add missing NBFormatError import in nbformat.reader.reads

opening unreadable notebook format failed with 'NameError: NBFormatError'
instead of the expected error.

IPython/nbformat/reader.py

@@ -1,107 +1,109 b''
 """API for reading notebooks.
 
 Authors:
 
 * Jonathan Frederic
 """
 
 #-----------------------------------------------------------------------------
 #  Copyright (C) 2013  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 json
 
 from . import v1
 from . import v2
 from . import v3
 
 versions = {
     1: v1,
     2: v2,
     3: v3,
     }
 
 #-----------------------------------------------------------------------------
 # Code
 #-----------------------------------------------------------------------------
 
 class NotJSONError(ValueError):
     pass
 
 def parse_json(s, **kwargs):
     """Parse a JSON string into a dict."""
     try:
         nb_dict = json.loads(s, **kwargs)
     except ValueError:
         # Limit the error message to 80 characters.  Display whatever JSON will fit.
         raise NotJSONError(("Notebook does not appear to be JSON: %r" % s)[:77] + "...")
     return nb_dict
 
 # High level API
 
 def get_version(nb):
     """Get the version of a notebook.
 
     Parameters
     ----------
     nb : dict
         NotebookNode or dict containing notebook data.
 
     Returns
     -------
     Tuple containing major (int) and minor (int) version numbers
     """
     major = nb.get('nbformat', 1)
     minor = nb.get('nbformat_minor', 0)
     return (major, minor)
 
 
 def reads(s, **kwargs):
     """Read a notebook from a json string and return the 
     NotebookNode object.
 
     This function properly reads notebooks of any version.  No version 
     conversion is performed.
 
     Parameters
     ----------
     s : unicode
         The raw unicode string to read the notebook from.
 
     Returns
     -------
     nb : NotebookNode
         The notebook that was read.
     """
+    from .current import NBFormatError
+    
     nb_dict = parse_json(s, **kwargs)
     (major, minor) = get_version(nb_dict)
     if major in versions:
         return versions[major].to_notebook_json(nb_dict, minor=minor)
     else:
         raise NBFormatError('Unsupported nbformat version %s' % major)
 
 
 def read(fp, **kwargs):
     """Read a notebook from a file and return the NotebookNode object.
 
     This function properly reads notebooks of any version.  No version 
     conversion is performed.
 
     Parameters
     ----------
     fp : file
         Any file-like object with a read method.
 
     Returns
     -------
     nb : NotebookNode
         The notebook that was read.
     """
     return reads(fp.read(), **kwargs)