code: def resolve_or_missing(self, key):
          
          if self._legacy_resolve_mode:
              rv = self.resolve(key)
              if isinstance(rv, Undefined):
                  rv = missing
              return rv
          return resolve_or_missing(self, key)
docs: Resolves a variable like :meth:`resolve` but returns the
          special `missing` value if it cannot be found.