code: def mutate_file(backup, context):
      
      with open(context.filename) as f:
          code = f.read()
      context.source = code
      if backup:
          with open(context.filename + '.bak', 'w') as f:
              f.write(code)
      result, number_of_mutations_performed = mutate(context)
      with open(context.filename, 'w') as f:
          f.write(result)
      return number_of_mutations_performed
docs: :type backup: bool
      :type context: Context