PolicyEngine Code Writing Style Guide

Essential patterns for writing clean, efficient PolicyEngine formulas.

Core Principles

1. Eliminate unnecessary intermediate variables
2. Use direct parameter/variable access
3. Return directly when possible
4. Combine boolean logic
5. Use correct period access (period vs period.this_year)
6. NO hardcoded values - use parameters or constants

---

Pattern 1: Direct Parameter Access

❌ Bad - Unnecessary intermediate variable

def formula(spm_unit, period, parameters):
    countable = spm_unit("tn_tanf_countable_resources", period)
    p = parameters(period).gov.states.tn.dhs.tanf.resource_limit
    resource_limit = p.amount  # ❌ Unnecessary
    return countable <= resource_limit

✅ Good - Direct access

def formula(spm_unit, period, parameters):
    countable = spm_unit("tn_tanf_countable_resources", period)
    p = parameters(period).gov.states.tn.dhs.tanf.resource_limit
    return countable <= p.amount

---

Pattern 2: Direct Return

❌ Bad - Unnecessary result variable

def formula(spm_unit, period, parameters):
    assets = spm_unit("spm_unit_assets", period.this_year)
    p = parameters(period).gov.states.tn.dhs.tanf.resource_limit
    vehicle_exemption = p.vehicle_exemption  # ❌ Unnecessary
    countable = max_(assets - vehicle_exemption, 0)  # ❌ Unnecessary
    return countable

✅ Good - Direct return

def formula(spm_unit, period, parameters):
    assets = spm_unit("spm_unit_assets", period.this_year)
    p = parameters(period).gov.states.tn.dhs.tanf.resource_limit
    return max_(assets - p.vehicle_exemption, 0)

---

Pattern 3: Combined Boolean Logic

❌ Bad - Too many intermediate booleans

def formula(spm_unit, period, parameters):
    person = spm_unit.members
    age = person("age", period.this_year)
    is_disabled = person("is_disabled", period.this_year)

    caretaker_is_60_or_older = spm_unit.any(age >= 60)  # ❌ Unnecessary
    caretaker_is_disabled = spm_unit.any(is_disabled)   # ❌ Unnecessary
    eligible = caretaker_is_60_or_older | caretaker_is_disabled  # ❌ Unnecessary

    return eligible

✅ Good - Combined logic

def formula(spm_unit, period, parameters):
    person = spm_unit.members
    age = person("age", period.this_year)
    is_disabled = person("is_disabled", period.this_year)

    return spm_unit.any((age >= 60) | is_disabled)

---

Pattern 4: Period Access - period vs period.this_year

# ❌ Bad
age = person("age", period)  # Gives age/12
monthly_income = person("employment_income", period.this_year) / MONTHS_IN_YEAR  # Redundant

# ✅ Good
age = person("age", period.this_year)  # Gets actual age
monthly_income = person("employment_income", period)  # Auto-converts to monthly

• Income/flows → Use period (want monthly from annual)
• Age/assets/counts/booleans → Use period.this_year (don't divide by 12)

See policyengine-period-patterns skill for the full decision tree and more examples.

---

Pattern 5: No Hardcoded Values

❌ Bad - Hardcoded numbers

def formula(spm_unit, period, parameters):
    size = spm_unit.nb_persons()
    capped_size = min_(size, 10)  # ❌ Hardcoded

    age = person("age", period.this_year)
    income = person("income", period) / 12  # ❌ Use MONTHS_IN_YEAR

    # ❌ Hardcoded thresholds
    if age >= 18 and age <= 65 and income < 2000:
        return True

✅ Good - Parameterized

def formula(spm_unit, period, parameters):
    p = parameters(period).gov.program
    capped_size = min_(spm_unit.nb_persons(), p.max_unit_size)  # ✅

    age = person("age", period.this_year)
    monthly_income = person("income", period)  # ✅ Auto-converts (no manual /12)

    age_eligible = (age >= p.age_min) & (age <= p.age_max)  # ✅
    income_eligible = monthly_income < p.income_threshold  # ✅

    return age_eligible & income_eligible

Use Framework Constants, Not Custom Parameters

PolicyEngine provides constants for universal conversion factors. Don't create parameters for these:

# ❌ BAD — created a weeks_per_month.yaml parameter:
weekly_subsidy * p.weeks_per_month

# ✅ GOOD — use framework constants:
weekly_subsidy * (WEEKS_IN_YEAR / MONTHS_IN_YEAR)

Available constants: MONTHS_IN_YEAR (12), WEEKS_IN_YEAR (52). Derive others from these.

CRITICAL: WEEKS_IN_YEAR is the integer 52, not 52.1429. Never use WEEKS_IN_YEAR * 7 for days per year — that gives 364, not 365. Use the literal 365 for days-per-year calculations.

