old_contents: from IPython.config.loader import Config
  from IPython.config.application import catch_config_error
  from IPython.utils.traitlets import Unicode
  from nbgrader.apps.customnbconvertapp import CustomNbConvertApp
  
  class SolutionApp(CustomNbConvertApp):
      
      name = Unicode(u'nbgrader-solution')
      description = Unicode(u'Prepare a solution version of an assignment')
  
      def _export_format_default(self):
          return 'notebook'
  
      def build_extra_config(self):
          self.extra_config = Config()
          self.extra_config.Exporter.preprocessors = [
              'nbgrader.preprocessors.IncludeHeaderFooter',
              'nbgrader.preprocessors.TableOfContents',
              'nbgrader.preprocessors.RenderSolutions',
              'nbgrader.preprocessors.ExtractTests',
              'IPython.nbconvert.preprocessors.ExecutePreprocessor'
          ]
          self.extra_config.RenderSolutions.solution = True
          self.config.merge(self.extra_config)
  
new_contents: from IPython.config.loader import Config
  from IPython.config.application import catch_config_error
  from IPython.utils.traitlets import Unicode
  from nbgrader.apps.customnbconvertapp import CustomNbConvertApp
  
  class SolutionApp(CustomNbConvertApp):
      
      name = Unicode(u'nbgrader-solution')
      description = Unicode(u'Prepare a solution version of an assignment')
  
      def _export_format_default(self):
          return 'notebook'
  
      def build_extra_config(self):
          self.extra_config = Config()
          self.extra_config.Exporter.preprocessors = [
              'nbgrader.preprocessors.IncludeHeaderFooter',
              'nbgrader.preprocessors.TableOfContents',
              'nbgrader.preprocessors.RenderSolutions',
              'nbgrader.preprocessors.ExtractTests',
              'IPython.nbconvert.preprocessors.ExecutePreprocessor'
          ]
          self.extra_config.RenderSolutions.solution = True
          self.extra_config.NbGraderApp.writer_class = 'IPython.nbconvert.writers.FilesWriter'
          self.config.merge(self.extra_config)
  
subject: Add files writer to solution app