code: def _init(self):
          '''
          Prepare to run (if not ready)
          '''
          num = 0
          for field in self._fields:
              field._initialize()
              self._need_second_pass |= field._need_second_pass
          for field in self._fields:
              num += field.num_mutations()
          self._calculate_mutations(num)
          self._initialize_default_buffer(f _init(self):
          '''
          Prepare to run (if not ready)
          '''
          num = 0
          for field in self._fields:
              field._initialize()
              self._need_second_pass |= field._need_second_pass
          for field in self._fields:
              num += field.num_mutations()
          self._calculate_mutations(num)
          self._initialize_default_buffer()
docs: Prepare to run (if not ready)