Helper for _AddMessageMethods().

def _AddHasFieldMethod(message_descriptor, cls):
  """Helper for _AddMessageMethods()."""

  is_proto3 = (message_descriptor.syntax == "proto3")
  error_msg = _Proto3HasError if is_proto3 else _Proto2HasError

  hassable_fields = {}
  for field in message_descriptor.fields:
    if field.label == _FieldDescriptor.LABEL_REPEATED:
      continue
    # For proto3, only submessages and fields inside a oneof have presence.
    if (is_proto3 and field.cpp_type != _FieldDescriptor.CPPTYPE_MESSAGE and
        not field.containing_oneof):
      continue
    hassable_fields[field.name] = field

  if not is_proto3:
    # Fields inside oneofs are never repeated (enforced by the compiler).
    for oneof in message_descriptor.oneofs:
      hassable_fields[oneof.name] = oneof

  def HasField(self, field_name):
    try:
      field = hassable_fields[field_name]
    except KeyError:
      raise ValueError(error_msg % field_name)

    if isinstance(field, descriptor_mod.OneofDescriptor):
      try:
        return HasField(self, self._oneofs[field].name)
      except KeyError:
        return False
    else:
      if field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE:
        value = self._fields.get(field)
        return value is not None and value._is_present_in_parent
      else:
        return field in self._fields

  cls.HasField = HasField