BUG: loc dropping levels when df has only one row

doc/source/whatsnew/v1.2.0.rst

@@ -637,6 +637,7 @@ Indexing
 - Bug in :meth:`DataFrame.loc` and :meth:`DataFrame.__getitem__`  raising ``KeyError`` when columns were :class:`MultiIndex` with only one level (:issue:`29749`)
 - Bug in :meth:`Series.__getitem__` and :meth:`DataFrame.__getitem__` raising blank ``KeyError`` without missing keys for :class:`IntervalIndex` (:issue:`27365`)
 - Bug in setting a new label on a :class:`DataFrame` or :class:`Series` with a :class:`CategoricalIndex` incorrectly raising ``TypeError`` when the new label is not among the index's categories (:issue:`38098`)
+- Bug in :meth:`DataFrame.loc` dropping levels of :class:`MultiIndex` when :class:`DataFrame` used as input has only one row (:issue:`10521`)
 
 Missing
 ^^^^^^^

pandas/core/indexing.py

@@ -838,8 +838,10 @@ def _getitem_nested_tuple(self, tup: Tuple):
             if self.name != "loc":
                 # This should never be reached, but lets be explicit about it
                 raise ValueError("Too many indices")
-            with suppress(IndexingError):
-                return self._handle_lowerdim_multi_index_axis0(tup)
+            if len(self.obj) > 1 or not any(isinstance(x, slice) for x in tup):
+                # GH#10521 IndexingError is not raised for slices for objs with one row
+                with suppress(IndexingError):
+                    return self._handle_lowerdim_multi_index_axis0(tup)
 
             # this is a series with a multi-index specified a tuple of
             # selectors

pandas/tests/indexing/multiindex/test_loc.py

@@ -695,3 +695,13 @@ def test_loc_getitem_index_differently_ordered_slice_none():
         columns=["a", "b"],
     )
     tm.assert_frame_equal(result, expected)
+
+
+def test_loc_getitem_drops_levels_for_one_row_dataframe():
+    # GH#10521
+    df = DataFrame({"a": ["a"], "b": ["b"], "c": ["a"], "d": 0}).set_index(
+        ["a", "b", "c"]
+    )
+    expected = df.copy()
+    result = df.loc["a", :, "a"]
+    tm.assert_frame_equal(result, expected)