new tests

Author: iamlemec

pandas/core/frame.py

@@ -3136,7 +3136,7 @@ def _setitem_multilevel(self, key, value):
                 key = (key,)
             if isinstance(value, DataFrame):
                 if len(key) + value.columns.nlevels != self.columns.nlevels:
-                    raise TypeError(
+                    raise ValueError(
                         "Must pass key/value pair that conforms with number of column "
                         "levels"
                     )

pandas/tests/indexing/multiindex/test_multiindex.py

@@ -159,3 +159,26 @@ def test_multiindex_frame_assign(self):
             axis=1,
         )
         tm.assert_frame_equal(result, expected)
+
+        # invalid usage
+        msg = "Must pass key/value pair that conforms with number of column levels"
+
+        # too few levels at level one
+        with pytest.raises(ValueError, match=msg):
+            df2["m"] = df0
+
+        # too few levels at level two - this appears to be desired
+        # with pytest.raises(ValueError, match=msg):
+        #     df2["m", "x"] = df0["a"]
+
+        # too many levels at level one
+        with pytest.raises(ValueError, match=msg):
+            df2["m"] = df2
+
+        # too many levels at level two
+        with pytest.raises(ValueError, match=msg):
+            df2["m", "x"] = df1
+
+        # too many levels at level three
+        with pytest.raises(ValueError, match=msg):
+            df2["m", "x", "a"] = df0