old_contents: from grow import Preprocessor
  from protorpc import messages
  
  
  class CustomPreprocessor(Preprocessor):
      KIND = 'custom_preprocessor'
  
      class Config(messages.Message):
          value = messages.StringField(1)
  
      def run(self, **kwargs):
          # To allow the test to check the result
          self.pod._custom_preprocessor_value = self.config.value
  
new_contents: import grow
  from protorpc import messages
  
  
  class CustomPreprocessor(grow.Preprocessor):
      KIND = 'custom_preprocessor'
  
      class Config(messages.Message):
          value = messages.StringField(1)
  
      def run(self, **kwargs):
          # To allow the test to check the result
          self.pod._custom_preprocessor_value = self.config.value
  
subject: Update preprocessor testdata to use grow.Preprocessor.