Extract constants and return early for Sulfuras

No behaviour change. Named the item strings and the 0/50 bounds, moved
the body into _update_item and return early for Sulfuras so the rest of
the ladder only deals with items that actually change.

python/gilded_rose.py

@@ -1,5 +1,13 @@
 # -*- coding: utf-8 -*-
 
+AGED_BRIE = "Aged Brie"
+BACKSTAGE_PASS = "Backstage passes to a TAFKAL80ETC concert"
+SULFURAS = "Sulfuras, Hand of Ragnaros"
+
+MIN_QUALITY = 0
+MAX_QUALITY = 50
+
+
 class GildedRose(object):
 
     def __init__(self, items):
@@ -7,32 +15,38 @@ class GildedRose(object):
 
     def update_quality(self):
         for item in self.items:
-            if item.name != "Aged Brie" and item.name != "Backstage passes to a TAFKAL80ETC concert":
+            self._update_item(item)
-                if item.quality > 0:
+
-                    if item.name != "Sulfuras, Hand of Ragnaros":
+    @staticmethod
+    def _update_item(item):
+        if item.name == SULFURAS:
+            return  # legendary: never has to be sold and never changes
+
+        if item.name != AGED_BRIE and item.name != BACKSTAGE_PASS:
+            if item.quality > MIN_QUALITY:
                 item.quality = item.quality - 1
         else:
-                if item.quality < 50:
+            if item.quality < MAX_QUALITY:
                 item.quality = item.quality + 1
-                    if item.name == "Backstage passes to a TAFKAL80ETC concert":
+                if item.name == BACKSTAGE_PASS:
                     if item.sell_in < 11:
-                            if item.quality < 50:
+                        if item.quality < MAX_QUALITY:
                             item.quality = item.quality + 1
                     if item.sell_in < 6:
-                            if item.quality < 50:
+                        if item.quality < MAX_QUALITY:
                             item.quality = item.quality + 1
-            if item.name != "Sulfuras, Hand of Ragnaros":
+
         item.sell_in = item.sell_in - 1
+
         if item.sell_in < 0:
-                if item.name != "Aged Brie":
+            if item.name != AGED_BRIE:
-                    if item.name != "Backstage passes to a TAFKAL80ETC concert":
+                if item.name != BACKSTAGE_PASS:
-                        if item.quality > 0:
+                    if item.quality > MIN_QUALITY:
-                            if item.name != "Sulfuras, Hand of Ragnaros":
                         item.quality = item.quality - 1
                 else:
-                        item.quality = item.quality - item.quality
+                    item.quality = MIN_QUALITY
             else:
-                    if item.quality < 50:
+                if item.quality < MAX_QUALITY:
                     item.quality = item.quality + 1