docs: def _compute(self):
          """
          Processes this fields `src` from `ctx.src`.
          """
          src_path = self.ctx.src_path
          if not src_path.exists:
              return NONE
          if src_path.is_null:
              return None
          try:
              if self.parse:
                  value = self.parse(src_path)
              else:
                  value = self._parse(src_path)
              return value
          except (SourceError, ValueError), ex:
              self.ctx.errors.invalid(str(ex))
              return ERROR