BUG: Fix IntervalIndex.to_tuples() with NA values

Author: jschendel

doc/source/whatsnew/v0.22.0.txt

@@ -263,6 +263,7 @@ Conversion
 - Adding a ``Period`` object to a ``datetime`` or ``Timestamp`` object will now correctly raise a ``TypeError`` (:issue:`17983`)
 - Fixed a bug where ``FY5253`` date offsets could incorrectly raise an ``AssertionError`` in arithmetic operatons (:issue:`14774`)
 - Bug in :meth:`Index.astype` with a categorical dtype where the resultant index is not converted to a :class:`CategoricalIndex` for all types of index (:issue:`18630`)
+- Bug in ``IntervalIndex.to_tuples()`` where NA values are returned as a tuple of NA values instead of the NA value itself (:issue:`18756`)
 
 
 Indexing

pandas/core/indexes/interval.py

@@ -546,7 +546,11 @@ def from_tuples(cls, data, closed='right', name=None, copy=False):
 
     def to_tuples(self):
         """Return an Index of tuples of the form (left, right)"""
-        return Index(_asarray_tuplesafe(zip(self.left, self.right)))
+        tuples = _asarray_tuplesafe(zip(self.left, self.right))
+        if self.hasnans:
+            # GH 18756
+            tuples = np.where(~self._isnan, tuples, np.nan)
+        return Index(tuples)
 
     @cache_readonly
     def _multiindex(self):

pandas/tests/indexes/test_interval.py

@@ -7,6 +7,7 @@
     Interval, IntervalIndex, Index, isna, notna, interval_range, Timestamp,
     Timedelta, compat, date_range, timedelta_range, DateOffset)
 from pandas.compat import lzip
+from pandas.core.common import _asarray_tuplesafe
 from pandas.tseries.offsets import Day
 from pandas._libs.interval import IntervalTree
 from pandas.tests.indexes.common import Base
@@ -1072,6 +1073,19 @@ def test_is_non_overlapping_monotonic(self, closed):
             idx = IntervalIndex.from_breaks(range(4), closed=closed)
             assert idx.is_non_overlapping_monotonic is True
 
+    @pytest.mark.parametrize('tuples', [
+        lzip(range(10), range(1, 11)),
+        lzip(range(10), range(1, 11)) + [np.nan],
+        lzip(date_range('20170101', periods=10),
+             date_range('20170101', periods=10)),
+        [np.nan] + lzip(date_range('20170101', periods=10),
+                        date_range('20170101', periods=10))])
+    def test_to_tuples(self, tuples):
+        # GH 18756
+        result = IntervalIndex.from_tuples(tuples).to_tuples()
+        expected = Index(_asarray_tuplesafe(tuples))
+        tm.assert_index_equal(result, expected)
+
 
 class TestIntervalRange(object):