Preserve None in Series unique

pandas/_libs/hashtable_class_helper.pxi.in

@@ -870,7 +870,12 @@ cdef class PyObjectHashTable(HashTable):
         for i in range(n):
             val = values[i]
             hash(val)
-            if not checknull(val):
+
+            # `val is None` below is exception to prevent mangling of None and
+            # other NA values; note however that other NA values (ex: pd.NaT
+            # and np.nan) will still get mangled, so many not be a permanent
+            # solution; see GH 20866
+            if not checknull(val) or val is None:
                 k = kh_get_pymap(self.table, <PyObject*>val)
                 if k == self.table.n_buckets:
                     kh_put_pymap(self.table, <PyObject*>val, &ret)

pandas/tests/test_algos.py

@@ -491,6 +491,14 @@ def test_tuple_with_strings(self, arg, expected):
         result = pd.unique(arg)
         tm.assert_numpy_array_equal(result, expected)
 
+    def test_obj_none_preservation(self):
+        # GH 20866
+        arr = np.array(['foo', None], dtype=object)
+        result = pd.unique(arr)
+        expected = np.array(['foo', None], dtype=object)
+
+        tm.assert_numpy_array_equal(result, expected, strict_nan=True)
+
 
 class TestIsin(object):