Is the variable ONLY a sum of other variables?
│
├─ YES → Use `adds` attribute (NO formula needed!)
│         adds = ["var1", "var2"]
│
└─ NO → Use `add()` function in formula
         (when you need max_, where, conditions, etc.)

class variable_name(Variable):
    value_type = float
    entity = Entity
    definition_period = PERIOD

    # Option 1: List of variables
    adds = ["variable1", "variable2", "variable3"]

    # Option 2: Parameter tree path
    adds = "gov.path.to.parameter.list"

class tanf_gross_earned_income(Variable):
    value_type = float
    entity = SPMUnit
    label = "TANF gross earned income"
    unit = USD
    definition_period = MONTH

    adds = ["employment_income", "self_employment_income"]
    # NO formula needed! Automatically:
    # 1. Gets each person's employment_income
    # 2. Gets each person's self_employment_income
    # 3. Sums all values across SPM unit members

class income_tax_refundable_credits(Variable):
    value_type = float
    entity = TaxUnit
    definition_period = YEAR

    adds = "gov.irs.credits.refundable"
    # Parameter file contains list like:
    # - earned_income_tax_credit
    # - child_tax_credit
    # - additional_child_tax_credit

class count_eligible_people(Variable):
    value_type = int
    entity = SPMUnit
    definition_period = YEAR

    adds = ["is_eligible_person"]
    # Automatically sums True (1) and False (0) across members

from policyengine_us.model_api import *

def formula(entity, period, parameters):
    result = add(entity, period, variable_list)

class adjusted_earned_income(Variable):
    value_type = float
    entity = SPMUnit
    definition_period = MONTH

    def formula(spm_unit, period, parameters):
        # Need max_() to clip negative values
        gross = add(spm_unit, period, ["employment_income", "self_employment_income"])
        return max_(0, gross)  # Prevent negative income

class household_benefits(Variable):
    value_type = float
    entity = Household
    definition_period = YEAR

    def formula(household, period, parameters):
        # Sum existing benefits
        BENEFITS = ["snap", "tanf", "ssi", "social_security"]
        existing = add(household, period, BENEFITS)

        # Add new benefit conditionally
        new_benefit = household("special_benefit", period)
        p = parameters(period).gov.special_benefit

        if p.include_in_total:
            return existing + new_benefit
        return existing

class total_deductions(Variable):
    value_type = float
    entity = TaxUnit
    definition_period = YEAR

    def formula(tax_unit, period, parameters):
        p = parameters(period).gov.irs.deductions

        # Get standard deductions using parameter list
        standard = add(tax_unit, period, p.standard_items)

        # Apply phase-out logic
        income = tax_unit("adjusted_gross_income", period)
        phase_out_rate = p.phase_out_rate
        phase_out_start = p.phase_out_start

        reduction = max_(0, (income - phase_out_start) * phase_out_rate)
        return max_(0, standard - reduction)

# WRONG - Never do this!
def formula(spm_unit, period, parameters):
    person = spm_unit.members
    employment = person("employment_income", period)
    self_emp = person("self_employment_income", period)
    return spm_unit.sum(employment + self_emp)  # ❌ BAD

# RIGHT - Clean and simple
adds = ["employment_income", "self_employment_income"]  # ✅ GOOD

# WRONG - Unnecessary complexity
def formula(spm_unit, period, parameters):
    return add(spm_unit, period, ["income1", "income2"])  # ❌ Overkill

# RIGHT - Simpler
adds = ["income1", "income2"]  # ✅ GOOD

class household_total_income(Variable):
    entity = Household  # Higher-level entity
    definition_period = YEAR

    adds = ["employment_income", "self_employment_income"]
    # employment_income is defined for Person (lower-level)
    # PolicyEngine automatically:
    # 1. Gets employment_income for each person in household
    # 2. Gets self_employment_income for each person
    # 3. Sums all values to household level

description: List of refundable tax credits
values:
  2024-01-01:
    - earned_income_tax_credit
    - child_tax_credit
    - additional_child_tax_credit

adds = "gov.irs.credits.refundable"
# Automatically sums all credits in the list