Investigate DataFrame.sort_values with pandas behavior.

import pandas as pd
pdf = pd.DataFrame(
    {
        "a": pd.Categorical([1, 2, 3, 1, 2, 3]),
        "b": pd.Categorical(
            ["b", "a", "c", "c", "b", "a"], categories=["c", "b", "d", "a"]
        ),
    },
)
pdf.groupby("a").apply(lambda x: x).sort_values(["a"])

Traceback (most recent call last):
...
ValueError: 'a' is both an index level and a column label, which is ambiguous.