Confusing HierarchyElement.__str__()

[mattwthompson]

I'm trying to investigate whether I or the toolkit is losing track of my residue information when I'm doing a conversion, and I've found that I've been led astray by the __repr__. It's called here; self.identifier not being set seems concerning, but they're there when I ask for them. Any ideas what I'm doing wrong here?

In [1]: from openff.toolkit import Molecule

In [2]: water = Molecule.from_smiles("O")

In [3]: water.add_hierarchy_scheme(("residue_name", "residue_number"), "residues")
Out[3]: HierarchyScheme with uniqueness_criteria '('residue_name', 'residue_number')', iterator_name 'residues', and 1 elements

In [4]: residue = water.residues[-1]

In [5]: str(residue)
Out[5]: "HierarchyElement ('None', 'None') of iterator 'residues' containing 3 atom(s)"

In [6]: residue.residue_name = "FOO"

In [7]: residue.residue_number = '7'

In [11]: water.residues[0].residue_name
Out[11]: 'FOO'

In [12]: str(residue)
Out[12]: "HierarchyElement ('None', 'None') of iterator 'residues' containing 3 atom(s)"

In [13]: residue.residue_number
Out[13]: '7'