[REVIEW] Fix inplace update of data and add Series.update

python/cudf/cudf/_lib/copying.pyx

@@ -558,11 +558,11 @@ def copy_if_else(object lhs, object rhs, Column boolean_mask):
             return _copy_if_else_column_column(lhs, rhs, boolean_mask)
         else:
             return _copy_if_else_column_scalar(
-                lhs, as_device_scalar(rhs, lhs.dtype), boolean_mask)
+                lhs, as_device_scalar(rhs), boolean_mask)
     else:
         if isinstance(rhs, Column):
             return _copy_if_else_scalar_column(
-                as_device_scalar(lhs, rhs.dtype), rhs, boolean_mask)
+                as_device_scalar(lhs), rhs, boolean_mask)
         else:
             if lhs is None and rhs is None:
                 return lhs

python/cudf/cudf/core/__init__.py

@@ -1,6 +1,6 @@
 # Copyright (c) 2018-2020, NVIDIA CORPORATION.
 
-from cudf.core import buffer, column, column_accessor, common
+from cudf.core import _internals, buffer, column, column_accessor, common
 from cudf.core.buffer import Buffer
 from cudf.core.dataframe import DataFrame, from_pandas, merge
 from cudf.core.index import (

python/cudf/cudf/core/_internals/__init__.py

@@ -0,0 +1 @@
+from cudf.core._internals.where import where