feat: Update Kids model and calculate cost of kids

- Update `Kids` model in `config.py` to include `fraction_of_spending` and `years_of_support` properties.
- Add new function `_calc_cost_of_kids` in `state_change.py` to calculate the cost of children based on the current date, spending, and `Kids` configuration.
- Implement tests in `test_state_change.py` to validate the calculation of the cost of kids.
- Add new field "Kids" in the `Results` class in `simulator.py` to display the cost of children in the simulation results.
- Update the sample configuration file `full_config.yml` to reflect the changes in the `Kids` model.

app/models/config.py

@@ -361,7 +361,8 @@ class Kids(BaseModel):
         birth_years (list[float])
     """
 
-    cost_of_kid: float
+    fraction_of_spending: float
+    years_of_support: int
     birth_years: list[float]
 
 

app/models/financial/state_change.py

@@ -8,7 +8,7 @@
 import numpy as np
 from app import util
 from app.data.constants import INTERVALS_PER_YEAR
-from app.models.config import Spending
+from app.models.config import Kids, Spending
 from app.models.financial.state import State
 from app.models.controllers import Controllers
 
@@ -37,8 +37,8 @@ def __float__(self):
 
 @dataclass
 class _Costs(util.FloatRepr):
-    spending: float = -1
-    kids: float = -1
+    spending: float
+    kids: float
     income_tax: float = -1
     medicare_tax: float = -1
     ss_tax: float = -1
@@ -64,6 +64,27 @@ def _calc_spending(state: State, config: Spending, is_working: bool) -> float:
     return base_amount
 
 
+def _calc_cost_of_kids(current_date: float, spending: float, config: Kids) -> float:
+    """Calculate the cost of children
+
+    Args:
+        current_date (float): date of state
+        spending (float): base spending in current state
+        config (Kids)
+
+    Returns:
+        float: cost of children for this interval
+    """
+    if config is None:
+        return 0
+    current_kids = [
+        year
+        for year in config.birth_years
+        if current_date - config.years_of_support < year <= current_date
+    ]
+    return len(current_kids) * spending * config.fraction_of_spending
+
+
 @dataclass
 class _NetTransactions(util.FloatRepr):
     income: _Income
@@ -97,12 +118,16 @@ def __init__(self, state: State, controllers: Controllers):
         )
 
         income = _Income(state, controllers)
+        spending = _calc_spending(
+            state=state,
+            config=state.user.spending,
+            is_working=(controllers.job_income.is_working(state.interval_idx)),
+        )
         costs = _Costs(
-            spending=_calc_spending(
-                state=state,
-                config=state.user.spending,
-                is_working=(controllers.job_income.is_working(state.interval_idx)),
-            )
+            spending=spending,
+            kids=_calc_cost_of_kids(
+                current_date=state.date, spending=spending, config=state.user.kids
+            ),
         )
         annuity = controllers.annuity.make_annuity_transaction(
             state=state,

app/models/simulator.py

@@ -124,6 +124,7 @@ def as_dataframes(self) -> list[pd.DataFrame]:
             "Pension",
             "Total Income",
             "Spending",
+            "Kids",
             "Total Costs",
             "Portfolio Return",
             "Annuity",
@@ -142,6 +143,7 @@ def as_dataframes(self) -> list[pd.DataFrame]:
                     interval.state_change_components.net_transactions.income.pension,
                     interval.state_change_components.net_transactions.income,
                     interval.state_change_components.net_transactions.costs.spending,
+                    interval.state_change_components.net_transactions.costs.kids,
                     interval.state_change_components.net_transactions.costs,
                     interval.state_change_components.net_transactions.portfolio_return,
                     interval.state_change_components.net_transactions.annuity,

tests/models/financial/test_state_change.py

@@ -4,9 +4,9 @@
 
 import pytest
 from app.data.constants import INTERVALS_PER_YEAR
-from app.models.config import Spending
+from app.models.config import Kids, Spending
 from app.models.financial.state import State
-from app.models.financial.state_change import _calc_spending
+from app.models.financial.state_change import _calc_cost_of_kids, _calc_spending
 
 
 class TestCalcSpending:
@@ -40,3 +40,51 @@ def test_after_working(self, first_state: State):
             * self.inflation
             * (1 + self.retirement_change)
         )
+
+
+class TestCalcCostOfKids:
+    spending = -100
+    cost_of_each_kid = -20
+    current_date = 2020
+    years_of_support = 18
+    birth_years = None
+    config = None
+
+    def calc_cost(self):
+        """Helper function to calculate the cost of kids"""
+        config = Kids(
+            fraction_of_spending=self.cost_of_each_kid / self.spending,
+            birth_years=self.birth_years,
+            years_of_support=self.years_of_support,
+        )
+        return _calc_cost_of_kids(
+            current_date=self.current_date,
+            spending=self.spending,
+            config=config,
+        )
+
+    def test_one_kid(self):
+        """Test that the cost of one kid is calculated correctly"""
+        self.birth_years = [2018]
+        cost_of_kid = self.calc_cost()
+        assert cost_of_kid == pytest.approx(self.cost_of_each_kid)
+
+    def test_multiple_kids(self):
+        """Test that the cost of multiple kids is calculated correctly"""
+        self.birth_years = [2018, 2019]
+        cost_of_kids = self.calc_cost()
+        assert cost_of_kids == pytest.approx(
+            len(self.birth_years) * self.cost_of_each_kid
+        )
+
+    def test_kid_not_born_yet(self):
+        """Test that the cost of a kid not born yet is zero"""
+        self.birth_years = [self.current_date + 1]
+        cost_of_kids = self.calc_cost()
+        assert cost_of_kids == pytest.approx(0)
+
+    def test_kid_too_old(self):
+        """Test that the cost of a kid that is older than `years_of_support` is zero"""
+        self.birth_years = [self.current_date - (self.years_of_support + 1)]
+        cost_of_kids = self.calc_cost()
+        assert cost_of_kids == pytest.approx(0)

tests/sample_configs/full_config.yml

@@ -63,7 +63,8 @@ spending:
 state: California
 
 kids:
-  cost_of_kid: 0.12
+  fraction_of_spending: 0.12
+  years_of_support: 18
   birth_years:
     - 2021
     - 2026