I have noticed an issue regarding get_items() method of datatypes.Composition class.
The scenario:
If you append any objects into a composite datatype and you later need to call get_items() (within the same execution thread),
you get an exception.
After looking into it, the problem occurs because get_items() expects object ids instead of instances.
Maybe a check could be included to see if the value is a string or an instance and act accordingly
Thomas