code: def _parse_default(self):
          
          # the configuration changes, so we invalidate the cached config
          self._full_config = None
  
          if self.scheme:
              self._default.update(self.scheme.build_defaults())
docs: Parse the `Schema` for the `Bison` instance to create the set of
          default values.
  
          If no defaults are specified in the `Schema`, the default dictionary
          will not contain anything.