Add unit tests for the existing rules

One test per rule from the requirements, replacing test_foo. Tier
boundaries are checked on both sides, e.g. backstage passes at 11 and 10,
so an off-by-one can't slip through. All pass against the original code
with 100% branch coverage.

python/tests/test_gilded_rose.py

@@ -1,16 +1,101 @@
 # -*- coding: utf-8 -*-
-import unittest
+"""Unit tests for the Gilded Rose inventory rules.
 
-from gilded_rose import Item, GildedRose
+Each test drives update_quality() for a single day on a single item so that a
+failure points at exactly one rule. The 30-day approval test covers the
+interaction of rules over time.
+"""
+import pytest
+
+from gilded_rose import GildedRose, Item
+
+AGED_BRIE = "Aged Brie"
+SULFURAS = "Sulfuras, Hand of Ragnaros"
+BACKSTAGE_PASS = "Backstage passes to a TAFKAL80ETC concert"
 
 
-class GildedRoseTest(unittest.TestCase):
-    def test_foo(self):
-        items = [Item("foo", 0, 0)]
-        gilded_rose = GildedRose(items)
-        gilded_rose.update_quality()
-        self.assertEqual("fixme", items[0].name)
+def update(name, sell_in, quality):
+    """Run one day of update_quality() and return (sell_in, quality)."""
+    item = Item(name, sell_in, quality)
+    GildedRose([item]).update_quality()
+    return item.sell_in, item.quality
 
-        
-if __name__ == '__main__':
-    unittest.main()
+
+class TestNormalItem:
+    def test_sell_in_and_quality_both_drop_by_one_each_day(self):
+        assert update("+5 Dexterity Vest", 10, 20) == (9, 19)
+
+    def test_quality_degrades_twice_as_fast_once_sell_by_date_has_passed(self):
+        assert update("+5 Dexterity Vest", 0, 20) == (-1, 18)
+
+    @pytest.mark.parametrize("sell_in", [5, 0, -3])
+    def test_quality_is_never_negative(self, sell_in):
+        _, quality = update("Elixir of the Mongoose", sell_in, 0)
+        assert quality == 0
+
+    def test_quality_of_one_after_sell_by_clamps_to_zero(self):
+        assert update("Elixir of the Mongoose", -1, 1) == (-2, 0)
+
+
+class TestAgedBrie:
+    def test_quality_increases_by_one_each_day(self):
+        assert update(AGED_BRIE, 2, 0) == (1, 1)
+
+    def test_quality_increases_twice_as_fast_once_sell_by_date_has_passed(self):
+        assert update(AGED_BRIE, 0, 10) == (-1, 12)
+
+    @pytest.mark.parametrize("sell_in, quality", [(5, 50), (0, 50), (0, 49)])
+    def test_quality_never_exceeds_fifty(self, sell_in, quality):
+        _, new_quality = update(AGED_BRIE, sell_in, quality)
+        assert new_quality == 50
+
+
+class TestSulfuras:
+    @pytest.mark.parametrize("sell_in", [10, 0, -1])
+    def test_never_changes(self, sell_in):
+        assert update(SULFURAS, sell_in, 80) == (sell_in, 80)
+
+
+class TestBackstagePasses:
+    # 11 is the day before the first tier boundary, so it guards the off-by-one.
+    @pytest.mark.parametrize("sell_in", [15, 11])
+    def test_quality_increases_by_one_when_more_than_ten_days_remain(self, sell_in):
+        assert update(BACKSTAGE_PASS, sell_in, 20) == (sell_in - 1, 21)
+
+    @pytest.mark.parametrize("sell_in", [10, 6])  # 6 is the day before the next tier
+    def test_quality_increases_by_two_when_ten_days_or_less_remain(self, sell_in):
+        assert update(BACKSTAGE_PASS, sell_in, 20) == (sell_in - 1, 22)
+
+    @pytest.mark.parametrize("sell_in", [5, 1])
+    def test_quality_increases_by_three_when_five_days_or_less_remain(self, sell_in):
+        assert update(BACKSTAGE_PASS, sell_in, 20) == (sell_in - 1, 23)
+
+    @pytest.mark.parametrize("sell_in, quality", [(15, 50), (10, 49), (5, 48)])
+    def test_quality_never_exceeds_fifty(self, sell_in, quality):
+        _, new_quality = update(BACKSTAGE_PASS, sell_in, quality)
+        assert new_quality == 50
+
+    @pytest.mark.parametrize("sell_in", [0, -1])
+    def test_quality_drops_to_zero_after_the_concert(self, sell_in):
+        _, quality = update(BACKSTAGE_PASS, sell_in, 40)
+        assert quality == 0
+
+
+def test_all_items_in_the_inventory_are_updated():
+    items = [
+        Item("+5 Dexterity Vest", 10, 20),
+        Item(AGED_BRIE, 2, 0),
+        Item(SULFURAS, 0, 80),
+        Item(BACKSTAGE_PASS, 5, 20),
+    ]
+    GildedRose(items).update_quality()
+    assert [(i.sell_in, i.quality) for i in items] == [
+        (9, 19),
+        (1, 1),
+        (0, 80),
+        (4, 23),
+    ]
+
+
+def test_an_empty_inventory_is_handled():
+    GildedRose([]).update_quality()  # must not raise