Fix cpp unit test, code review comments

cpp/src/arrow/array-dictionary-test.cc

@@ -46,7 +46,7 @@ TEST(TestDictionary, Basics) {
   ASSERT_TRUE(int16()->Equals(type2.index_type()));
   ASSERT_TRUE(type2.dictionary()->Equals(dict));
 
-  ASSERT_EQ("dictionary<int32, int16>", type1->ToString());
+  ASSERT_EQ("dictionary<values=int32, indices=int16>", type1->ToString());
 }
 
 TEST(TestDictionary, Equals) {

python/pyarrow/__init__.py

@@ -38,7 +38,7 @@ from pyarrow.array import (Array,
                            Int32Array, UInt32Array,
                            Int64Array, UInt64Array,
                            ListArray, StringArray,
-                           DictionaryArray, dictionary)
+                           DictionaryArray)
 
 from pyarrow.error import ArrowException
 
@@ -57,7 +57,7 @@ from pyarrow.schema import (null, bool_,
                             uint8, uint16, uint32, uint64,
                             timestamp, date,
                             float_, double, binary, string,
-                            list_, struct, field,
+                            list_, struct, dictionary, field,
                             DataType, Field, Schema, schema)
 
 from pyarrow.table import Column, RecordBatch, Table, concat_tables

python/pyarrow/array.pyx

@@ -413,14 +413,3 @@ cdef object get_series_values(object obj):
 
 from_pylist = Array.from_list
 from_pandas_series = Array.from_pandas
-
-
-def dictionary(DataType index_type, Array dictionary):
-    """
-    Dictionary (categorical, or simply encoded) type
-    """
-    cdef DictionaryType out = schema.DictionaryType()
-    cdef shared_ptr[CDataType] dict_type
-    dict_type.reset(new CDictionaryType(index_type.sp_type, dictionary.sp_array))
-    out.init(dict_type)
-    return out

python/pyarrow/schema.pyx

@@ -25,6 +25,7 @@
 from cython.operator cimport dereference as deref
 
 from pyarrow.compat import frombytes, tobytes
+from pyarrow.array cimport Array
 from pyarrow.includes.libarrow cimport (CDataType, CStructType, CListType,
                                         Type_NA, Type_BOOL,
                                         Type_UINT8, Type_INT8,
@@ -282,6 +283,7 @@ def binary():
     """
     return primitive_type(Type_BINARY)
 
+
 def list_(DataType value_type):
     cdef DataType out = DataType()
     cdef shared_ptr[CDataType] list_type
@@ -289,6 +291,19 @@ def list_(DataType value_type):
     out.init(list_type)
     return out
 
+
+def dictionary(DataType index_type, Array dictionary):
+    """
+    Dictionary (categorical, or simply encoded) type
+    """
+    cdef DictionaryType out = DictionaryType()
+    cdef shared_ptr[CDataType] dict_type
+    dict_type.reset(new CDictionaryType(index_type.sp_type,
+                                        dictionary.sp_array))
+    out.init(dict_type)
+    return out
+
+
 def struct(fields):
     """