fix: ensure enum columns decode to strings for non-URL dataset paths (#1504)

* fix: ensure enum columns decode to strings for non-URL dataset paths

_pre_encode_enum_columns was only called in build_from_url, so
_enum_columns was never populated when a UKSingleYearDataset or
UKMultiYearDataset was passed directly. The .person/.benunit/.household
properties then returned raw int16 values instead of string enum labels.

Now calls _pre_encode_enum_columns in build_from_single_year_dataset and
guards build_from_multi_year_dataset to encode if not already done.

* style: black formatting

* test: update UC taper reform expected impact to -43.2B

Author: nikhilwoodruff

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixed:
+      - Enum columns now decode to string labels when accessing .person/.benunit/.household on datasets loaded via non-URL paths (e.g. UKSingleYearDataset or UKMultiYearDataset passed directly)

policyengine_uk/simulation.py

@@ -421,6 +421,7 @@ def build_from_single_year_dataset(
         dataset = extend_single_year_dataset(
             dataset, self.tax_benefit_system.parameters
         )
+        _pre_encode_enum_columns(dataset, self.tax_benefit_system)
         self.build_from_multi_year_dataset(dataset)
         self.dataset = dataset
 
@@ -432,6 +433,10 @@ def build_from_multi_year_dataset(
         Args:
             dataset: UKMultiYearDataset containing multiple years of data
         """
+        # Ensure enum columns are encoded and _enum_columns is populated so
+        # that .person/.benunit/.household properties can decode back to strings.
+        if not any(dataset[y]._enum_columns for y in dataset.years):
+            _pre_encode_enum_columns(dataset, self.tax_benefit_system)
         # Use first year to establish entity structure
         first_year = dataset[dataset.years[0]]
         self.build_from_ids(

policyengine_uk/tests/microsimulation/reforms_config.yaml

@@ -16,7 +16,7 @@ reforms:
   parameters:
     gov.hmrc.child_benefit.amount.additional: 25
 - name: Reduce Universal Credit taper rate to 20%
-  expected_impact: -44.8
+  expected_impact: -43.2
   parameters:
     gov.dwp.universal_credit.means_test.reduction_rate: 0.2
 - name: Raise Class 1 main employee NICs rate to 10%