Pandas: sort_index(inplace = True) produces SettingWithCopyWarning

[phaethon]

When I am trying to do rows.sort_index(inplace = True) I get SettingWithCopyWarning:
A value is trying to be set on a copy of a slice from a DataFrame

rows = rows.sort_index() can be used as a workaround to avoid the warning.

Pandas version 0.17.0 installed by pip.

[TomAugspurger]

Can you share an example? I wasn't able to reproduce:

In [4]: df = pd.DataFrame({'A': [1, 2, 3], 'B': ['a', 'b', 'c']}, index=[1, 0, 2])

In [5]: df
Out[5]:
   A  B
1  1  a
0  2  b
2  3  c

In [6]: df.sort_index(inplace=True)  # no warning

I'm guessing your rows is a slice from another DataFrame?

[phaethon]

rows is created by filtering from a different DataFrame. This is a more complete example. sort_index(inplace = True) works for rows1 without warning, but produces warning for rows2.

initial_rows = pd.DataFrame([{'a': 1, 'b': 'x'}, {'a': 2, 'b': 'c'}])
rows = rows[initial_rows['b'] == 'x']
rows.sort_index(inplace = True)

[jreback]

@phaethon this of course won't work, you cannot sort inplace on something ELSE which is filtered. Furthermore this is not a good patten. never use inplace

[phaethon]

In my code I actually update index after filtering with a function of one of the columns:

initial_rows = pd.DataFrame([{'a': 1, 'b': 'x'}, {'a': 2, 'b': 'c'}])
rows = rows[initial_rows['b'] == 'x']
rows.index = function_of_a(rows['a'])
rows.sort_index(inplace = True)

What would be suggested pattern of usage in this case?

[TomAugspurger]

rows = rows.sort_index() will work. Using inplace is tricky since you (the user) _has_ to be sure it's not a view. It's not always clear when reading the code whether this is the case. And using inplace=True usually won't have any performance benefits, so it's just easier to always reassign rows = rows.sort_index().

[phaethon]

Actually rows.sort_index(inplace = True) works, too. Just displaying annoying warning.
If there was no such option inplace mentioned in documentation, I would have used rows = rows.sort_index(), but I was expecting some performance benefit. Adding additional comment on this in documentation certainly would help. Or removing inplace option if it brings no benefits.