ARROW-2515 [Python] Add DictionaryValue class, fixing bugs with nested dictionaries

python/pyarrow/array.pxi

@@ -918,15 +918,6 @@ cdef class BinaryArray(Array):
 
 cdef class DictionaryArray(Array):
 
-    cdef getitem(self, int64_t i):
-        cdef Array dictionary = self.dictionary
-        index = self.indices[i]
-        if index is NA:
-            return index
-        else:
-            return box_scalar(dictionary.type, dictionary.sp_array,
-                              index.as_py())
-
     def dictionary_encode(self):
         return self
 

python/pyarrow/scalar.pxi

@@ -402,6 +402,29 @@ cdef class StructValue(ArrayValue):
             zip(child_names, wrapped_arrays)
         }
 
+cdef class DictionaryValue(ArrayValue):
+
+    def as_py(self):
+        return self.dictionary_value.as_py()
+
+    property index_value:
+
+        def __get__(self):
+            cdef CDictionaryArray* darr
+
+            darr = <CDictionaryArray*>(self.sp_array.get())
+            indices = pyarrow_wrap_array(darr.indices())
+            return indices[self.index]
+
+    property dictionary_value:
+
+        def __get__(self):
+            cdef CDictionaryArray* darr
+
+            darr = <CDictionaryArray*>(self.sp_array.get())
+            dictionary = pyarrow_wrap_array(darr.dictionary())
+            return dictionary[self.index_value.as_py()]
+
 
 cdef dict _scalar_classes = {
     _Type_BOOL: BooleanValue,
@@ -428,6 +451,7 @@ cdef dict _scalar_classes = {
     _Type_FIXED_SIZE_BINARY: FixedSizeBinaryValue,
     _Type_DECIMAL: DecimalValue,
     _Type_STRUCT: StructValue,
+    _Type_DICTIONARY: DictionaryValue,
 }
 
 

python/pyarrow/tests/test_array.py

@@ -841,3 +841,13 @@ def test_struct_array_flatten():
     xs, ys = a[1:].flatten()
     assert xs.to_pylist() == [None, None]
     assert ys.to_pylist() == [None, 2.5]
+
+
+def test_nested_dictionary_array():
+    dict_arr = pa.DictionaryArray.from_arrays([0, 1, 0], ['a', 'b'])
+    list_arr = pa.ListArray.from_arrays([0, 2, 3], dict_arr)
+    assert list_arr.to_pylist() == [['a', 'b'], ['a']]
+
+    dict_arr = pa.DictionaryArray.from_arrays([0, 1, 0], ['a', 'b'])
+    dict_arr2 = pa.DictionaryArray.from_arrays([0, 1, 2, 1, 0], dict_arr)
+    assert dict_arr2.to_pylist() == ['a', 'b', 'a', 'b', 'a']

python/pyarrow/tests/test_scalars.py

@@ -180,6 +180,7 @@ def test_timestamp(self):
 
     def test_dictionary(self):
         colors = ['red', 'green', 'blue']
+        colors_dict = {'red': 0, 'green': 1, 'blue': 2}
         values = pd.Series(colors * 4)
 
         categorical = pd.Categorical(values, categories=colors)
@@ -188,6 +189,8 @@ def test_dictionary(self):
                                            categorical.categories)
         for i, c in enumerate(values):
             assert v[i].as_py() == c
+            assert v[i].dictionary_value == c
+            assert v[i].index_value == colors_dict[c]
 
     def test_int_hash(self):
         # ARROW-640