Implement ltrim method

fakeredis.py

@@ -365,7 +365,15 @@ def rpushx(self, name, value):
             return
 
     def ltrim(self, name, start, end):
-        raise NotImplementedError()
+        try:
+            val = self._db[name]
+        except KeyError:
+            return
+        if end == -1:
+            end = None
+        else:
+            end += 1
+        self._db[name] = val[start:end]
 
     def lindex(self, name, index):
         try:

test_fakeredis.py

@@ -371,6 +371,18 @@ def test_rpushx(self):
         self.assertEqual(self.redis.lrange('foo', 0, -1), ['one', 'two'])
         self.assertEqual(self.redis.lrange('bar', 0, -1), [])
 
+    def test_ltrim(self):
+        self.redis.rpush('foo', 'one')
+        self.redis.rpush('foo', 'two')
+        self.redis.rpush('foo', 'three')
+        self.redis.rpush('foo', 'four')
+
+        self.redis.ltrim('foo', 1, 3)
+        self.assertEqual(self.redis.lrange('foo', 0, -1), ['two', 'three',
+                                                           'four'])
+        self.redis.ltrim('foo', 1, -1)
+        self.assertEqual(self.redis.lrange('foo', 0, -1), ['three', 'four'])
+
     def test_lindex(self):
         self.redis.rpush('foo', 'one')
         self.redis.rpush('foo', 'two')