Add decimal column handling in copy_type_metadata

python/cudf/cudf/__init__.py

@@ -40,7 +40,12 @@
     merge,
 )
 from cudf.core.algorithms import factorize
-from cudf.core.dtypes import CategoricalDtype, Decimal64Dtype
+from cudf.core.dtypes import (
+    CategoricalDtype,
+    Decimal64Dtype,
+    ListDtype,
+    StructDtype,
+)
 from cudf.core.groupby import Grouper
 from cudf.core.ops import (
     add,

python/cudf/cudf/core/column/column.py

@@ -1402,6 +1402,8 @@ def _copy_type_metadata(self: T, other: ColumnBase) -> ColumnBase:
           of `other`  and the categories of `self`.
         * when both `self` and `other` are StructColumns, rename the fields
           of `other` to the field names of `self`.
+        * when both `self` and `other` are DecimalColumns, copy the precision
+          from self.dtype to other.dtype
         * when `self` and `other` are nested columns of the same type,
           recursively apply this function on the children of `self` to the
           and the children of `other`.
@@ -1425,6 +1427,11 @@ def _copy_type_metadata(self: T, other: ColumnBase) -> ColumnBase:
         ):
             other = other._rename_fields(self.dtype.fields.keys())
 
+        if isinstance(other, cudf.core.column.DecimalColumn) and isinstance(
+            self, cudf.core.column.DecimalColumn
+        ):
+            other.dtype.precision = self.dtype.precision
+
         if type(self) is type(other):
             if self.base_children and other.base_children:
                 base_children = tuple(

python/cudf/cudf/core/column/decimal.py

@@ -1,26 +1,27 @@
 # Copyright (c) 2021, NVIDIA CORPORATION.
 
-import cudf
+from typing import cast
+
 import cupy as cp
 import numpy as np
 import pyarrow as pa
 from pandas.api.types import is_integer_dtype
-from typing import cast
 
+import cudf
 from cudf import _lib as libcudf
-from cudf.core.buffer import Buffer
-from cudf.core.column import ColumnBase
-from cudf.core.dtypes import Decimal64Dtype
-from cudf.utils.utils import pa_mask_buffer_to_mask
-
-from cudf._typing import Dtype
 from cudf._lib.strings.convert.convert_fixed_point import (
     from_decimal as cpp_from_decimal,
 )
-from cudf.core.column import as_column
+from cudf._typing import Dtype
+from cudf.core.buffer import Buffer
+from cudf.core.column import ColumnBase, as_column
+from cudf.core.dtypes import Decimal64Dtype
+from cudf.utils.utils import pa_mask_buffer_to_mask
 
 
 class DecimalColumn(ColumnBase):
+    dtype: Decimal64Dtype
+
     @classmethod
     def from_arrow(cls, data: pa.Array):
         dtype = Decimal64Dtype.from_arrow(data.type)

python/cudf/cudf/core/dtypes.py

@@ -14,7 +14,11 @@
 from cudf._typing import Dtype
 
 
-class CategoricalDtype(ExtensionDtype):
+class _BaseDtype(ExtensionDtype):
+    pass
+
+
+class CategoricalDtype(_BaseDtype):
 
     ordered: Optional[bool]
 
@@ -121,7 +125,7 @@ def deserialize(cls, header, frames):
         return cls(categories=categories, ordered=ordered)
 
 
-class ListDtype(ExtensionDtype):
+class ListDtype(_BaseDtype):
     _typ: pa.ListType
     name: str = "list"
 
@@ -180,7 +184,7 @@ def __hash__(self):
         return hash(self._typ)
 
 
-class StructDtype(ExtensionDtype):
+class StructDtype(_BaseDtype):
 
     name = "struct"
 
@@ -231,7 +235,7 @@ def __hash__(self):
         return hash(self._typ)
 
 
-class Decimal64Dtype(ExtensionDtype):
+class Decimal64Dtype(_BaseDtype):
 
     name = "decimal"
     _metadata = ("precision", "scale")

python/cudf/cudf/tests/test_indexing.py

@@ -1401,3 +1401,14 @@ def test_iloc_before_zero_terminate(arg, pobj):
     gobj = cudf.from_pandas(pobj)
 
     assert_eq(pobj.iloc[arg], gobj.iloc[arg])
+
+
+def test_iloc_decimal():
+    sr = cudf.Series(["1.00", "2.00", "3.00", "4.00"]).astype(
+        cudf.Decimal64Dtype(scale=2, precision=3)
+    )
+    got = sr.iloc[[3, 2, 1, 0]]
+    expect = cudf.Series(["4.00", "3.00", "2.00", "1.00"],).astype(
+        cudf.Decimal64Dtype(scale=2, precision=3)
+    )
+    assert_eq(expect.reset_index(drop=True), got.reset_index(drop=True))

python/cudf/cudf/tests/utils.py

@@ -74,6 +74,13 @@ def assert_eq(left, right, **kwargs):
     without switching between assert_frame_equal/assert_series_equal/...
     functions.
     """
+    # dtypes that we support but Pandas doesn't will convert to
+    # `object`. Check equality before that happens:
+    if kwargs.get("check_dtype", True):
+        if hasattr(left, "dtype") and hasattr(right, "dtype"):
+            if isinstance(left.dtype, cudf.core.dtypes._BaseDtype):
+                assert_eq(left.dtype, right.dtype)
+
     if hasattr(left, "to_pandas"):
         left = left.to_pandas()
     if hasattr(right, "to_pandas"):