Fix for comparisons of categorical and an scalar not in categories

doc/source/whatsnew/v0.16.1.txt

@@ -123,3 +123,5 @@ Bug Fixes
 - Bug in which ``SparseDataFrame`` could not take `nan` as a column name (:issue:`8822`)
 
 - Bug in unequal comparisons between a ``Series`` of dtype `"category"` and a scalar (e.g. ``Series(Categorical(list("abc"), categories=list("cba"), ordered=True)) > "b"``, which wouldn't use the order of the categories but use the lexicographical order. (:issue:`9848`)
+
+- Bug in unequal comparisons between categorical data and a scalar, which was not in the categories (e.g. ``Series(Categorical(list("abc"), ordered=True)) > "d"``. This returned ``False`` for all elements, but now raises a TypeError. Equality comparisons also now return ``False`` for ``==`` and ``True`` for ``!=``. (:issue:`9848`)

pandas/core/categorical.py

@@ -61,7 +61,14 @@ def f(self, other):
                 i = self.categories.get_loc(other)
                 return getattr(self._codes, op)(i)
             else:
-                return np.repeat(False, len(self))
+                if op == '__eq__':
+                    return np.repeat(False, len(self))
+                elif op == '__ne__':
+                    return np.repeat(True, len(self))
+                else:
+                    msg  = "Cannot compare a Categorical for op {op} with a scalar, " \
+                           "which is not a category."
+                    raise TypeError(msg.format(op=op))
         else:
 
             # allow categorical vs object dtype array comparisons for equality

pandas/tests/test_categorical.py

@@ -1087,6 +1087,20 @@ def test_reflected_comparison_with_scalars(self):
         self.assert_numpy_array_equal(cat > cat[0], [False, True, True])
         self.assert_numpy_array_equal(cat[0] < cat, [False, True, True])
 
+    def test_comparison_with_unknown_scalars(self):
+        # https://github.com/pydata/pandas/issues/9836#issuecomment-92123057 and following
+        # comparisons with scalars not in categories should raise for unequal comps, but not for
+        # equal/not equal
+        cat = pd.Categorical([1, 2, 3], ordered=True)
+
+        self.assertRaises(TypeError, lambda: cat < 4)
+        self.assertRaises(TypeError, lambda: cat > 4)
+        self.assertRaises(TypeError, lambda: 4 < cat)
+        self.assertRaises(TypeError, lambda: 4 > cat)
+
+        self.assert_numpy_array_equal(cat == 4 , [False, False, False])
+        self.assert_numpy_array_equal(cat != 4 , [True, True, True])
+
 
 class TestCategoricalAsBlock(tm.TestCase):
     _multiprocess_can_split_ = True
@@ -2440,6 +2454,19 @@ def f():
             cat > "b"
         self.assertRaises(TypeError, f)
 
+        # https://github.com/pydata/pandas/issues/9836#issuecomment-92123057 and following
+        # comparisons with scalars not in categories should raise for unequal comps, but not for
+        # equal/not equal
+        cat = Series(Categorical(list("abc"), ordered=True))
+
+        self.assertRaises(TypeError, lambda: cat < "d")
+        self.assertRaises(TypeError, lambda: cat > "d")
+        self.assertRaises(TypeError, lambda: "d" < cat)
+        self.assertRaises(TypeError, lambda: "d" > cat)
+
+        self.assert_series_equal(cat == "d" , Series([False, False, False]))
+        self.assert_series_equal(cat != "d" , Series([True, True, True]))
+
 
         # And test NaN handling...
         cat = Series(Categorical(["a","b","c", np.nan]))