Pandas 3.0: TypeError: unhashable type 'StringArray' in parameter_node_at_instant.py

[MaxGhenis]

Bug Description

When using pandas 3.0, StringArray objects can be passed to ParameterNodeAtInstant.__getitem__, but StringArray is not hashable and can't be used as a dictionary key.

Error

TypeError: unhashable type: 'StringArray'

at policyengine_core/parameters/parameter_node_at_instant.py:61:

return self._children[key]

Root Cause

In pandas 3.0, string columns use StringDtype by default, producing StringArray objects instead of numpy object arrays. The check on line 57 only handles numpy.ndarray:

if isinstance(key, numpy.ndarray):
    return parameters.VectorialParameterNodeAtInstant.build_from_node(self)[key]
return self._children[key]  # Fails if key is StringArray

Proposed Fix

Also check for pandas ExtensionArrays or convert StringArray to numpy array before using as dict key:

import pandas as pd

if isinstance(key, (numpy.ndarray, pd.api.extensions.ExtensionArray)):
    return parameters.VectorialParameterNodeAtInstant.build_from_node(self)[key]
return self._children[key]

Related

This is separate from #427 which fixed enum encoding with StringDtype Series.