Improve extend method in LimitedHistory to enforce memory depth

- Updated the extend method in the LimitedHistory class to properly truncate oldest entries when the combined history exceeds the defined memory depth.

- Added a corresponding unit test in axelrod/tests/unit/test_history.py to validate this behavior.

- Formatted all committed files using black for consistent code style.

Author: LindyZh

axelrod/history.py

@@ -117,8 +117,8 @@ def __init__(self, memory_depth, plays=None, coplays=None):
         memory_depth, int:
             length of history to retain
         """
-        super().__init__(plays=plays, coplays=coplays)
         self.memory_depth = memory_depth
+        super().__init__(plays=plays, coplays=coplays)
 
     def flip_plays(self):
         """Creates a flipped plays history for use with DualTransformer."""
@@ -138,3 +138,11 @@ def append(self, play, coplay):
             first_play, first_coplay = self._plays.pop(0), self._coplays.pop(0)
             self._actions[first_play] -= 1
             self._state_distribution[(first_play, first_coplay)] -= 1
+
+    def extend(self, new_plays, new_coplays):
+        """A function that emulates list.extend, respecting the stated memory depth."""
+        self._plays.extend(new_plays)
+        self._coplays.extend(new_coplays)
+        if len(self._plays) > self.memory_depth:
+            self._plays = self._plays[-self.memory_depth :]
+            self._coplays = self._coplays[-self.memory_depth :]

axelrod/tests/unit/test_history.py

@@ -123,3 +123,14 @@ def test_memory_depth(self):
             h.state_distribution,
             Counter({(D, D): 1, (C, D): 1, (D, C): 1, (C, C): 0}),
         )
+
+    def test_extend(self):
+        h1 = LimitedHistory(3, plays=[C, C, D], coplays=[C, C, C])
+        self.assertEqual(list(h1), [C, C, D])
+        h1.extend([C, C], [D, D])
+        self.assertEqual(list(h1), [D, C, C])
+        h1.extend([D, C], [D, D])
+        self.assertEqual(list(h1), [C, D, C])
+        h1.memory_depth = 4
+        h1.extend([D, C], [D, D])
+        self.assertEqual(list(h1), [D, C, D, C])