sort_index behavior differs for the same DataFrame? #9212
Description
I feel like I've encountered a bug. In the following scenario, the first sort_index call behaves as expected, but the second does not. Does someone know what the difference is here?
In [1]: import pandas as pd In [2]: pd.__version__ Out[2]: '0.15.2' In [3]: tuples = [(' foo', 'bar'), ('foo', 'bar'), (' foo ()', 'bar')] In [4]: cols = pd.MultiIndex.from_tuples(tuples) In [5]: df = pd.DataFrame(index=cols, data={'baz': [0, 1, 2]}) In [6]: df Out[6]: baz foo bar 0 foo bar 1 foo () bar 2 In [7]: df.sort_index() Out[7]: baz foo bar 0 foo () bar 2 foo bar 1 In [8]: tuples = [(' foo', 'bar'), ('foo', 'bar')] In [9]: cols = pd.MultiIndex.from_tuples(tuples) In [10]: df = pd.DataFrame(index=cols, data={'baz': [0, 1]}) In [11]: df Out[11]: baz foo bar 0 foo bar 1 In [12]: df.ix[(' foo ()', 'bar'), 'baz'] = 2 In [13]: df Out[13]: baz foo bar 0 foo bar 1 foo () bar 2 In [14]: df.sort_index() Out[14]: baz foo bar 0 foo bar 1 foo () bar 2