ENH: drop_level argument for xs

doc/source/release.rst

@@ -48,6 +48,7 @@ pandas 0.13
     overlapping color and style arguments (:issue:`4402`)
   - Significant table writing performance improvements in ``HDFStore``
   - JSON date serialisation now performed in low-level C code.
+  - Add ``drop_level`` argument to xs (:issue:`4180`)
   - ``Index.copy()`` and ``MultiIndex.copy()`` now accept keyword arguments to
     change attributes (i.e., ``names``, ``levels``, ``labels``)
     (:issue:`4039`)

pandas/core/frame.py

@@ -2063,7 +2063,7 @@ def _sanitize_column(self, key, value):
     def _series(self):
         return self._data.get_series_dict()
 
-    def xs(self, key, axis=0, level=None, copy=True):
+    def xs(self, key, axis=0, level=None, copy=True, drop_level=True):
         """
         Returns a cross-section (row(s) or column(s)) from the DataFrame.
         Defaults to cross-section on the rows (axis=0).
@@ -2079,6 +2079,8 @@ def xs(self, key, axis=0, level=None, copy=True):
             which levels are used. Levels can be referred by label or position.
         copy : boolean, default True
             Whether to make a copy of the data
+        drop_level, default True
+            If False, returns object with same levels as self.
 
         Examples
         --------
@@ -2130,11 +2132,13 @@ def xs(self, key, axis=0, level=None, copy=True):
         Returns
         -------
         xs : Series or DataFrame
+
         """
         axis = self._get_axis_number(axis)
         labels = self._get_axis(axis)
         if level is not None:
-            loc, new_ax = labels.get_loc_level(key, level=level)
+            loc, new_ax = labels.get_loc_level(key, level=level,
+                                    drop_level=drop_level)
 
             if not copy and not isinstance(loc, slice):
                 raise ValueError('Cannot retrieve view (copy=False)')
@@ -2168,7 +2172,8 @@ def xs(self, key, axis=0, level=None, copy=True):
 
         index = self.index
         if isinstance(index, MultiIndex):
-            loc, new_index = self.index.get_loc_level(key)
+            loc, new_index = self.index.get_loc_level(key,
+                                    drop_level=drop_level)
         else:
             loc = self.index.get_loc(key)
 

pandas/core/index.py

@@ -2560,7 +2560,7 @@ def get_loc(self, key):
         else:
             return self._get_level_indexer(key, level=0)
 
-    def get_loc_level(self, key, level=0):
+    def get_loc_level(self, key, level=0, drop_level=True):
         """
         Get integer location slice for requested label or tuple
 
@@ -2572,7 +2572,9 @@ def get_loc_level(self, key, level=0):
         -------
         loc : int or slice object
         """
-        def _drop_levels(indexer, levels):
+        def _maybe_drop_levels(indexer, levels, drop_level):
+            if not drop_level:
+                return self[indexer]
             # kludgearound
             new_index = self[indexer]
             levels = [self._get_level_number(i) for i in levels]
@@ -2593,7 +2595,8 @@ def _drop_levels(indexer, levels):
                     loc = mask
 
                 result = loc if result is None else result & loc
-            return result, _drop_levels(result, level)
+
+            return result, _maybe_drop_levels(result, level, drop_level)
 
         level = self._get_level_number(level)
 
@@ -2606,7 +2609,7 @@ def _drop_levels(indexer, levels):
             try:
                 if key in self.levels[0]:
                     indexer = self._get_level_indexer(key, level=level)
-                    new_index = _drop_levels(indexer, [0])
+                    new_index = _maybe_drop_levels(indexer, [0], drop_level)
                     return indexer, new_index
             except TypeError:
                 pass
@@ -2625,7 +2628,7 @@ def _drop_levels(indexer, levels):
                         raise KeyError(key)
                     ilevels = [i for i in range(len(key))
                                if key[i] != slice(None, None)]
-                    return indexer, _drop_levels(indexer, ilevels)
+                    return indexer, _maybe_drop_levels(indexer, ilevels, drop_level)
             else:
                 indexer = None
                 for i, k in enumerate(key):
@@ -2652,10 +2655,10 @@ def _drop_levels(indexer, levels):
                     indexer = slice(None, None)
                 ilevels = [i for i in range(len(key))
                            if key[i] != slice(None, None)]
-                return indexer, _drop_levels(indexer, ilevels)
+                return indexer, _maybe_drop_levels(indexer, ilevels, drop_level)
         else:
             indexer = self._get_level_indexer(key, level=level)
-            new_index = _drop_levels(indexer, [level])
+            new_index = _maybe_drop_levels(indexer, [level], drop_level)
             return indexer, new_index
 
     def _get_level_indexer(self, key, level=0):

pandas/tests/test_frame.py

@@ -7261,6 +7261,18 @@ def test_xs_duplicates(self):
         exp = df.irow(2)
         assert_series_equal(cross, exp)
 
+    def test_xs_keep_level(self):
+        df = DataFrame({'day': {0: 'sat', 1: 'sun'},
+                        'flavour': {0: 'strawberry', 1: 'strawberry'},
+                        'sales': {0: 10, 1: 12},
+                        'year': {0: 2008, 1: 2008}}).set_index(['year','flavour','day'])
+        result = df.xs('sat', level='day', drop_level=False)
+        expected = df[:1]
+        assert_frame_equal(result, expected)
+
+        result = df.xs([2008, 'sat'], level=['year', 'day'], drop_level=False)
+        assert_frame_equal(result, expected)
+
     def test_pivot(self):
         data = {
             'index': ['A', 'B', 'C', 'C', 'B', 'A'],