loc() does not swap two rows in multi-index pandas dataframe

df = pd.DataFrame(np.arange(12).reshape((4, 3)),
    index=[['a', 'a', 'b', 'b'], [1, 2, 1, 2]],
    columns=[['Ohio', 'Ohio', 'Colorado'],
    ['Green', 'Red', 'Green']])

df.loc[['b','a'],:]  # does not swap

# df.reindex(index=['b', 'a'], level=0) # This works

Problem description

df.loc[[‘b’,‘a’],:] does not swap the rows ‘a’ and ‘b’, nor does for a Series.

Expected Output

The output should be the same as the one obtained via df.reindex(index=[‘b’, ‘a’], level=0)

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None python: 3.6.3.final.0 python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel byteorder: little LC_ALL: None LANG: None LOCALE: None.None

pandas: 0.20.3 pytest: 3.2.1 pip: 10.0.1 setuptools: 36.5.0.post20170921 Cython: 0.26.1 numpy: 1.14.0 scipy: 0.19.1 xarray: None IPython: 6.1.0 sphinx: 1.6.3 patsy: 0.4.1 dateutil: 2.6.1 pytz: 2017.2 blosc: None bottleneck: 1.2.1 tables: 3.4.2 numexpr: 2.6.2 feather: None matplotlib: 2.1.0 openpyxl: 2.4.8 xlrd: 1.1.0 xlwt: 1.3.0 xlsxwriter: 1.0.2 lxml: 4.1.0 bs4: 4.6.0 html5lib: 0.999999999 sqlalchemy: 1.1.13 pymysql: None psycopg2: None jinja2: 2.10 s3fs: None pandas_gbq: None pandas_datareader: 0.7.0

[eisthfroyalblue]

For a single-indexed table, we can reindex it using .loc() instead of .reindex()

df = pd.DataFrame([[1,2,3],[4,5,6]], index=['a','b'])

df.loc[['b','a']]

Out[9]: 
   0  1  2
b  4  5  6
a  1  2  3

In his book, “Python for Data Analysis 2ed” Wes McKinney says “you can reindex more succinctly by label-indexing with loc, and many users prefer to use it”, although he used only a single-index table for illustration.

This may be not a bug. however it is natural for one to expect the same effect for the multi-index table.

[pl-phan]

I too got mixed up, by using .loc[mylist] on a multi-index dataframe, it did not preserve the order of mylist. I realized it way later than I should have.

Surely it is strange that it preserves the order for single-index, but not multi-index.

Even if it doesn’t qualifies as a bug, is it possible to add a warning ? Only when .loc[key] is used on a multi-index structure, if key is an iterable, just to warn that the order might not be preserved (and maybe recommending using .reindex(index=key, level=0) instead)

I tried to locate in the code where the warning should be written, without success. 😦