US model import fails offline due to import-time PyPI lookup

[MaxGhenis]

Summary

The US model performs a live PyPI metadata lookup at import time (src/policyengine/tax_benefit_models/us/model.py:52), which breaks offline, firewalled, and sandboxed environments. The UK model handles this gracefully with a try/except fallback (src/policyengine/tax_benefit_models/uk/model.py:45).

Impact

• import policyengine.tax_benefit_models.us fails without network access
• This cascades to test collection (tests/conftest.py imports US fixtures globally), so even unit tests that don't need US model data can't run offline
• The paper's inline code example and the README smoke test both fail in no-network environments
• A JOSS reviewer flagged this as a major issue when reviewing in a sandboxed environment

Suggested fix

1. Make the US metadata lookup fault-tolerant, mirroring the UK implementation pattern
2. Consider lazy-loading the US model metadata (fetch on first use, not at import time)
3. Move heavyweight US imports in tests/conftest.py and tests/fixtures/filtering_fixtures.py inside test functions or fixtures so offline unit tests can still collect

References

• UK graceful handling: src/policyengine/tax_benefit_models/uk/model.py:45
• US import-time lookup: src/policyengine/tax_benefit_models/us/model.py:52
• Test collection dependency: tests/conftest.py:4, tests/fixtures/filtering_fixtures.py:11