ENH: add IntervalIndex.get_loc_exact to look for exact matches only

asv_bench/benchmarks/indexing.py

@@ -229,6 +229,10 @@ def time_getitem_list(self, monotonic):
     def time_loc_list(self, monotonic):
         monotonic.loc[80000:]
 
+    def time_get_loc_exact(self, monotonic):
+        interval = monotonic.index[80000]
+        monotonic.index.get_loc_exact(interval)
+
 
 class PanelIndexing(object):
 

doc/source/whatsnew/v0.23.0.txt

@@ -323,6 +323,7 @@ Other Enhancements
 
 - ``IntervalIndex.astype`` now supports conversions between subtypes when passed an ``IntervalDtype`` (:issue:`19197`)
 - :class:`IntervalIndex` and its associated constructor methods (``from_arrays``, ``from_breaks``, ``from_tuples``) have gained a ``dtype`` parameter (:issue:`19262`)
+- New :meth:`IntervalIndex.get_loc_exact` has been added to find exact Interval matches only (:issue:`19349`)
 
 .. _whatsnew_0230.api_breaking:
 

pandas/core/indexes/base.py

@@ -2536,6 +2536,21 @@ def get_loc(self, key, method=None, tolerance=None):
             raise KeyError(key)
         return loc
 
+    def get_loc_exact(self, key, method=None):
+        """Get integer location, slice or boolean mask for exact
+        matches only.
+
+        This method dispatches to :meth:`get_loc`. The use for
+        ``get_loc_exact`` is mainly in :class:`IntervalIndex`,
+        when a exact match is needed.
+
+        See Also
+        --------
+        get_loc
+        pandas.IntervalIndex.get_loc_exact
+        """
+        return self.get_loc(key, method=method)
+
     def get_value(self, series, key):
         """
         Fast lookup of value from 1-dimensional ndarray. Only use this if you

pandas/core/indexes/interval.py

@@ -899,15 +899,11 @@ def _searchsorted_monotonic(self, label, side, exclude_label=False):
 
     def _get_loc_only_exact_matches(self, key):
         if isinstance(key, Interval):
-
             if not self.is_unique:
                 raise ValueError("cannot index with a slice Interval"
                                  " and a non-unique index")
-
-            # TODO: this expands to a tuple index, see if we can
-            # do better
-            return Index(self._multiindex.values).get_loc(key)
-        raise KeyError
+            return self.get_loc_exact(key)
+        raise KeyError(key)
 
     def _find_non_overlapping_monotonic_bounds(self, key):
         if isinstance(key, IntervalMixin):
@@ -970,6 +966,10 @@ def get_loc(self, key, method=None):
         >>> overlapping_index = pd.IntervalIndex([i2, i3])
         >>> overlapping_index.get_loc(1.5)
         array([0, 1], dtype=int64)
+
+        See Also
+        --------
+        get_loc_exact : Exact matches only
         """
         self._check_method(method)
 
@@ -1003,6 +1003,59 @@ def get_loc(self, key, method=None):
             else:
                 return self._engine.get_loc(key)
 
+    def get_loc_exact(self, key, method=None):
+        """Get integer location, slice or boolean mask for exact
+        Interval matches only.
+
+        Parameters
+        ----------
+        key : Interval
+            The label we want to find locations for. Must have type
+            :class:`Interval`
+        method : {None}, optional
+            * default: matches where the label exactly matches a given
+              :class:`Interval`.
+
+        Returns
+        -------
+        loc : int if unique index, slice if monotonic index, else mask
+
+        Raises
+        ------
+        KeyError if key is not found
+
+        Examples
+        ---------
+        >>> i1, i2 = pd.Interval(0, 1), pd.Interval(1, 2)
+        >>> index = pd.IntervalIndex([i1, i2])
+        >>> index.get_loc_exact(i1)
+        0
+
+        If an exact match is not found, a KeyError is raised
+
+        >>> index.get_loc_exact(pd.Interval(0.5, 1.5))
+        KeyError: Interval(0.5, 1.5, closed='right')
+
+        If a label is in several locations, you get all the relevant
+        locations.
+
+        >>> index = pd.IntervalIndex([i1, i2, i1])
+        >>> index.get_loc_exact(i1)
+        array([0, 2], dtype=int64)
+
+        See Also
+        --------
+        get_loc
+        """
+
+        all_matches = self.get_loc(key, method=method)
+        exact_matches = self[all_matches] == key
+        if np.all(exact_matches):
+            return all_matches
+        elif np.any(exact_matches):
+            return all_matches[exact_matches]
+        raise KeyError(key)
+
     def get_value(self, series, key):
         if com.is_bool_indexer(key):
             loc = key

pandas/tests/indexes/interval/test_interval.py

@@ -497,6 +497,24 @@ def test_get_loc_interval(self):
         pytest.raises(KeyError, self.index.get_loc,
                       Interval(-1, 0, 'left'))
 
+    def test_get_loc_exact(self):
+        # GH19353
+        assert self.index.get_loc_exact(Interval(0, 1)) == 0
+        with pytest.raises(KeyError):
+            self.index.get_loc_exact(1)
+        with pytest.raises(KeyError):
+            self.index.get_loc_exact(Interval(0, 1, 'left'))
+        with pytest.raises(KeyError):
+            self.index.get_loc_exact(Interval(0, 0.5))
+        with pytest.raises(KeyError):
+            self.index.get_loc_exact(Interval(2, 3))
+        with pytest.raises(KeyError):
+            self.index.get_loc_exact(Interval(-1, 0, 'left'))
+
+        # The below tests if get_loc_exact interferes with caching
+        # used for index.get_loc. See #19353#issuecomment-364295029
+        assert self.index.get_loc(Interval(0, 1)) == 0
+
     # To be removed, replaced by test_interval_new.py (see #16316, #16386)
     def test_get_indexer(self):
         actual = self.index.get_indexer([-1, 0, 0.5, 1, 1.5, 2, 3])