changes ac. to comments

Author: topper-123

doc/source/whatsnew/v1.0.0.rst

@@ -666,7 +666,8 @@ Other API changes
 - When testing pandas, the new minimum required version of pytest is 5.0.1 (:issue:`29664`)
 - :meth:`Series.str.__iter__` was deprecated and will be removed in future releases (:issue:`28277`).
 - Added ``<NA>`` to the list of default NA values for :meth:`read_csv` (:issue:`30821`)
-
+- a ``TypeError`` is now raised if attempting to call :meth:`DataFrame.swaplevels` and the axis is not
+  a :class:`MultiIndex`. Previously a ``AttributeError`` was raised (:issue:`31126`)
 
 .. _whatsnew_100.api.documentation:
 

pandas/core/frame.py

@@ -396,6 +396,7 @@ class DataFrame(NDFrame):
     2  7  8  9
     """
 
+    _internal_names_set = {"columns", "index"} | NDFrame._internal_names_set
     _typ = "dataframe"
 
     @property
@@ -5273,6 +5274,10 @@ def swaplevel(self, i=-2, j=-1, axis=0) -> "DataFrame":
         result = self.copy()
 
         axis = self._get_axis_number(axis)
+
+        if not isinstance(result._get_axis(axis), ABCMultiIndex):  # pragma: no cover
+            raise TypeError("Can only swap levels on a hierarchical axis.")
+
         if axis == 0:
             assert isinstance(result.index, ABCMultiIndex)
             result.index = result.index.swaplevel(i, j)
@@ -8481,11 +8486,9 @@ def isin(self, values) -> "DataFrame":
     index: "Index" = properties.AxisProperty(
         axis=1, doc="The index (row labels) of the DataFrame."
     )
-    NDFrame._internal_names_set.add("index")
     columns: "Index" = properties.AxisProperty(
         axis=0, doc="The column labels of the DataFrame."
     )
-    NDFrame._internal_names_set.add("columns")
 
     # ----------------------------------------------------------------------
     # Add plotting methods to DataFrame

pandas/core/series.py

@@ -178,6 +178,7 @@ class Series(base.IndexOpsMixin, generic.NDFrame):
 
     _name: Optional[Hashable]
     _metadata: List[str] = ["name"]
+    _internal_names_set = {"index"} | generic.NDFrame._internal_names_set
     _accessors = {"dt", "cat", "str", "sparse"}
     _deprecations = (
         base.IndexOpsMixin._deprecations
@@ -4499,7 +4500,6 @@ def to_period(self, freq=None, copy=True) -> "Series":
     index: "Index" = properties.AxisProperty(
         axis=0, doc="The index (axis labels) of the Series."
     )
-    generic.NDFrame._internal_names_set.add("index")
 
     # ----------------------------------------------------------------------
     # Accessor Methods

pandas/tests/test_multilevel.py

@@ -957,6 +957,10 @@ def test_swaplevel(self):
         exp = self.frame.swaplevel("first", "second").T
         tm.assert_frame_equal(swapped, exp)
 
+        msg = "Can only swap levels on a hierarchical axis."
+        with pytest.raises(TypeError, match=msg):
+            DataFrame(range(3)).swaplevel()
+
     def test_reorder_levels(self):
         result = self.ymd.reorder_levels(["month", "day", "year"])
         expected = self.ymd.swaplevel(0, 1).swaplevel(1, 2)