Fix regression in .ix fallback with IntervalIndex (pandas-dev#27926)

Author: jorisvandenbossche

doc/source/whatsnew/v0.25.1.rst

@@ -84,6 +84,7 @@ Indexing
 - Bug in partial-string indexing returning a NumPy array rather than a ``Series`` when indexing with a scalar like ``.loc['2015']`` (:issue:`27516`)
 - Break reference cycle involving :class:`Index` and other index classes to allow garbage collection of index objects without running the GC. (:issue:`27585`, :issue:`27840`)
 - Fix regression in assigning values to a single column of a DataFrame with a ``MultiIndex`` columns (:issue:`27841`).
+- Fix regression in ``.ix`` fallback with an ``IntervalIndex`` (:issue:`27865`).
 -
 
 Missing

pandas/core/indexing.py

@@ -124,14 +124,17 @@ def __getitem__(self, key):
             key = tuple(com.apply_if_callable(x, self.obj) for x in key)
             try:
                 values = self.obj._get_value(*key)
-            except (KeyError, TypeError, InvalidIndexError):
+            except (KeyError, TypeError, InvalidIndexError, AttributeError):
                 # TypeError occurs here if the key has non-hashable entries,
                 #  generally slice or list.
                 # TODO(ix): most/all of the TypeError cases here are for ix,
                 #  so this check can be removed once ix is removed.
                 # The InvalidIndexError is only catched for compatibility
                 #  with geopandas, see
                 #  https://github.com/pandas-dev/pandas/issues/27258
+                # TODO: The AttributeError is for IntervalIndex which
+                #  incorrectly implements get_value, see
+                #  https://github.com/pandas-dev/pandas/issues/27865
                 pass
             else:
                 if is_scalar(values):

pandas/tests/indexing/test_ix.py

@@ -343,3 +343,13 @@ def test_ix_duplicate_returns_series(self):
             r = df.ix[0.2, "a"]
         e = df.loc[0.2, "a"]
         tm.assert_series_equal(r, e)
+
+    def test_ix_intervalindex(self):
+        # https://github.com/pandas-dev/pandas/issues/27865
+        df = DataFrame(
+            np.random.randn(5, 2),
+            index=pd.IntervalIndex.from_breaks([-np.inf, 0, 1, 2, 3, np.inf]),
+        )
+        result = df.ix[0:2, 0]
+        expected = df.iloc[0:2, 0]
+        tm.assert_series_equal(result, expected)