PolicyEngine Testing Patterns

Comprehensive patterns and standards for creating PolicyEngine tests.

Quick Reference

File Structure

policyengine_us/tests/policy/baseline/gov/states/[state]/[agency]/[program]/
├── [variable_name].yaml       # Unit test for specific variable
├── [another_variable].yaml    # Another unit test
└── integration.yaml           # Integration test (NEVER prefixed)

Period Restrictions

• ✅ 2024-01 - First month only
• ✅ 2024 - Whole year
• ❌ 2024-04 - Other months NOT supported
• ❌ 2024-01-01 - Full dates NOT supported

Naming Convention

• Files: variable_name.yaml (matches variable exactly)
• Integration: Always integration.yaml (never prefixed)
• Cases: Case 1, description. (numbered, comma, period)
• People: person1, person2 (never descriptive names)

---

1. Test File Organization

File Naming Rules

Unit tests - Named after the variable they test:

✅ CORRECT:
az_liheap_eligible.yaml    # Tests az_liheap_eligible variable
az_liheap_benefit.yaml      # Tests az_liheap_benefit variable

❌ WRONG:
test_az_liheap.yaml         # Wrong prefix
liheap_tests.yaml           # Wrong pattern

Integration tests - Always named integration.yaml:

✅ CORRECT:
integration.yaml            # Standard name

❌ WRONG:
az_liheap_integration.yaml  # Never prefix integration
program_integration.yaml    # Never prefix integration

Folder Structure

Follow state/agency/program hierarchy:

gov/
└── states/
    └── [state_code]/
        └── [agency]/
            └── [program]/
                ├── eligibility/
                │   └── income_eligible.yaml
                ├── income/
                │   └── countable_income.yaml
                └── integration.yaml

---

2. Period Format Restrictions

Critical: Only Two Formats Supported

PolicyEngine test system ONLY supports:

• 2024-01 - First month of year
• 2024 - Whole year

Never use:

• 2024-04 - April (will fail)
• 2024-10 - October (will fail)
• 2024-01-01 - Full date (will fail)

Handling Mid-Year Policy Changes

If policy changes April 1, 2024:

# Option 1: Test with first month
period: 2024-01  # Tests January with new policy

# Option 2: Test next year
period: 2025-01  # When policy definitely active

---

3. Test Naming Conventions

Case Names

Use numbered cases with descriptions:

✅ CORRECT:
- name: Case 1, single parent with one child.
- name: Case 2, two parents with two children.
- name: Case 3, income at threshold.

❌ WRONG:
- name: Single parent test
- name: Test case for family
- name: Case 1 - single parent  # Wrong punctuation

Person Names

Use generic sequential names:

✅ CORRECT:
people:
  person1:
    age: 30
  person2:
    age: 10
  person3:
    age: 8

❌ WRONG:
people:
  parent:
    age: 30
  child1:
    age: 10

Output Format

Use simplified format without entity key:

✅ CORRECT:
output:
  tx_tanf_eligible: true
  tx_tanf_benefit: 250

❌ WRONG:
output:
  tx_tanf_eligible:
    spm_unit: true  # Don't nest under entity

---

4. Which Variables Need Tests

Variables That DON'T Need Tests

Skip tests for simple composition variables using only adds or subtracts:

# NO TEST NEEDED - just summing
class tx_tanf_countable_income(Variable):
    adds = ["earned_income", "unearned_income"]

# NO TEST NEEDED - simple arithmetic
class net_income(Variable):
    adds = ["gross_income"]
    subtracts = ["deductions"]

Variables That NEED Tests

Create tests for variables with:

• Conditional logic (where, select, if)
• Calculations/transformations
• Business logic
• Deductions/disregards
• Eligibility determinations

# NEEDS TEST - has logic
class tx_tanf_income_eligible(Variable):
    def formula(spm_unit, period, parameters):
        return where(enrolled, passes_test, other_test)

---

5. Period Conversion in Tests

Complete Input/Output Rules

The key rule: Input matches the larger of (variable period, test period). Output matches the test period.

| Variable Def | Test Period | Input Value | Output Value | |--------------|-------------|-------------|--------------| | YEAR | YEAR | Yearly | Yearly | | YEAR | MONTH | Yearly (always!) | Monthly (÷12) | | MONTH | YEAR | Yearly (÷12 per month) | Yearly (sum of 12) | | MONTH | MONTH | Monthly | Monthly |

YEAR Variable Examples

# YEAR variable + YEAR period
- name: Case 1, yearly test.
  period: 2024
  input:
    employment_income: 12_000  # Yearly input
  output:
    employment_income: 12_000  # Yearly output

# YEAR variable + MONTH period
- name: Case 2, monthly test with yearly variable.
  period: 2024-01
  input:
    employment_income: 12_000  # Still yearly input!
  output:
    employment_income: 1_000   # Monthly output (12_000/12)

MONTH Variable Examples

# MONTH variable + YEAR period
- name: Case 3, yearly test with monthly variable.
  period: 2024
  input:
    some_monthly_var: 1_200  # Yearly total (divided by 12 = 100/month)
  output:
    some_monthly_var: 1_200  # Yearly sum

# MONTH variable + MONTH period
- name: Case 4, monthly test with monthly variable.
  period: 2024-01
  input:
    some_monthly_var: 100  # Monthly input (just January)
  output:
    some_monthly_var: 100  # Monthly output

Formula Design: Never Multiply by 12

Critical: Let PolicyEngine handle period conversion automatically. Never use * MONTHS_IN_YEAR or / 12 in formulas.

