Add DecimalDtype to cuDF (#6675)

CHANGELOG.md

@@ -13,6 +13,7 @@
 - PR #6592 Add `cudf.to_numeric` function
 - PR #6598 Add strings::contains API with target column parameter
 - PR #6638 Add support for `pipe` API
+- PR #6675 Add DecimalDtype to cuDF
 
 ## Improvements
 

python/cudf/cudf/core/dtypes.py

@@ -1,5 +1,6 @@
 # Copyright (c) 2020, NVIDIA CORPORATION.
 
+import decimal
 import pickle
 
 import numpy as np
@@ -211,3 +212,32 @@ def __eq__(self, other):
 
     def __repr__(self):
         return f"StructDtype({self.fields})"
+
+
+class DecimalDtype(ExtensionDtype):
+
+    name = "decimal"
+    _metadata = ("precision", "scale")
+
+    def __init__(self, precision, scale):
+        self._typ = pa.decimal128(precision, scale)
+
+    @property
+    def precision(self):
+        return self._typ.precision
+
+    @property
+    def scale(self):
+        return self._typ.scale
+
+    @property
+    def type(self):
+        # might need to account for precision and scale here
+        return decimal.Decimal
+
+    def to_arrow(self):
+        return self._typ
+
+    @classmethod
+    def from_arrow(cls, typ):
+        return cls(typ.precision, typ.scale)

python/cudf/cudf/tests/test_dtypes.py

@@ -6,7 +6,12 @@
 import pytest
 
 import cudf
-from cudf.core.dtypes import CategoricalDtype, ListDtype, StructDtype
+from cudf.core.dtypes import (
+    CategoricalDtype,
+    ListDtype,
+    StructDtype,
+    DecimalDtype,
+)
 from cudf.tests.utils import assert_eq
 
 
@@ -128,3 +133,9 @@ def test_struct_dtype_fields(fields):
     fields = {"a": "int32", "b": StructDtype({"c": "int64", "d": "int32"})}
     dt = StructDtype(fields)
     assert_eq(dt.fields, fields)
+
+
+def test_decimal_dtype():
+    dt = DecimalDtype(4, 2)
+    assert dt.to_arrow() == pa.decimal128(4, 2)
+    assert dt == DecimalDtype.from_arrow(pa.decimal128(4, 2))