sort_index is not working with MultiIndex

[garawalid]

DataFrame.sort_index() is not working when the DataFrame uses MultiIndex.

Reproduce the error:

import pandas as pd
import databricks.koalas as ks

pdf = pd.DataFrame({
    ('x', 'a', '1'): [1, 2, 3],
    ('x', 'b', '2'): [4, 5, 6],
    ('y.z', 'c.d', '3'): [7, 8, 9],
    ('x', 'b', '4'): [10, 11, 12],
}, index=[0, 1, 3])

kdf = ks.from_pandas(pdf)

kdf.sort_index()

Traceback (most recent call last):
  File "/home/xxx/databricks/koalas/tests/bug.py", line 15, in <module>
    kdf.sort_index()
  File "/home/xxx/databricks/koalas/frame.py", line 4295, in sort_index
    inplace=inplace, na_position=na_position)
  File "/home/xxx/databricks/koalas/frame.py", line 4191, in sort_values
    for colname, asc in zip(by, ascending)]
  File "/home/xxx/databricks/koalas/frame.py", line 4191, in <listcomp>
    for colname, asc in zip(by, ascending)]
  File "/home/xxx/databricks/koalas/frame.py", line 6343, in __getitem__
    return self._pd_getitem(key)
  File "/home/xxx/databricks/koalas/frame.py", line 6280, in _pd_getitem
    return self._get_from_multiindex_column((key,))
  File "/home/xxx/databricks/koalas/frame.py", line 6235, in _get_from_multiindex_column
    raise KeyError(k)
KeyError: '__index_level_0__'

[garawalid]

cc @ueshin

[ueshin]

Thanks for the report. I'll work on this.

[NA-Dev]

I am sorting a dataframe with multiIndex where one is a string, and one is a date. The sorting is not working properly, as I can see dates are still out of order. Python 3.9 Pandas 1.4.2

[itholic]

@NA-Dev Would you mind filing a ticket to Apache Spark JIRA ??

Koalas is ported into PySpark as pandas API on Spark.