# ✅ CORRECT - PolicyEngine handles conversion
class my_yearly_benefit(Variable):
    definition_period = YEAR

    def formula(tax_unit, period, parameters):
        monthly_premium = tax_unit("slcsp", period)  # Auto-sums 12 months
        return where(eligible, monthly_premium, 0)   # No * 12!

# ❌ WRONG - Double counting
class my_yearly_benefit(Variable):
    definition_period = YEAR

    def formula(tax_unit, period, parameters):
        monthly_premium = tax_unit("slcsp", period)  # Already summed!
        return where(eligible, monthly_premium * MONTHS_IN_YEAR, 0)  # Bug!

How PolicyEngine Converts Periods

When a YEAR formula calls a MONTH variable:

my_yearly_benefit<2024> calls slcsp<2024>
  → slcsp<2024-01> = 500
  → slcsp<2024-02> = 500
  → ... (all 12 months)
  → slcsp<2024> = 6000 (sum)

When a MONTH formula calls a YEAR variable:

my_monthly_calc<2024-01> calls employment_income<2024-01>
  → employment_income<2024> = 12000
  → employment_income<2024-01> = 1000 (12000/12)

Summary

• Input: Match the variable's definition_period (or yearly if test period is YEAR)
• Output: Always matches the test period
• Formulas: Never manually multiply/divide by 12

---

6. Numeric Formatting

Always Use Underscore Separators

✅ CORRECT:
employment_income: 50_000
cash_assets: 1_500

❌ WRONG:
employment_income: 50000
cash_assets: 1500

---

7. Integration Test Quality Standards

Inline Calculation Comments

Document every calculation step:

- name: Case 2, earnings with deductions.
  period: 2025-01
  input:
    people:
      person1:
        employment_income: 3_000  # $250/month
  output:
    # Person-level arrays
    tx_tanf_gross_earned_income: [250, 0]
    # Person1: 3,000/12 = 250

    tx_tanf_earned_after_disregard: [87.1, 0]
    # Person1: 250 - 120 = 130
    # Disregard: 130/3 = 43.33
    # After: 130 - 43.33 = 86.67 ≈ 87.1

Comprehensive Scenarios

Include 5-7 scenarios covering:

1. Basic eligible case
2. Earnings with deductions
3. Edge case at threshold
4. Mixed enrollment status
5. Special circumstances (SSI, immigration)
6. Ineligible case

Verify Intermediate Values

Check 8-10 values per test:

output:
  # Income calculation chain
  program_gross_income: 250
  program_earned_after_disregard: 87.1
  program_deductions: 200
  program_countable_income: 0

  # Eligibility chain
  program_income_eligible: true
  program_resources_eligible: true
  program_eligible: true

  # Final benefit
  program_benefit: 320

---

8. Common Variables to Use

Always Available

# Demographics
age: 30
is_disabled: false
is_pregnant: false

# Income
employment_income: 50_000
self_employment_income: 10_000
social_security: 12_000
ssi: 9_000

# Benefits
snap: 200
tanf: 150
medicaid: true

# Location
state_code: CA
county_code: "06037"  # String for FIPS

Variables That DON'T Exist

Never use these (not in PolicyEngine):

• heating_expense
• utility_expense
• utility_shut_off_notice
• past_due_balance
• bulk_fuel_amount
• weatherization_needed

---

9. Enum Verification

Always Check Actual Enum Values

Before using enums in tests:

# Find enum definition
grep -r "class ImmigrationStatus" --include="*.py"

# Check actual values
class ImmigrationStatus(Enum):
    CITIZEN = "Citizen"
    LEGAL_PERMANENT_RESIDENT = "Legal Permanent Resident"  # NOT "PERMANENT_RESIDENT"
    REFUGEE = "Refugee"

✅ CORRECT:
immigration_status: LEGAL_PERMANENT_RESIDENT

❌ WRONG:
immigration_status: PERMANENT_RESIDENT  # Doesn't exist

---

10. Test Quality Checklist

Before submitting tests:

• [ ] All variables exist in PolicyEngine
• [ ] Period format is 2024-01 or 2024 only
• [ ] Numbers use underscore separators
• [ ] Integration tests have calculation comments
• [ ] 5-7 comprehensive scenarios in integration.yaml
• [ ] Enum values verified against actual definitions
• [ ] Output values realistic, not placeholders
• [ ] File names match variable names exactly

---

Common Test Patterns

Income Eligibility

- name: Case 1, income exactly at threshold.
  period: 2024-01
  input:
    people:
      person1:
        employment_income: 30_360  # Annual limit
  output:
    program_income_eligible: true  # At threshold = eligible

Priority Groups

- name: Case 2, elderly priority.
  period: 2024-01
  input:
    people:
      person1:
        age: 65
  output:
    program_priority_group: true

Categorical Eligibility

- name: Case 3, SNAP categorical.
  period: 2024-01
  input:
    spm_units:
      spm_unit:
        snap: 200  # Receives SNAP
  output:
    program_categorical_eligible: true

---

For Agents

When creating tests:

1. Check existing variables before using any in tests
2. Use only supported periods (2024-01 or 2024)
3. Document calculations in integration tests
4. Verify enum values against actual code
5. Follow naming conventions exactly
6. Include edge cases at thresholds
7. Test realistic scenarios not placeholders