Regulatory conversion factors take precedence. When a regulation cites a specific factor (e.g., "multiply by 4.3"), use that exact value even if a framework constant is close (WEEKS_IN_YEAR / MONTHS_IN_YEAR ≈ 4.333). The discrepancy may affect benefit amounts at boundary conditions.

---

Pattern 6: Streamline Variable Access

❌ Bad - Redundant steps

def formula(spm_unit, period, parameters):
    unit_size = spm_unit.nb_persons()  # ❌ Unnecessary
    max_size = 10  # ❌ Hardcoded
    capped_size = min_(unit_size, max_size)

    p = parameters(period).gov.states.tn.dhs.tanf.benefit
    spa = p.standard_payment_amount[capped_size]  # ❌ Unnecessary
    dgpa = p.differential_grant_payment_amount[capped_size]  # ❌ Unnecessary

    eligible = spm_unit("eligible_for_dgpa", period)
    return where(eligible, dgpa, spa)

✅ Good - Streamlined

def formula(spm_unit, period, parameters):
    p = parameters(period).gov.states.tn.dhs.tanf.benefit
    capped_size = min_(spm_unit.nb_persons(), p.max_unit_size)
    eligible = spm_unit("eligible_for_dgpa", period)

    return where(
        eligible,
        p.differential_grant_payment_amount[capped_size],
        p.standard_payment_amount[capped_size]
    )

---

When to Keep Intermediate Variables

✅ Keep when value is used multiple times

def formula(tax_unit, period, parameters):
    p = parameters(period).gov.irs.credits
    filing_status = tax_unit("filing_status", period)

    # ✅ Used multiple times - keep as variable
    threshold = p.phase_out.start[filing_status]

    income = tax_unit("adjusted_gross_income", period)
    excess = max_(0, income - threshold)
    reduction = (excess / p.phase_out.width) * threshold

    return max_(0, threshold - reduction)

✅ Keep when calculation is complex

def formula(spm_unit, period, parameters):
    p = parameters(period).gov.program
    gross_earned = spm_unit("gross_earned_income", period)

    # ✅ Complex multi-step calculation - break it down
    work_expense_deduction = min_(gross_earned * p.work_expense_rate, p.work_expense_max)
    after_work_expense = gross_earned - work_expense_deduction

    earned_disregard = after_work_expense * p.earned_disregard_rate
    countable_earned = after_work_expense - earned_disregard

    dependent_care = spm_unit("dependent_care_expenses", period)

    return max_(0, countable_earned - dependent_care)

✅ Break out complex expressions inside function calls

Don't inline complex calculations inside where(), max_(), or other function calls - give them descriptive names.

# ❌ BAD - Complex expression inlined in where()
return where(
    above_trigger,
    reduced_payment,
    max_(maximum_benefit - countable_income, 0),  # Hard to read
)

# ✅ GOOD - Break out into named variable
standard_payment = max_(maximum_benefit - countable_income, 0)
return where(
    above_trigger,
    reduced_payment,
    standard_payment,  # Clear what this represents
)

Another example:

# ❌ BAD - Multiple complex inlined expressions
return where(
    income > add(spm_unit, period, ["earned", "unearned"]) * p.rate,
    max_(benefit - (income * p.reduction_rate), 0),
    benefit,
)

# ✅ GOOD - Named variables explain the logic
gross_income = add(spm_unit, period, ["earned", "unearned"])
income_threshold = gross_income * p.rate
reduced_benefit = max_(benefit - (income * p.reduction_rate), 0)

return where(
    income > income_threshold,
    reduced_benefit,
    benefit,
)

Rule: If it's more than a simple variable or parameter access, give it a name.

---

Pattern 8: Use add() > 0 Instead of spm_unit.any()

When checking if ANY member has a boolean property, use add() > 0 instead of spm_unit.members + spm_unit.any().

# ❌ LESS PREFERRED - verbose pattern:
person = spm_unit.members
has_citizen = spm_unit.any(
    person("is_citizen_or_legal_immigrant", period)
)

# ✅ BETTER - cleaner add() > 0 pattern:
immigration_eligible = add(spm_unit, period, ["is_citizen_or_legal_immigrant"]) > 0

Why this is better:

• Avoids intermediate person = spm_unit.members variable
• Consistent with add() patterns used elsewhere
• More descriptive variable name (immigration_eligible vs has_citizen)
• Single line instead of multiple

More examples:

# Check if any member is disabled
has_disabled_member = add(spm_unit, period, ["is_disabled"]) > 0

# Check if any member is elderly
has_elderly_member = add(spm_unit, period, ["is_elderly"]) > 0

# Check if any child is present
has_child = add(spm_unit, period, ["is_child"]) > 0

---

Pattern 9: Use Existing SPMUnit-Level Variables Directly

If an spm_unit-level variable exists, use it directly. Don't access through person level.

# ❌ BAD - Unnecessary person-level access when spm_unit variable exists:
person = spm_unit.members
demographic = person("is_person_demographic_tanf_eligible", period)
# Then aggregating back to spm_unit...

