BUG: Fix interval constructor to handle left=right case

doc/source/whatsnew/v0.25.0.rst

@@ -617,13 +617,12 @@ Strings
 - Improved error message when passing :class:`Series` of wrong dtype to :meth:`Series.str.cat` (:issue:`22722`)
 -
 
-
 Interval
 ^^^^^^^^
 
 - Construction of :class:`Interval` is restricted to numeric, :class:`Timestamp` and :class:`Timedelta` endpoints (:issue:`23013`)
 - Fixed bug in :class:`Series`/:class:`DataFrame` not displaying ``NaN`` in :class:`IntervalIndex` with missing values (:issue:`25984`)
--
+- :class:Interval now refuses to create intervals like `[0,0)` or `(0,0]` (:issue:`26893`)
 
 Indexing
 ^^^^^^^^

pandas/_libs/interval.pyx

@@ -255,6 +255,9 @@ cdef class Interval(IntervalMixin):
             raise ValueError(msg)
         if not left <= right:
             raise ValueError('left side of interval must be <= right side')
+        if left == right and closed not in ('both', 'neither'):
+            msg='both/neither sides must be closed when left == right'
+            raise ValueError(msg)
         if (isinstance(left, Timestamp) and
                 not tz_compare(left.tzinfo, right.tzinfo)):
             # GH 18538

pandas/core/reshape/tile.py

@@ -476,9 +476,9 @@ def _format_labels(bins, precision, right=True,
     if right and include_lowest:
         # we will adjust the left hand side by precision to
         # account that we are all right closed
-        v = adjust(labels[0].left)
+        v = adjust(labels.left[0])
 
-        i = IntervalIndex([Interval(v, labels[0].right, closed='right')])
+        i = IntervalIndex([Interval(v, labels.right[0], closed='right')])
         labels = i.append(labels[1:])
 
     return labels

pandas/tests/arrays/interval/test_interval.py

@@ -65,3 +65,30 @@ def test_repr_matches():
     a = repr(idx)
     b = repr(idx.values)
     assert a.replace("Index", "Array") == b
+
+
+def test_point_interval_illegal():
+    match = "both/neither sides must be closed when left == right"
+    for closed in ('left', 'right'):
+        with pytest.raises(ValueError, match=match):
+            pd.Interval(0, 0, closed)
+
+
+@pytest.mark.parametrize('left_interval, right_interval, result',
+                         [
+                             ((0, 1, "left"), (0, 0, "neither"), False),
+                             ((0, 1, "left"), (0, 0, "both"), True),
+                             ((0, 1, "right"), (1, 1, "neither"), False),
+                             ((0, 1, "right"), (1, 1, "both"), True),
+                             ((0, 1, "both"), (0, 0, "neither"), False),
+                             ((0, 1, "both"), (0, 0, "neither"), False),
+                             ((0, 1, "neither"), (1, 1, "both"), False),
+                             ((0, 1, "neither"), (1, 1, "both"), False),
+                         ])
+def test_point_interval(left_interval, right_interval, result):
+    pd.Interval(0, 0, 'neither')  # no exception
+    pd.Interval(0, 0, 'both')  # no exception
+
+    a = Interval(*left_interval)
+    b = Interval(*right_interval)
+    assert result == a.overlaps(b)

pandas/tests/indexes/interval/test_interval.py

@@ -73,7 +73,7 @@ def test_properties(self, closed):
         tm.assert_numpy_array_equal(np.asarray(index), expected)
 
     @pytest.mark.parametrize('breaks', [
-        [1, 1, 2, 5, 15, 53, 217, 1014, 5335, 31240, 201608],
+        [1, 2, 5, 15, 53, 217, 1014, 5335, 31240, 201608],
         [-np.inf, -100, -10, 0.5, 1, 1.5, 3.8, 101, 202, np.inf],
         pd.to_datetime(['20170101', '20170202', '20170303', '20170404']),
         pd.to_timedelta(['1ns', '2ms', '3s', '4M', '5H', '6D'])])
@@ -90,6 +90,22 @@ def test_length(self, closed, breaks):
         expected = Index(iv.length if notna(iv) else iv for iv in index)
         tm.assert_index_equal(result, expected)
 
+    @pytest.mark.parametrize('breaks', [
+        [1, 1, 2, 2]])
+    @pytest.mark.parametrize('closed', ['neither', 'both'])
+    def test_length_with_point_intervals(self, closed, breaks):
+        # GH 18789
+        index = IntervalIndex.from_breaks(breaks, closed=closed)
+        result = index.length
+        expected = Index(iv.length for iv in index)
+        tm.assert_index_equal(result, expected)
+
+        # with NA
+        index = index.insert(1, np.nan)
+        result = index.length
+        expected = Index(iv.length if notna(iv) else iv for iv in index)
+        tm.assert_index_equal(result, expected)
+
     def test_with_nans(self, closed):
         index = self.create_index(closed=closed)
         assert index.hasnans is False

pandas/tests/scalar/interval/test_interval.py

@@ -77,7 +77,7 @@ def test_hash(self, interval):
         (Timedelta('5S'), Timedelta('1H'), Timedelta('59M55S'))])
     def test_length(self, left, right, expected):
         # GH 18789
-        iv = Interval(left, right)
+        iv = Interval(left, right, 'both')
         result = iv.length
         assert result == expected
 
@@ -89,7 +89,7 @@ def test_length(self, left, right, expected):
     @pytest.mark.parametrize('tz', (None, 'UTC', 'CET', 'US/Eastern'))
     def test_length_timestamp(self, tz, left, right, expected):
         # GH 18789
-        iv = Interval(Timestamp(left, tz=tz), Timestamp(right, tz=tz))
+        iv = Interval(Timestamp(left, tz=tz), Timestamp(right, tz=tz), 'both')
         result = iv.length
         expected = Timedelta(expected)
         assert result == expected