Household-impact tests returning 0 (resolved)

What went wrong¶

policyengine-core’s changelog for 3.24.4 describes the bug: its cache invalidation after apply_reform “wiped every variable’s _memory_storage._arrays, which also wiped user-provided dataset inputs loaded via set_input. Country-package subclasses calling set_input during construction and then applying a structural reform (the policyengine-uk pattern) silently lost their datasets.” In the failing tests, inputs such as age, employment_income and would_claim_* were gone before any calculation ran, so tax and benefits came out as zero.

Two other explanations were considered and do not fit the harness code at policyengine.py#280’s head (6772ecc):

• The dataset-required change (policyengine-uk 2.76.0, where Simulation() with no situation, no dataset and no POLICYENGINE_UK_DEFAULT_DATASET raises a ValueError). The harness passed an explicit dataset (Microsimulation(dataset=...) in src/policyengine/tax_benefit_models/uk/model.py), so it never reached that path.

• The default period (default_input_period and default_calculation_period, 2025 in both 2.74.0 and 2.88.0). The harness passed period= to every calculation, and the failing tests built 2026 households.

Reproduction from the UK side¶

Two comparable scenarios, run directly against policyengine-uk main for 2025 (the failing tests used 2026 and a one-adult, two-child family):

from policyengine_uk import Simulation

YEAR = 2025

# Scenario 1: single adult with employment income.
s1 = {
    "people": {"adult": {"age": {YEAR: 30}, "employment_income": {YEAR: 50_000}}},
    "benunits": {"benunit": {"members": ["adult"]}},
    "households": {"household": {"members": ["adult"]}},
}
sim = Simulation(situation=s1)
print("income_tax:", sim.calculate("income_tax", YEAR).sum())
# -> 7486.00

# Scenario 2: couple with one child.
s2 = {
    "people": {
        "a1": {"age": {YEAR: 35}, "employment_income": {YEAR: 30_000}},
        "a2": {"age": {YEAR: 33}},
        "c1": {"age": {YEAR: 5}},
    },
    "benunits": {"benunit": {"members": ["a1", "a2", "c1"]}},
    "households": {"household": {"members": ["a1", "a2", "c1"]}},
}
sim = Simulation(situation=s2)
print("child_benefit:", sim.calculate("child_benefit", YEAR).sum())
# -> 1354.60

Re-checked on 23 October 2026 on policyengine-uk 2.148.2 (policyengine-core 3.33.2): both scenarios return £7,486 and £1,354.60. Both also return non-zero values for every year from 2022 to 2025. The income-tax figure is the same each year because the personal allowance (£12,570), the basic rate limit (£37,700) and the 20% basic rate are unchanged from 2021-22; child benefit follows the eldest-child rate (£1,133.60 in 2022, £1,248.00 in 2023, £1,331.20 in 2024).

References¶

• Issue: #1628.

• Where the failure showed: PolicyEngine/policyengine.py#280 (closed without merging, superseded by #282).

• The fix: PolicyEngine/policyengine-core#475 (core 3.24.4) and PolicyEngine/policyengine.py#283 (core floor 3.25.0).

• Default-period contract: policyengine_uk/simulation.py (default_input_period, default_calculation_period).

• Default-dataset env var contract: POLICYENGINE_UK_DEFAULT_DATASET, defined in policyengine_uk/tax_benefit_system.py and read in policyengine_uk/simulation.py.