Changes according to comments

Author: topper-123

doc/source/whatsnew/v1.1.0.rst

@@ -137,7 +137,9 @@ Other API changes
 Backwards incompatible API changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 - :meth:`DataFrame.swaplevels` now raises a  ``TypeError`` if the axis is not a :class:`MultiIndex`.
-  Previously a ``AttributeError`` was raised (:issue:`31126`)
+  Previously an ``AttributeError`` was raised (:issue:`31126`)
+- :meth:`DataFrame.xs` now raises a  ``TypeError`` if a ``level`` keyword is supplied and the axis is not a :class:`MultiIndex`.
+  Previously an ``AttributeError`` was raised (:issue:`33610`)
 - :meth:`DataFrameGroupby.mean` and :meth:`SeriesGroupby.mean` (and similarly for :meth:`~DataFrameGroupby.median`, :meth:`~DataFrameGroupby.std` and :meth:`~DataFrameGroupby.var`)
   now raise a  ``TypeError`` if a not-accepted keyword argument is passed into it.
   Previously a ``UnsupportedFunctionCall`` was raised (``AssertionError`` if ``min_count`` passed into :meth:`~DataFrameGroupby.median`) (:issue:`31485`)

pandas/core/generic.py

@@ -590,6 +590,8 @@ def swapaxes(self: FrameOrSeries, axis1, axis2, copy=True) -> FrameOrSeries:
         if copy:
             new_values = new_values.copy()
 
+        # ignore needed because of NDFrame constructor is different than
+        # DataFrame/Series constructors.
         return self._constructor(new_values, *new_axes).__finalize__(  # type: ignore
             self, method="swapaxes"
         )
@@ -3490,7 +3492,8 @@ class   animal   locomotion
         axis = self._get_axis_number(axis)
         labels = self._get_axis(axis)
         if level is not None:
-            assert isinstance(labels, MultiIndex), type(labels)
+            if not isinstance(labels, MultiIndex):
+                raise TypeError("Index must be a MultiIndex")
             loc, new_ax = labels.get_loc_level(key, level=level, drop_level=drop_level)
 
             # create the tuple of the indexer

pandas/tests/indexing/multiindex/test_xs.py

@@ -243,3 +243,15 @@ def test_series_getitem_multiindex_xs_by_label():
 
     result = s.xs("one", level="L2")
     tm.assert_series_equal(result, expected)
+
+
+def test_xs_levels_raises():
+    df = DataFrame({"A": [1, 2, 3]})
+
+    msg = "Index must be a MultiIndex"
+    with pytest.raises(TypeError, match=msg):
+        df.xs(0, level="as")
+
+    s = df.A
+    with pytest.raises(TypeError, match=msg):
+        s.xs(0, level="as")