ENH: Implement subtraction for object-dtype Index (#21981)

Author: jbrockmendel

doc/source/whatsnew/v0.24.0.txt

@@ -370,6 +370,7 @@ Other API Changes
 - Invalid construction of ``IntervalDtype`` will now always raise a ``TypeError`` rather than a ``ValueError`` if the subdtype is invalid (:issue:`21185`)
 - Trying to reindex a ``DataFrame`` with a non unique ``MultiIndex`` now raises a ``ValueError`` instead of an ``Exception`` (:issue:`21770`)
 - :meth:`PeriodIndex.tz_convert` and :meth:`PeriodIndex.tz_localize` have been removed (:issue:`21781`)
+- :class:`Index` subtraction will attempt to operate element-wise instead of raising ``TypeError`` (:issue:`19369`)
 
 .. _whatsnew_0240.deprecations:
 

pandas/core/indexes/base.py

@@ -2650,8 +2650,10 @@ def __iadd__(self, other):
  return self + other
 
  def __sub__(self, other):
- raise TypeError("cannot perform __sub__ with this index type: "
- "{typ}".format(typ=type(self).__name__))
+ return Index(np.array(self) - other)
+
+ def __rsub__(self, other):
+ return Index(other - np.array(self))
 
  def __and__(self, other):
  return self.intersection(other)

pandas/tests/indexes/test_base.py

@@ -3,7 +3,7 @@
 import pytest
 
 from datetime import datetime, timedelta
-
+from decimal import Decimal
 from collections import defaultdict
 
 import pandas.util.testing as tm
@@ -864,13 +864,47 @@ def test_add(self):
  expected = Index(['1a', '1b', '1c'])
  tm.assert_index_equal('1' + index, expected)
 
- def test_sub(self):
+ def test_sub_fail(self):
  index = self.strIndex
  pytest.raises(TypeError, lambda: index - 'a')
  pytest.raises(TypeError, lambda: index - index)
  pytest.raises(TypeError, lambda: index - index.tolist())
  pytest.raises(TypeError, lambda: index.tolist() - index)
 
+ def test_sub_object(self):
+ # GH#19369
+ index = pd.Index([Decimal(1), Decimal(2)])
+ expected = pd.Index([Decimal(0), Decimal(1)])
+
+ result = index - Decimal(1)
+ tm.assert_index_equal(result, expected)
+
+ result = index - pd.Index([Decimal(1), Decimal(1)])
+ tm.assert_index_equal(result, expected)
+
+ with pytest.raises(TypeError):
+ index - 'foo'
+
+ with pytest.raises(TypeError):
+ index - np.array([2, 'foo'])
+
+ def test_rsub_object(self):
+ # GH#19369
+ index = pd.Index([Decimal(1), Decimal(2)])
+ expected = pd.Index([Decimal(1), Decimal(0)])
+
+ result = Decimal(2) - index
+ tm.assert_index_equal(result, expected)
+
+ result = np.array([Decimal(2), Decimal(2)]) - index
+ tm.assert_index_equal(result, expected)
+
+ with pytest.raises(TypeError):
+ 'foo' - index
+
+ with pytest.raises(TypeError):
+ np.array([True, pd.Timestamp.now()]) - index
+
  def test_map_identity_mapping(self):
  # GH 12766
  # TODO: replace with fixture