JSON format.
 
 Authors:
 
 * Brian Granger
 """
 
 #-----------------------------------------------------------------------------
 #  Copyright (C) 2008-2011  The IPython Development Team
 #
 #  Distributed under the terms of the BSD License.  The full license is in
 #  the file COPYING, distributed as part of this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 import copy
 import json
 
 from .nbbase import from_dict
 from .rwbase import (
     NotebookReader, NotebookWriter, restore_bytes, rejoin_lines, split_lines
 )
 
 from IPython.utils import py3compat
 
 #-----------------------------------------------------------------------------
 # Code
 #-----------------------------------------------------------------------------
 
 class BytesEncoder(json.JSONEncoder):
     """A JSON encoder that accepts b64 (and other *ascii*) bytestrings."""
     def default(self, obj):
         if isinstance(obj, bytes):
             return obj.decode('ascii')
         return json.JSONEncoder.default(self, obj)
 
 
 class JSONReader(NotebookReader):
 
     def reads(self, s, **kwargs):
         nb = json.loads(s, **kwargs)
         nb = self.to_notebook(nb, **kwargs)
         return nb
 
     def to_notebook(self, d, **kwargs):
         return rejoin_lines(from_dict(d))
 
 
 class JSONWriter(NotebookWriter):
 
     def writes(self, nb, **kwargs):
         kwargs['cls'] = BytesEncoder
         kwargs['indent'] = 1
         kwargs['sort_keys'] = True
         kwargs['separators'] = (',',': ')
+        nb = copy.deepcopy(nb)
+        # don't write transient values to disk
+        for key in ('orig_nbformat', 'orig_nbformat_minor'):
+            nb.pop(key, None)
         if kwargs.pop('split_lines', True):
-            nb = split_lines(copy.deepcopy(nb))
+            nb = split_lines(nb)
         return py3compat.str_to_unicode(json.dumps(nb, **kwargs), 'utf-8')
     
 
 _reader = JSONReader()
 _writer = JSONWriter()
 
 reads = _reader.reads
 read = _reader.read
 to_notebook = _reader.to_notebook
 write = _writer.write
 writes = _writer.writes