# ✅ GOOD - Use the spm_unit-level variable directly:
demographic_eligible = spm_unit("is_demographic_tanf_eligible", period)

Before writing code, check:

1. Does an spm_unit-level variable already exist?
2. If yes, use spm_unit("variable_name", period) directly
3. Only use spm_unit.members when you need person-level data that must be aggregated

# ✅ GOOD - Using existing spm_unit variables:
income_eligible = spm_unit("ar_tea_income_eligible", period)
resource_eligible = spm_unit("ar_tea_resource_eligible", period)
demographic_eligible = spm_unit("is_demographic_tanf_eligible", period)

return income_eligible & resource_eligible & demographic_eligible

---

Complete Example: Before vs After

❌ Before - Multiple Issues

def formula(person, period, parameters):
    # Wrong period access
    age = person("age", period)  # ❌ age/12
    assets = person("assets", period)  # ❌ assets/12
    annual_income = person("employment_income", period.this_year)
    monthly_income = annual_income / 12  # ❌ Use MONTHS_IN_YEAR

    # Hardcoded values
    min_age = 18  # ❌
    max_age = 64  # ❌
    asset_limit = 10000  # ❌
    income_limit = 2000  # ❌

    # Unnecessary intermediate variables
    age_check = (age >= min_age) & (age <= max_age)
    asset_check = assets <= asset_limit
    income_check = monthly_income <= income_limit
    eligible = age_check & asset_check & income_check

    return eligible

✅ After - Clean and Correct

def formula(person, period, parameters):
    p = parameters(period).gov.program.eligibility

    # Correct period access
    age = person("age", period.this_year)
    assets = person("assets", period.this_year)
    monthly_income = person("employment_income", period)

    # Direct return with combined logic
    return (
        (age >= p.age_min) & (age <= p.age_max) &
        (assets <= p.asset_limit) &
        (monthly_income <= p.income_threshold)
    )

---

Pattern 7: Minimal Comments

Code Should Be Self-Documenting

Variable names and structure should explain the code - not comments.

❌ Bad - Verbose explanatory comments

def formula(spm_unit, period, parameters):
    # Wisconsin disregards all earned income of dependent children (< 18)
    # Calculate earned income for adults only
    is_adult = spm_unit.members("age", period.this_year) >= 18  # Hard-coded!
    adult_earned = spm_unit.sum(
        spm_unit.members("tanf_gross_earned_income", period) * is_adult
    )

    # All unearned income is counted (including children's)
    gross_unearned = add(spm_unit, period, ["tanf_gross_unearned_income"])

    # NOTE: Wisconsin disregards many additional income sources that
    # are not separately tracked in PolicyEngine (educational aid, etc.)
    return max_(total_income - disregards, 0)

✅ Good - Clean self-documenting code

def formula(spm_unit, period, parameters):
    p = parameters(period).gov.states.wi.dcf.tanf.income

    is_adult = spm_unit.members("age", period.this_year) >= p.adult_age_threshold
    adult_earned = spm_unit.sum(
        spm_unit.members("tanf_gross_earned_income", period) * is_adult
    )
    gross_unearned = add(spm_unit, period, ["tanf_gross_unearned_income"])
    child_support = add(spm_unit, period, ["child_support_received"])

    return max_(adult_earned + gross_unearned - child_support, 0)

Comment Rules

1. NO comments explaining what code does - variable names should be clear
2. OK: Brief NOTE about implementation decisions (one line):

   # NOTE: Disregard rate varies by calendar month
   

3. NO multi-line explanations of what the code calculates

---

Pattern 10: Remove Redundant Operations

Don't cap what's already capped

When a formula algebraically guarantees result <= upper_bound, remove the redundant min_():

# ❌ BAD — redundant (payment_standard - positive_income ≤ payment_standard always):
benefit = min_(max_(payment_standard - countable_income, 0), payment_standard)

# ✅ GOOD — max_(x, 0) already ensures result ≤ payment_standard:
benefit = max_(payment_standard - countable_income, 0)

Only use min_() when the bound can actually bind (e.g., capping subsidy at actual expenses or provider charges).

---

Quick Checklist

Before finalizing code:

• No hardcoded numbers (use parameters or constants like MONTHS_IN_YEAR)
• Correct period access:
  • Income/flows use period
  • Age/assets/counts/booleans use period.this_year
• No single-use intermediate variables FOR SIMPLE VALUES
• BUT: Break out complex expressions inside where(), max_() into named variables
• Direct parameter access (p.amount not amount = p.amount)
• Direct returns when possible
• Combined boolean logic when possible
• Minimal comments (code should be self-documenting)
• No redundant min_() / max_() when algebraically guaranteed
• Framework constants used correctly (WEEKS_IN_YEAR = 52, not 52.14)
• Regulatory conversion factors used when regulation specifies an exact value