Two problems...

1. There is no way to encode Code with explicit empty scope to BSON type 0x0F. Code with explicit empty scope (Code("", {})) is always stored as BSON type 0x0D.
2. json_util always exports Code with $scope.

The solution is probably to set bson.code.Code.__scope to None when Code was created without an explicit scope, then have the bson encoder and json_util treat None and {} separately. Also add a warning in the changelog, since bson.code.Code.scope currently always returns a dict.