Specify that both by and level should not be specified in groupby - GH40378

pandas/core/frame.py

@@ -8332,6 +8332,19 @@ def update(
 1  Falcon      370.0
 2  Parrot       24.0
 3  Parrot       26.0
+
+When using ``.groupby()`` on a multiple index dataframe,
+do not specify both ``by`` and ``level``.
+The argument validation should be done in instead be done in
+``.groupby()``, using the name of the specific index.
+
+>>> df = pandas.DataFrame({"col1": ["a", "b", "c"]})
+
+>>> df.index = pandas.MultiIndex.from_arrays([["a", "a", "b"],
+...                                           [1, 2, 1]],
+...                                           names=["x", "y"])
+>>> df.groupby(["col1", "x"]) # Fine
+>>> df.groupby("col1", level=0)  # TypeError
 """
     )
     @Appender(_shared_docs["groupby"] % _shared_doc_kwargs)