fill missing metadata when upgrading v2->v3

IPython/nbformat/v3/convert.py

@@ -1,88 +1,91 b''
 """Code for converting notebooks to and from the v2 format."""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 from .nbbase import (
     new_code_cell, new_text_cell, new_worksheet, new_notebook, new_output,
     nbformat, nbformat_minor
 )
 
 from IPython.nbformat import v2
 
 def _unbytes(obj):
     """There should be no bytes objects in a notebook
 
     v2 stores png/jpeg as b64 ascii bytes
     """
     if isinstance(obj, dict):
         for k,v in obj.items():
             obj[k] = _unbytes(v)
     elif isinstance(obj, list):
         for i,v in enumerate(obj):
             obj[i] = _unbytes(v)
     elif isinstance(obj, bytes):
         # only valid bytes are b64-encoded ascii
         obj = obj.decode('ascii')
     return obj
 
 def upgrade(nb, from_version=2, from_minor=0):
     """Convert a notebook to v3.
 
     Parameters
     ----------
     nb : NotebookNode
         The Python representation of the notebook to convert.
     from_version : int
         The original version of the notebook to convert.
     from_minor : int
         The original minor version of the notebook to convert (only relevant for v >= 3).
     """
     if from_version == 2:
         # Mark the original nbformat so consumers know it has been converted.
         nb.nbformat = nbformat
         nb.nbformat_minor = nbformat_minor
         
         nb.orig_nbformat = 2
         nb = _unbytes(nb)
+        for ws in nb['worksheets']:
+            for cell in ws['cells']:
+                cell.setdefault('metadata', {})
         return nb
     elif from_version == 3:
         if from_minor != nbformat_minor:
             nb.orig_nbformat_minor = from_minor
         nb.nbformat_minor = nbformat_minor
         return nb
     else:
         raise ValueError('Cannot convert a notebook directly from v%s to v3.  ' \
                 'Try using the IPython.nbformat.convert module.' % from_version)
  
 
 def heading_to_md(cell):
     """turn heading cell into corresponding markdown"""
     cell.cell_type = "markdown"
     level = cell.pop('level', 1)
     cell.source = '#'*level + ' ' + cell.source
  
  
 def raw_to_md(cell):
     """let raw passthrough as markdown"""
     cell.cell_type = "markdown"
  
 
 def downgrade(nb):
     """Convert a v3 notebook to v2.
 
     Parameters
     ----------
     nb : NotebookNode
         The Python representation of the notebook to convert.
     """
     if nb.nbformat != 3:
         return nb
     nb.nbformat = 2
     for ws in nb.worksheets:
         for cell in ws.cells:
             if cell.cell_type == 'heading':
                 heading_to_md(cell)
             elif cell.cell_type == 'raw':
                 raw_to_md(cell)
     return nb
  No newline at end of file