change Data [] behavior to return None or [] for missing keys

Previously threw exceptions. This differs from python dict or
sequence behavior but is now consistent with get and getRange.

python/test/test_data.py

@@ -99,9 +99,9 @@ def test_brackets(self):
         assert_equals(0, vals[0][0])
         assert_true(array_equal(self.ary1, vals[0][1]))
 
-        assert_raises(KeyError, self.images.__getitem__, 2)  # equiv: self.images[2]
+        assert_is_none(self.images[2])
 
-        assert_raises(IndexError, self.images.__getitem__, slice(2, 3))  # equiv: self.images[2:3]
+        assert_equals([], self.images[2:3])
 
 
 class TestSeriesGetters(PySparkTestCase):
@@ -250,11 +250,8 @@ def test_brackets(self):
         assert_true(array_equal(self.dataLocal[0][1], vals[0][1]))
         assert_true(array_equal(self.dataLocal[1][1], vals[1][1]))
 
-        # trying to getitem a key that doesn't exist raises KeyError
-        # this differs from `get` behavior but is consistent with python dict
-        # see object.__getitem__ in https://docs.python.org/2/reference/datamodel.html
-        assert_raises(KeyError, self.series.__getitem__, (25, 17))  # equiv: self.series[(25, 17)]
+        # trying to getitem a key that doesn't exist returns None
+        assert_is_none(self.series[(25, 17)])
 
-        # passing a range that is completely out of bounds throws IndexError
-        # note that if a range is only partly out of bounds, it will return what elements the slice does include
-        assert_raises(IndexError, self.series.__getitem__, [slice(2, 3), slice(None)])  # series[2:3,:]
+        # passing a range that is completely out of bounds returns []
+        assert_equals([], self.series[2:3, :])

python/thunder/rdds/data.py

@@ -249,14 +249,9 @@ def __getitem__(self, item):
                 isRangeQuery = True
 
         if isRangeQuery:
-            retVals = self.getRange(item)
-            if not retVals:
-                raise IndexError("No keys found for slice(s): '%s'" % str(item))
+            return self.getRange(item)
         else:
-            retVals = self.get(item)
-            if retVals is None:
-                raise KeyError("No key found matching '%s'" % str(item))
-        return retVals
+            return self.get(item)
 
     def values(self):
         """ Return values, ignoring keys