Add OrderHistory.first/last_order() methods

- get the `datetime` of the first or last order within a pixel
- unify some fixtures in `tests.forecasts.timify.conftest`

tests/forecasts/timify/test_order_history.py

@@ -1,17 +1,13 @@
 """Test the basic functionalities in the `OrderHistory` class."""
 
+import datetime as dt
+
 import pytest
 
 from tests import config as test_config
 from urban_meal_delivery.forecasts import timify
 
 
-@pytest.fixture
-def order_history(grid):
-    """An `OrderHistory` object."""
-    return timify.OrderHistory(grid=grid, time_step=test_config.LONG_TIME_STEP)
-
-
 class TestSpecialMethods:
     """Test the special methods in `OrderHistory`."""
 
@@ -32,12 +28,29 @@ class TestProperties:
 
         assert result == time_step
 
+    def test_totals(self, order_history, order_totals):
+        """Test `OrderHistory.totals` property.
+
+        The result of the `OrderHistory.aggregate_orders()` method call
+        is cached in the `OrderHistory.totals` property.
+
+        Note: `OrderHistory.aggregate_orders()` is not called as
+        `OrderHistory._data` is already set in the `order_history` fixture.
+        """
+        result = order_history.totals
+
+        assert result is order_totals
+
     def test_totals_is_cached(self, order_history, monkeypatch):
         """Test `OrderHistory.totals` property.
 
         The result of the `OrderHistory.aggregate_orders()` method call
         is cached in the `OrderHistory.totals` property.
+
+        Note: We make `OrderHistory.aggregate_orders()` return a `sentinel`
+        that is cached into `OrderHistory._data`, which must be unset first.
         """
+        monkeypatch.setattr(order_history, '_data', None)
         sentinel = object()
         monkeypatch.setattr(order_history, 'aggregate_orders', lambda: sentinel)
 
@@ -46,3 +59,34 @@ class TestProperties:
 
         assert result1 is result2
         assert result1 is sentinel
+
+
+class TestMethods:
+    """Test various methods in `OrderHistory`."""
+
+    def test_first_order_at_existing_pixel(self, order_history, good_pixel_id):
+        """Test `OrderHistory.first_order_at()` with good input."""
+        result = order_history.first_order_at(good_pixel_id)
+
+        assert result == test_config.START
+
+    def test_first_order_at_non_existing_pixel(self, order_history, good_pixel_id):
+        """Test `OrderHistory.first_order_at()` with bad input."""
+        with pytest.raises(
+            LookupError, match='`pixel_id` is not in the `grid`',
+        ):
+            order_history.first_order_at(-1)
+
+    def test_last_order_at_existing_pixel(self, order_history, good_pixel_id):
+        """Test `OrderHistory.last_order_at()` with good input."""
+        result = order_history.last_order_at(good_pixel_id)
+
+        one_time_step = dt.timedelta(minutes=test_config.LONG_TIME_STEP)
+        assert result == test_config.END - one_time_step
+
+    def test_last_order_at_non_existing_pixel(self, order_history, good_pixel_id):
+        """Test `OrderHistory.last_order_at()` with bad input."""
+        with pytest.raises(
+            LookupError, match='`pixel_id` is not in the `grid`',
+        ):
+            order_history.last_order_at(-1)