old_contents: """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
          return cell, resources
  
new_contents: """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
          return cell, resources
  
subject: Use cell.prompt_number rather than cell['prompt_number']