Include each variable's quantity type in the country metadata

changelog.d/metadata-variable-quantity-type.added.md

@@ -0,0 +1 @@
+Include each variable's quantity type ("stock" or "flow") in the `/{country_id}/metadata` variable descriptions as `quantityType`.

policyengine_api/country.py

@@ -350,6 +350,9 @@ def build_variables(self) -> dict:
                 "entity": variable.entity.key,
                 "valueType": variable.value_type.__name__,
                 "definitionPeriod": variable.definition_period,
+                # "stock" for a balance or value at one point in time, "flow"
+                # for an amount over a period.
+                "quantityType": variable.quantity_type,
                 "name": variable_name,
                 "label": variable.label,
                 "category": variable.category,

tests/unit/test_country.py

@@ -195,6 +195,39 @@ def get_population(self, entity_plural):
         )
 
 
+class TestVariableQuantityType:
+    """Each variable's metadata says whether it is a stock or a flow."""
+
+    @pytest.mark.parametrize(
+        ("country_id", "variable_name", "quantity_type"),
+        [
+            # Money over a period: core's default for a float variable.
+            ("us", "employment_income", "flow"),
+            ("uk", "employment_income", "flow"),
+            # A value at one point in time, declared in the model.
+            ("us", "household_vehicles_value", "stock"),
+            ("uk", "mortgage_debt", "stock"),
+            # Core's default for a yes-or-no variable.
+            ("us", "is_blind", "stock"),
+        ],
+    )
+    def test__variable_metadata_includes_the_models_quantity_type(
+        self, country_id, variable_name, quantity_type
+    ):
+        variable_metadata = COUNTRIES[country_id].metadata["variables"][variable_name]
+
+        assert variable_metadata["quantityType"] == quantity_type
+
+    def test__every_variable_has_a_stock_or_flow_quantity_type(self):
+        for country_id, country in COUNTRIES.items():
+            quantity_types = {
+                variable_metadata["quantityType"]
+                for variable_metadata in country.metadata["variables"].values()
+            }
+
+            assert quantity_types <= {"stock", "flow"}, country_id
+
+
 class TestUKCountryMetadata:
     """Tests for UK country metadata, specifically local authority loading."""