added bowling example

tdd/bowling.py

@@ -0,0 +1,45 @@
+
+class BowlingCase:
+    def __init__(self):
+        self.first_throw = None
+        self.second_throw = None
+        self.xtra_throw = None
+
+    def add_score(self, pins_down):
+        if self.first_throw is None:
+            self.first_throw = pins_down
+        else:
+            self.second_throw = pins_down
+
+    def is_completed(self):
+        if self.first_throw == 10:
+            return True
+        return isinstance(self.first_throw, int) and isinstance(self.second_throw, int)
+
+    def is_spare(self):
+        return self.first_throw != 10 and self.score() == 10
+
+    def requires_extra_throw(self):
+        if self.is_spare():
+            return self.xtra_throw is None
+        return False
+
+    def set_extra_throw(self, throw):
+        self.xtra_throw = throw
+
+    def score(self):
+        return (self.first_throw or 0) + (self.second_throw or 0)
+
+
+class BowlingGame:
+    def __init__(self):
+        self._score = [BowlingCase() for i in range(10)]
+
+    def throw(self, pin_downs):
+        for case in self._score:
+            if not case.is_completed():
+                case.add_score(pin_downs)
+                return
+
+    def score(self):
+        return sum([case.score() for case in self._score])