COMPAT: Pandas 0.21.0 type coercion

See pandas-dev/pandas#17767
and pandas-dev/pandas#16821

Author: TomAugspurger

dask/dataframe/core.py

@@ -1,6 +1,6 @@
 from __future__ import absolute_import, division, print_function
 
-from collections import Iterator
+from collections import Iterator, Iterable
 from functools import wraps, partial
 import operator
 from operator import getitem
@@ -206,6 +206,12 @@ def _scalar_binary(op, self, other, inv=False):
         return Scalar(dsk, name, meta)
 
 
+# Register Scalar with Iterable so that binops with pandas types
+# Go through correctly, rather than forcing object typecast
+# See https://github.com/pandas-dev/pandas/issues/17767
+Iterable.register(Scalar)
+
+
 class _Frame(Base):
     """ Superclass for DataFrame and Series
 

dask/dataframe/tests/test_arithmetics_reduction.py

@@ -456,6 +456,15 @@ def test_scalar_arithmetics():
     assert_eq(~(l == r), ~(el == er))
 
 
+def test_scalar_arithmetics_dtypes():
+    s = dd.core.Scalar({('s', 0): 10}, 's', 'i8')
+    pdf = pd.DataFrame({'a': [1, 2, 3, 4, 5, 6, 7],
+                        'b': [7, 6, 5, 4, 3, 2, 1]})
+    result = (pdf + s).dtypes
+    expected = pdf.dtypes
+    assert_eq(result, expected)
+
+
 def test_scalar_arithmetics_with_dask_instances():
     s = dd.core.Scalar({('s', 0): 10}, 's', 'i8')
     e = 10