ENH - first pass at modifying set operations on indexes. Dont ignore …

…empty indexes, and allow more cross index operaions
pandas-dev · TomAugspurger · May 21, 2019 · Nov 7, 2018 · Nov 8, 2018 · Nov 8, 2018
commit c2cf2697cdeb27322ef38aa3ed66c54485df9142
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
@@ -2762,20 +2762,18 @@ def union(self, other):
         self._assert_can_do_setop(other)
         other = ensure_index(other)
 
-        if len(other) == 0 or self.equals(other):
+        if self.equals(other):
             return self._get_consensus_name(other)
 
-        if len(self) == 0:
-            return other._get_consensus_name(self)
-
         # TODO: is_dtype_union_equal is a hack around
         # 1. buggy set ops with duplicates (GH #13432)
         # 2. CategoricalIndex lacking setops (GH #10186)
         # Once those are fixed, this workaround can be removed
         if not is_dtype_union_equal(self.dtype, other.dtype):
             this = self.astype('O')
             other = other.astype('O')
-            return this.union(other)
+            # force object dtype, for empty index cases
+            return this.union(other).astype('O')
 
         # TODO(EA): setops-refactor, clean all this up
         if is_period_dtype(self) or is_datetime64tz_dtype(self):

diff --git a/pandas/core/indexes/interval.py b/pandas/core/indexes/interval.py
@@ -1030,7 +1030,14 @@ def equals(self, other):
 
     def _setop(op_name):
         def func(self, other):
-            other = self._as_like_interval_index(other)
+            try:
+                other = self._as_like_interval_index(other)
+            except (TypeError, ValueError):
+                # Currently this will cause difference operations to return
+                # object dtype as opposed to IntervalIndex, unlike other Index
+                # objects that return the same type when using `difference` on
+                # mismatched types
+                return getattr(self.astype('O'), op_name)(other)
 
             # GH 19016: ensure set op will not return a prohibited dtype
             subtypes = [self.dtype.subtype, other.dtype.subtype]

diff --git a/pandas/core/indexes/period.py b/pandas/core/indexes/period.py
@@ -660,10 +660,9 @@ def join(self, other, how='left', level=None, return_indexers=False,
     def _assert_can_do_setop(self, other):
         super(PeriodIndex, self)._assert_can_do_setop(other)
 
-        if not isinstance(other, PeriodIndex):
-            raise ValueError('can only call with other PeriodIndex-ed objects')
-
-        if self.freq != other.freq:
+        # *Can't* use PeriodIndexes of different freqs
+        # *Can* use PeriodIndex/DatetimeIndex
+        if isinstance(other, PeriodIndex) and self.freq != other.freq:
             msg = DIFFERENT_FREQ_INDEX.format(self.freqstr, other.freqstr)
             raise IncompatibleFrequency(msg)