docs: def map(self, value=NONE):
          """
          Executes the steps used to "map" this fields value from `ctx.src` to a
          value.
  
          :param value: optional **pre-computed** value.
  
          :return: The successfully mapped value or:
  
              - NONE if one was not found
              - ERROR if the field was present in `ctx.src` but invalid.
  
          """
          with self.ctx(field=self, parent=self):
              value = self._map(value)
          if self.attach_parent and value not in IGNORE:
              if hasattr(self.ctx, 'parent'):
                  value.parent = weakref.proxy(self.ctx.parent)
              else:
                  value.parent = None
          return value