BUG: Categorical.remove_categories(np.nan) fails when underlying dtyp…

…e is float (GH pandas-dev#10156)

doc/source/whatsnew/v0.16.2.txt

@@ -163,6 +163,8 @@ Bug Fixes
 - Bug in GroupBy.get_group raises ValueError when group key contains NaT (:issue:`6992`)
 - Bug in ``SparseSeries`` constructor ignores input data name (:issue:`10258`)
 
+- Bug in ``Categorical.remove_categories`` causing a ValueError when removing the ``NaN`` category if underlying dtype is floating-point (:issue:`10156`)
+
 - Bug where infer_freq infers timerule (WOM-5XXX) unsupported by to_offset (:issue:`9425`)
 - Bug in ``DataFrame.to_hdf()`` where table format would raise a seemingly unrelated error for invalid (non-string) column names. This is now explicitly forbidden. (:issue:`9057`)
 - Bug to handle masking empty ``DataFrame``(:issue:`10126`)

pandas/core/categorical.py

@@ -749,11 +749,19 @@ def remove_categories(self, removals, inplace=False):
         """
         if not is_list_like(removals):
             removals = [removals]
-        removals = set(list(removals))
-        not_included = removals - set(self._categories)
+
+        removal_set = set(list(removals))
+        not_included = removal_set - set(self._categories)
+        new_categories = [ c for c in self._categories if c not in removal_set ]
+
+        # GH 10156
+        if any(isnull(removals)):
+            not_included = [x for x in not_included if notnull(x)]
+            new_categories = [x for x in new_categories if notnull(x)]
+
         if len(not_included) != 0:
             raise ValueError("removals must all be in old categories: %s" % str(not_included))
-        new_categories = [ c for c in self._categories if c not in removals ]
+
         return self.set_categories(new_categories, ordered=self.ordered, rename=False,
                                    inplace=inplace)
 

pandas/tests/test_categorical.py

@@ -854,6 +854,28 @@ def test_nan_handling(self):
         self.assert_numpy_array_equal(c.categories , np.array(["a","b",np.nan],dtype=np.object_))
         self.assert_numpy_array_equal(c._codes , np.array([0,2,-1,0]))
 
+        # Remove null categories (GH 10156)
+        cases = [
+            ([1.0, 2.0, np.nan], [1.0, 2.0]),
+            (['a', 'b', None], ['a', 'b']),
+            ([pd.Timestamp('2012-05-01'), pd.NaT], [pd.Timestamp('2012-05-01')])
+        ]
+
+        null_values = [np.nan, None, pd.NaT]
+
+        for with_null, without in cases:
+            base = Categorical([], with_null)
+            expected = Categorical([], without)
+
+            for nullval in null_values:
+                result = base.remove_categories(nullval)
+                self.assert_categorical_equal(result, expected)
+
+        # Different null values are indistinguishable
+        for i, j in [(0, 1), (0, 2), (1, 2)]:
+            nulls = [null_values[i], null_values[j]]
+            self.assertRaises(ValueError, lambda: Categorical([], categories=nulls))
+
 
     def test_isnull(self):
         exp = np.array([False, False, True])