policyengine-testing-patterns

What this skill does

# 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

### Error Margin
Choose the margin based on the output type:
- **Boolean outputs** (`true`/`false`, eligibility, flags): **no error margin at all** — booleans are exact, no rounding. Omit `absolute_error_margin` entirely.
- **Currency outputs** (benefits, income, amounts): `absolute_error_margin: 0.01`
- **Rate/percentage outputs**: `absolute_error_margin: 0.001`
- **Never use 1** — a margin of 1 makes `true` (1) and `false` (0) indistinguishable, rendering the test meaningless

### 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:
```yaml
# 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:
```yaml
✅ 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
```

### Adding Cases to Existing Test Files

**CRITICAL: Always append new test cases at the bottom of the file.** Never insert cases in the middle of existing tests.

```yaml
# Existing file has Cases 1-3
# ✅ CORRECT - Add Case 4 at the bottom:
- name: Case 3, income above threshold.
  ...

- name: Case 4, new edge case scenario.
  ...

# ❌ WRONG - Inserting between existing cases and renumbering:
- name: Case 1, ...
- name: Case 2, new case inserted here.    # Renumbered!
- name: Case 3, was previously Case 2.     # Renumbered!
```

**Why:** Inserting in the middle forces renumbering of existing cases, which creates noisy diffs and makes review harder. Appending at the bottom keeps existing cases untouched.

### Person Names

Use generic sequential names:
```yaml
✅ 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:
```yaml
✅ 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`:
```python
# 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

```python
# 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

```yaml
# 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

```yaml
# 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
```

See **policyengine-period-patterns** skill for the full explanation of period auto-conversion.

---

## 6. Numeric Formatting

### Always Use Underscore Separators

```yaml
✅ 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:
```yaml
- 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:
```yaml
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

  # Fi