BUG: alignment bug when setting a subset of rows and attempting to switch columns

[johne13]

Originally posted at stackexchange:
http://stackoverflow.com/questions/25811529/setting-values-on-a-subset-of-rows-indexing-boolean-setting/25812825#25812825
Jeff suggested posting here also.

df = DataFrame({'L':   ['left', 'right', 'left', 'right'],
                'R':   ['right', 'left', 'right', 'left'],
                'idx': [False, True, False, True],
                'num': np.arange(4) })

df1 = df.copy()
df2 = df.copy()
df3 = df.copy()
df4 = df.copy()

#1 nothing happens (expected result)
df1.loc[df1.idx,['L','R']] = df1.loc[df1.idx,['R','L']]

#2 something weird happens
df2.loc[df2.idx,['L','R']] = df2[['R','L']]

Results below:

In [572]: df
Out[572]: 
       L      R    idx  num
0   left  right  False    0
1  right   left   True    1
2   left  right  False    2
3  right   left   True    3

In [573]: df1
Out[573]: 
       L      R    idx  num
0   left  right  False    0
1  right   left   True    1
2   left  right  False    2
3  right   left   True    3

In [574]: df2
Out[574]: 
       L      R    idx  num
0   left  right  False    0
1   left  right   True    1
2   left  right  False    2
3  right   left   True    3

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.7.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 26 Stepping 5, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.14.1.dev
nose: 1.3.3
Cython: 0.20.1
numpy: 1.8.1
scipy: 0.14.0
statsmodels: 0.5.0
IPython: 2.1.0
sphinx: 1.2.2
patsy: 0.2.1
scikits.timeseries: None
dateutil: 1.5
pytz: 2014.3
bottleneck: None
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.3.1
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: 0.5.5
lxml: 3.3.5
bs4: 4.3.1
html5lib: None
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: 0.9.4
pymysql: None
psycopg2: None