Use cell.prompt_number rather than cell['prompt_number']

IPython/nbconvert/preprocessors/clearoutput.py

@@ -1,28 +1,28 b''
 """Module containing a preprocessor that removes the outputs from code cells"""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 from .base import Preprocessor
 
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
 class ClearOutputPreprocessor(Preprocessor):
     """
     Removes the output from all code cells in a notebook.
     """
 
     def preprocess_cell(self, cell, resources, cell_index):
         """
         Apply a transformation on each cell. See base.py for details.
         """
         if cell.cell_type == 'code':
             cell.outputs = []
-            cell['prompt_number'] = None
+            cell.prompt_number = None
         return cell, resources